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RAISONNEMENT<br />
& INCOHÉRENCE<br />
Sébasti<strong>en</strong> Konieczny
RAISONNEMENT<br />
& INCOHÉRENCE<br />
Sébasti<strong>en</strong> Konieczny<br />
<strong>Habilitation</strong> <strong>à</strong> <strong>diriger</strong> <strong><strong>de</strong>s</strong> <strong>recherches</strong><br />
Composition du jury<br />
Gerhard Brewka (rapporteur)<br />
Professeur <strong>à</strong> l’Université <strong>de</strong> Leipzig<br />
Marie-Christine Rousset (rapporteur)<br />
Professeur <strong>à</strong> l’Université <strong>de</strong> Gr<strong>en</strong>oble<br />
Torst<strong>en</strong> Schaub (rapporteur)<br />
Professeur <strong>à</strong> l’Université <strong>de</strong> Potsdam<br />
Salem B<strong>en</strong>ferhat<br />
Professeur <strong>à</strong> l’Université d’Artois<br />
Andreas Herzig<br />
Directeur <strong>de</strong> <strong>Recherche</strong> au CNRS - IRIT<br />
David Makinson<br />
Professeur <strong>à</strong> la London School of Economics<br />
Pierre Marquis<br />
Professeur <strong>à</strong> l’Université d’Artois
Ce qui est créé par l’esprit est plus vivant que la matière.<br />
(Charles Bau<strong>de</strong>laire)
REMERCIEMENTS<br />
Je voudrais tout d’abord remercier très vivem<strong>en</strong>t Gerhard Brewka, Marie-Christine<br />
Rousset et Torst<strong>en</strong> Schaub pour l’intérêt qu’ils ont porté <strong>à</strong> mon travail <strong>en</strong> acceptant<br />
d’<strong>en</strong> être rapporteurs, et pour leurs remarques et les discussions que nous avons lors<br />
<strong><strong>de</strong>s</strong> différ<strong>en</strong>tes confér<strong>en</strong>ces où nous nous croisons.<br />
Je remercie David Makinson <strong>de</strong> l’honneur qu’il me fait <strong>en</strong> acceptant <strong>de</strong> participer<br />
<strong>à</strong> ce jury. Ses articles sur les relations d’infér<strong>en</strong>ce non monotones et sur la révision<br />
<strong>de</strong> croyances, que j’ai lus p<strong>en</strong>dant mon DEA, m’ont beaucoup impressionné (ils<br />
m’impressionn<strong>en</strong>t toujours autant), et sont pour beaucoup dans le choix <strong>de</strong> ce sujet <strong>de</strong><br />
recherche.<br />
Mon passage <strong>à</strong> l’IRIT (Toulouse) <strong>de</strong> 2001 <strong>à</strong> 2004 m’a été extrêmem<strong>en</strong>t bénéfique,<br />
<strong>en</strong> particulier les discussions avec Leila Amgoud, Salem B<strong>en</strong>ferhat, Philippe Besnard,<br />
Didier Dubois, Hélène Fargier, Andreas Herzig, Jérôme Lang et H<strong>en</strong>ri Pra<strong>de</strong>. Je ne pouvais<br />
les mettre tous dans ce jury, j’ai donc <strong>de</strong>mandé <strong>à</strong> Andreas Herzig <strong>de</strong> les représ<strong>en</strong>ter.<br />
Cela me fait très plaisir qu’il ait accepté, non seulem<strong>en</strong>t parce que les nombreuses discussions<br />
que nous avons eues m’ont beaucoup apporté, mais égalem<strong>en</strong>t parce que le<br />
travail que nous avons effectué sur les opérateurs <strong>de</strong> conflu<strong>en</strong>ce a été initié par une<br />
question qu’Andreas m’a posée lors d’un séminaire <strong>à</strong> Dagsthul.<br />
Je suis très heureux que Salem B<strong>en</strong>ferhat ait accepté <strong>de</strong> faire partie <strong>de</strong> ce jury.<br />
J’apprécie beaucoup les conversations, sci<strong>en</strong>tifiques ou autres, que nous avons régulièrem<strong>en</strong>t.<br />
Un grand merci <strong>en</strong>fin <strong>à</strong> Pierre Marquis, d’abord pour avoir accepté <strong>de</strong> faire partie<br />
<strong>de</strong> ce jury, mais surtout pour notre (longue <strong>à</strong> prés<strong>en</strong>t) collaboration, et pour ces nombreuses<br />
réunions où nous ne sommes jamais d’accord !<br />
Je remercie égalem<strong>en</strong>t l’<strong>en</strong>semble <strong>de</strong> mes co-auteurs, sans qui une majorité <strong>de</strong> ces<br />
travaux n’aurai<strong>en</strong>t pas vu le jour. Merci <strong>en</strong> particulier <strong>à</strong> Patricia Everaere, avec qui<br />
travailler est toujours un plaisir.<br />
Je voudrais <strong>à</strong> prés<strong>en</strong>t remercier celui qui a été, et qui reste, mon maître, Ramón<br />
Pino Pérez. Ramón m’a guidé durant mon DEA et ma thèse. Il m’a beaucoup appris.<br />
Et je continue <strong>à</strong> appr<strong>en</strong>dre <strong>à</strong> chacune <strong>de</strong> nos r<strong>en</strong>contres.<br />
Je souhaite remercier l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> collègues du laboratoire, et surtout ceux que<br />
je croise quotidi<strong>en</strong>nem<strong>en</strong>t <strong>à</strong> la faculté, pour la bonne ambiance <strong>de</strong> travail. Merci <strong>en</strong><br />
particulier <strong>à</strong> Bertrand, Gilles, Jean-Luc, Pierre, Salem, Stéphane, Sylvain et Sylvie pour<br />
les pauses divertissantes. Et bravo <strong>à</strong> Sylvain pour t<strong>en</strong>ir aussi longtemps dans le bureau !<br />
Je voudrais remercier mes amis, et plus particulièrem<strong>en</strong>t Bruno, Sonia et Clau<strong>de</strong>,<br />
ainsi que toute ma famille pour leur souti<strong>en</strong> constant, malgré le fait que dans la vie <strong>de</strong><br />
tous les jours aussi je suis plus doué pour la logique que pour la psychologie...<br />
Merci <strong>à</strong> Isabelle pour tout, et pour le reste.<br />
Ce docum<strong>en</strong>t a bénéficié <strong><strong>de</strong>s</strong> remarques et critiques judicieuses <strong>de</strong> Philippe Besnard,<br />
Pierre Marquis, Ramón Pino Pérez, Patricia Everaere, Sylvain Lagrue, Isabelle<br />
Dieval et Emmanuel G<strong>en</strong>ot, ainsi que <strong>de</strong> l’ai<strong>de</strong> technique (et TEXnique) <strong>de</strong> Bertrand<br />
Mazure et Bruno Beaufils.
TABLE DES MATIÈRES<br />
I Synthèse <strong><strong>de</strong>s</strong> travaux <strong>de</strong> recherche 3<br />
1 Introduction 5<br />
1.1 Co-auteurs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5<br />
1.2 Prés<strong>en</strong>tation <strong>de</strong> la problématique . . . . . . . . . . . . . . . . . . . . 6<br />
1.2.1 Logique propositionnelle . . . . . . . . . . . . . . . . . . . . 6<br />
1.2.2 Bases <strong>de</strong> croyances . . . . . . . . . . . . . . . . . . . . . . . 6<br />
1.2.3 Représ<strong>en</strong>tation <strong>de</strong> l’incertitu<strong>de</strong> . . . . . . . . . . . . . . . . . 7<br />
1.3 Résolution <strong>de</strong> conflits logiques . . . . . . . . . . . . . . . . . . . . . 8<br />
1.4 Plan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9<br />
1.5 Notations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10<br />
2 Démarche sci<strong>en</strong>tifique 11<br />
2.1 Démarche . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11<br />
2.2 Infér<strong>en</strong>ce non monotone . . . . . . . . . . . . . . . . . . . . . . . . 13<br />
2.3 Révision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14<br />
2.4 Vote . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14<br />
2.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14<br />
3 Raisonnem<strong>en</strong>t <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce 17<br />
3.1 Infér<strong>en</strong>ce <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce . . . . . . . . . . . . . . . . . . 17<br />
3.1.1 Affaiblir les hypothèses . . . . . . . . . . . . . . . . . . . . 19<br />
3.1.2 Affaiblir la logique . . . . . . . . . . . . . . . . . . . . . . . 20<br />
3.2 Mesure <strong>de</strong> l’incohér<strong>en</strong>ce . . . . . . . . . . . . . . . . . . . . . . . . 24<br />
3.2.1 Mesures basées sur les formules . . . . . . . . . . . . . . . . 25<br />
3.2.2 Mesures basées sur les variables . . . . . . . . . . . . . . . . 26<br />
3.2.3 Valeurs d’incohér<strong>en</strong>ce . . . . . . . . . . . . . . . . . . . . . 28<br />
4 Révision 31<br />
4.1 Le cadre AGM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31<br />
4.1.1 Postulats AGM . . . . . . . . . . . . . . . . . . . . . . . . . 33<br />
4.1.2 Théorèmes <strong>de</strong> représ<strong>en</strong>tation . . . . . . . . . . . . . . . . . . 36<br />
4.1.3 Li<strong>en</strong>s avec les logiques non monotones . . . . . . . . . . . . 40<br />
4.2 Révision itérée . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
4.3 Itération dans le cadre AGM . . . . . . . . . . . . . . . . . . . . . . 44<br />
4.4 Révision itérée et états épistémiques . . . . . . . . . . . . . . . . . . 45<br />
4.5 Opérateurs d’amélioration . . . . . . . . . . . . . . . . . . . . . . . 45<br />
5 Fusion 47<br />
5.1 Caractérisation <strong>de</strong> la fusion . . . . . . . . . . . . . . . . . . . . . . . 47<br />
5.2 Familles d’opérateurs <strong>de</strong> fusion . . . . . . . . . . . . . . . . . . . . . 50<br />
5.2.1 Opérateurs <strong>à</strong> base <strong>de</strong> modèles . . . . . . . . . . . . . . . . . 51<br />
5.2.2 Opérateurs <strong>à</strong> base <strong>de</strong> formules . . . . . . . . . . . . . . . . . 52<br />
5.2.3 Opérateurs DA 2 . . . . . . . . . . . . . . . . . . . . . . . . . 54<br />
5.2.4 Opérateurs disjonctifs . . . . . . . . . . . . . . . . . . . . . 54<br />
5.2.5 Opérateurs <strong>à</strong> base <strong>de</strong> conflits . . . . . . . . . . . . . . . . . . 56<br />
5.2.6 Opérateurs <strong>à</strong> base <strong>de</strong> défauts . . . . . . . . . . . . . . . . . . 57<br />
5.2.7 Opérateurs <strong>à</strong> base <strong>de</strong> similarité . . . . . . . . . . . . . . . . . 58<br />
5.3 Fusion <strong>de</strong> croyances versus fusion <strong>de</strong> buts . . . . . . . . . . . . . . . 59<br />
5.4 Manipulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60<br />
5.5 Fusion dans d’autres cadres . . . . . . . . . . . . . . . . . . . . . . . 63<br />
5.5.1 Fusion prioritaire, fusion et révision itérée . . . . . . . . . . . 63<br />
5.5.2 Fusion <strong>de</strong> bases pondérées . . . . . . . . . . . . . . . . . . . 64<br />
5.5.3 Fusion <strong>en</strong> logique du premier ordre . . . . . . . . . . . . . . 65<br />
5.5.4 Fusion <strong>de</strong> programmes logiques . . . . . . . . . . . . . . . . 65<br />
5.5.5 Fusion <strong>de</strong> réseaux <strong>de</strong> contraintes . . . . . . . . . . . . . . . . 66<br />
5.5.6 Fusion <strong>de</strong> systèmes d’argum<strong>en</strong>tation . . . . . . . . . . . . . . 66<br />
6 Négociation 69<br />
6.1 Fusion itérée . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70<br />
6.2 Fusion <strong>de</strong> croyances comme un jeu <strong>en</strong>tre sources . . . . . . . . . . . 70<br />
6.3 Opérateurs <strong>de</strong> conflu<strong>en</strong>ce . . . . . . . . . . . . . . . . . . . . . . . . 71<br />
6.4 Vers une caractérisation <strong>de</strong> la conciliation . . . . . . . . . . . . . . . 71<br />
7 Conclusion et Perspectives 73<br />
7.1 Mesures <strong>de</strong> conflit . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74<br />
7.2 Fusion et choix social . . . . . . . . . . . . . . . . . . . . . . . . . . 75<br />
7.3 Economie et représ<strong>en</strong>tation <strong><strong>de</strong>s</strong> connaissances . . . . . . . . . . . . . 76<br />
II Articles 77<br />
8 Liste <strong><strong>de</strong>s</strong> articles 79<br />
9 Shapley inconsist<strong>en</strong>cy values 81<br />
10 Measuring inconsist<strong>en</strong>cy through minimal inconsist<strong>en</strong>t sets 93<br />
11 Improvem<strong>en</strong>t operators 105<br />
12 On iterated revision in the AGM framework 117
13 DA 2 merging operators 131<br />
14 On the merging of Dung’s argum<strong>en</strong>tation systems 171<br />
15 Belief base merging as a game 209<br />
16 Conflu<strong>en</strong>ce operators 231<br />
17 Bibliographie 247<br />
18 Publications 257<br />
19 In<strong>de</strong>x 263
Première partie<br />
Synthèse <strong><strong>de</strong>s</strong> travaux <strong>de</strong><br />
recherche
1.1 Co-auteurs<br />
1.2 Prés<strong>en</strong>tation <strong>de</strong> la problématique<br />
1.3 Résolution <strong>de</strong> conflits logiques<br />
1.4 Plan<br />
1.5 Notations<br />
Chapitre I<br />
INTRODUCTION<br />
Le tout est plus grand que la somme <strong><strong>de</strong>s</strong> parties<br />
(Aristote)<br />
Ce docum<strong>en</strong>t est un rapport d’habilitation <strong>à</strong> <strong>diriger</strong> <strong><strong>de</strong>s</strong> <strong>recherches</strong>. Il prés<strong>en</strong>te une<br />
gran<strong>de</strong> partie <strong>de</strong> mes travaux <strong>de</strong> recherche, qui ont comme thème commun la<br />
résolution <strong>de</strong> conflits logiques (le raisonnem<strong>en</strong>t <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ces).<br />
Pour <strong><strong>de</strong>s</strong> raisons <strong>de</strong> cohér<strong>en</strong>ce je ne détaillerai pas les travaux que j’ai réalisé sur<br />
d’autres thèmes, tels que la théorie <strong>de</strong> la décision [53], la théorie <strong><strong>de</strong>s</strong> jeux [34, 57, 70]<br />
et la planification multi-ag<strong>en</strong>ts [33, 69].<br />
1.1. Co-auteurs<br />
Je ne conçois pas la recherche comme une activité solitaire. Il me semble que les<br />
échanges lors <strong>de</strong> réunions sci<strong>en</strong>tifiques permett<strong>en</strong>t <strong>de</strong> faire naître <strong><strong>de</strong>s</strong> idées et résultats<br />
que chacun <strong><strong>de</strong>s</strong> participants n’aurait pas eus seul. Bref, <strong>en</strong> recherche comme ailleurs,<br />
je suis convaincu que Le tout est plus grand que la somme <strong><strong>de</strong>s</strong> parties.<br />
La plupart <strong>de</strong> mes travaux ont été réalisés <strong>en</strong> collaboration avec d’autres collègues.<br />
Dans la suite j’utiliserai un nous qui n’est donc pas une figure <strong>de</strong> style, mais qui illustre<br />
ce travail <strong>en</strong> commun. Je ne citerai pas explicitem<strong>en</strong>t les noms <strong><strong>de</strong>s</strong> co-auteurs correspondant<br />
<strong>à</strong> chaque travail, pour ne pas alourdir la prés<strong>en</strong>tation, mais ces noms sont<br />
facilem<strong>en</strong>t i<strong>de</strong>ntifiables dans les articles correspondants. Ces co-auteurs sont 1 : Pierre<br />
Marquis (CRIL, L<strong>en</strong>s), Ramón Pino Pérez (Université <strong><strong>de</strong>s</strong> An<strong><strong>de</strong>s</strong>, Mérida, V<strong>en</strong>ezuela),<br />
Patricia Everaere (LIFL, Lille), Jérôme Lang (LAMSADE, Paris), Ramzi B<strong>en</strong> Larbi<br />
(CRIL, L<strong>en</strong>s), Anthony Hunter (University College London, Royaume Uni), Didier<br />
Dubois (IRIT, Toulouse), H<strong>en</strong>ri Pra<strong>de</strong> (IRIT, Toulouse), Olivier Gauwin (LIFL, Lille),<br />
1. Classés par nombre d’articles <strong>en</strong> commun.<br />
5
Salem B<strong>en</strong>ferhat (CRIL, L<strong>en</strong>s), Odile Papini (LSIS, Marseille), Stéphane Janot (LIFL,<br />
Lille), Hassan Bezzazi (LIFL, Lille), Sylvie Coste-Marquis (CRIL, L<strong>en</strong>s), Caroline Devred<br />
(LERIA, Angers), Marie-Christine Lagasquie-Schiex (IRIT, Toulouse), Andreas<br />
Herzig (IRIT, Toulouse), Philippe Besnard (IRIT, Toulouse), Laur<strong>en</strong>t Perrussel (IRIT,<br />
Toulouse), Eric Grégoire (CRIL, L<strong>en</strong>s).<br />
Dans la suite, nous indiquons les citations <strong>de</strong> nos articles avec <strong><strong>de</strong>s</strong> référ<strong>en</strong>ces numériques<br />
(par exemple [35]). Les citations d’articles d’autres auteurs sont alphanumériques<br />
(par exemple [Mak94]).<br />
1.2. Prés<strong>en</strong>tation <strong>de</strong> la problématique<br />
Nous nous intéressons au problème du raisonnem<strong>en</strong>t <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce.<br />
Cette incohér<strong>en</strong>ce peut avoir différ<strong>en</strong>tes causes, que nous détaillons ci-après, mais elle<br />
est toujours due <strong>à</strong> l’incertitu<strong>de</strong> inhér<strong>en</strong>te au fait que les ag<strong>en</strong>ts raisonn<strong>en</strong>t <strong>à</strong> partir<br />
<strong>de</strong> croyances qui sont par nature incertaines et donc pot<strong>en</strong>tiellem<strong>en</strong>t fausses. Nous<br />
nous intéressons aux approches qualitatives, qui ne nécessit<strong>en</strong>t pas <strong>de</strong> disposer d’un<br />
<strong>en</strong>semble très important d’informations. Nous nous focalisons <strong>en</strong> particulier sur les<br />
approches logiques, où les croyances <strong><strong>de</strong>s</strong> ag<strong>en</strong>ts sont exprimées <strong>en</strong> logique propositionnelle.<br />
1.2.1. Logique propositionnelle<br />
Un langage <strong>de</strong> représ<strong>en</strong>tation doit satisfaire <strong>à</strong> un certain nombre <strong>de</strong> contraintes, dont<br />
certaines sont antinomiques. Parmi les nombreuses propriétés que l’on peut att<strong>en</strong>dre<br />
d’un langage <strong>de</strong> représ<strong>en</strong>tation, on peut au moins m<strong>en</strong>tionner :<br />
• la compréh<strong>en</strong>sibilité : il faut que le langage soit facilem<strong>en</strong>t compréh<strong>en</strong>sible par<br />
un humain ;<br />
• l’expressivité : il faut que le langage permette <strong>de</strong> représ<strong>en</strong>ter <strong><strong>de</strong>s</strong> informations<br />
complexes ;<br />
• la concision : il faut que le langage permette <strong>de</strong> représ<strong>en</strong>ter <strong>de</strong> manière économique<br />
(<strong>en</strong> terme d’espace) ces informations complexes ;<br />
• l’efficacité : il faut que le langage puisse être facilem<strong>en</strong>t manipulable pour les<br />
tâches <strong>de</strong> raisonnem<strong>en</strong>t qui l’utiliseront (complexité algorithmique).<br />
Il existe <strong>de</strong> nombreux langages <strong>de</strong> représ<strong>en</strong>tation. La logique propositionnelle est<br />
un <strong><strong>de</strong>s</strong> langages prés<strong>en</strong>tant un bon compromis <strong>en</strong>tre ces critères. C’est un langage qui<br />
est facilem<strong>en</strong>t compréh<strong>en</strong>sible pour un humain, qui offre une expressivité correcte et<br />
une bonne concision. Il existe <strong>en</strong>fin <strong><strong>de</strong>s</strong> métho<strong><strong>de</strong>s</strong> <strong>de</strong> calcul efficaces <strong>en</strong> pratique pour<br />
tester la cohér<strong>en</strong>ce ou pour calculer les infér<strong>en</strong>ces possibles <strong>à</strong> partir d’un <strong>en</strong>semble <strong>de</strong><br />
formules.<br />
1.2.2. Bases <strong>de</strong> croyances<br />
Une croyance est une information que l’ag<strong>en</strong>t croit actuellem<strong>en</strong>t, c’est-<strong>à</strong>-dire une<br />
information qu’il considère comme vraie, mais qui ne l’est pas forcém<strong>en</strong>t, et qui peut<br />
donc être invalidée si l’ag<strong>en</strong>t se r<strong>en</strong>d compte qu’il a fait une erreur.<br />
6
On appelle base <strong>de</strong> croyances un <strong>en</strong>semble d’informations, qui est représ<strong>en</strong>té, dans<br />
la plupart <strong>de</strong> nos travaux, par un <strong>en</strong>semble <strong>de</strong> formules propositionnelles.<br />
Il est courant <strong>en</strong> intellig<strong>en</strong>ce artificielle, et plus généralem<strong>en</strong>t <strong>en</strong> logiques <strong><strong>de</strong>s</strong> croyances,<br />
<strong>de</strong> distinguer <strong>en</strong>tre croyances et connaissances. Nous v<strong>en</strong>ons <strong>de</strong> définir les croyances<br />
comme <strong><strong>de</strong>s</strong> informations que l’ag<strong>en</strong>t croit vraies, mais sans <strong>en</strong> être assuré. Les<br />
connaissances sont <strong><strong>de</strong>s</strong> informations que l’ag<strong>en</strong>t sait vraies, c’est-<strong>à</strong>-dire qu’il est assuré<br />
que ces informations sont vraies. Cela implique <strong>en</strong> particulier qu’elles ne pourront plus<br />
être remises <strong>en</strong> question par la suite. Il nous semble que cette hypothèse <strong>de</strong> disposer<br />
d’informations dont on est assuré qu’elles soi<strong>en</strong>t vraies est irréaliste. Il s’agit donc simplem<strong>en</strong>t<br />
d’une hypothèse simplificatrice utile pour représ<strong>en</strong>ter la plupart <strong><strong>de</strong>s</strong> problèmes<br />
d’intellig<strong>en</strong>ce artificielle. Mais dans les faits, l’ag<strong>en</strong>t dispose juste d’informations dont<br />
certaines sont beaucoup plus certaines (et pas juste « certaines ») que d’autres. Nous<br />
avons récemm<strong>en</strong>t proposé <strong>de</strong> représ<strong>en</strong>ter cela <strong>à</strong> l’ai<strong>de</strong> <strong>de</strong> « niveaux <strong>de</strong> croyances »[40],<br />
qui permett<strong>en</strong>t d’avoir <strong><strong>de</strong>s</strong> gradualités <strong>en</strong>tre croyances <strong>de</strong> même niveau, mais égalem<strong>en</strong>t<br />
<strong>de</strong> représ<strong>en</strong>ter <strong><strong>de</strong>s</strong> niveaux <strong>de</strong> croyances différ<strong>en</strong>ts (et donc <strong>de</strong> généraliser cette<br />
distinction croyances/connaissances).<br />
1.2.3. Représ<strong>en</strong>tation <strong>de</strong> l’incertitu<strong>de</strong><br />
Les croyances dont dispose l’ag<strong>en</strong>t sont donc <strong><strong>de</strong>s</strong> informations plus ou moins certaines.<br />
Il existe plusieurs manières <strong>de</strong> représ<strong>en</strong>ter cette incertitu<strong>de</strong>. En fait, on dispose<br />
d’une palette allant <strong>de</strong> représ<strong>en</strong>tations purem<strong>en</strong>t quantitatives <strong>de</strong> l’incertitu<strong>de</strong> <strong>à</strong> <strong><strong>de</strong>s</strong><br />
représ<strong>en</strong>tations purem<strong>en</strong>t qualitatives. Les approches quantitatives permett<strong>en</strong>t <strong>de</strong> disposer<br />
d’une information plus fine, mais la contrepartie est qu’elles nécessit<strong>en</strong>t automatiquem<strong>en</strong>t<br />
plus d’informationsque les approches qualitatives.<br />
Parmi les approches principales, on peut distinguer les :<br />
• Probabilités : il est possible d’associer une probabilité <strong>à</strong> chaque information.<br />
• Possibilités : il est possible d’associer un <strong>de</strong>gré <strong>de</strong> plausibilité <strong>à</strong> chaque information.<br />
C’est une information plus fruste que les distributions <strong>de</strong> probabilité (une<br />
distribution <strong>de</strong> possibilité peut être considérée comme la borne inférieure d’un<br />
<strong>en</strong>semble <strong>de</strong> distributions <strong>de</strong> probabilité).<br />
• Fonctions ordinales conditionnelles (ou possibilités qualitatives) : l<strong>à</strong> <strong>en</strong>core,<br />
on associe un <strong>de</strong>gré <strong>de</strong> plausibilité <strong>à</strong> chaque information, mais on n’autorise<br />
que <strong><strong>de</strong>s</strong> opérations « qualitatives » sur ces <strong>de</strong>grés, comme le min et le max par<br />
exemple. Ce cadre permet tout <strong>de</strong> même d’exprimer <strong><strong>de</strong>s</strong> int<strong>en</strong>sités dans les plausibilités.<br />
• Pré-ordres : il n’est plus possible d’associer une information numérique (un <strong>de</strong>gré)<br />
<strong>à</strong> chaque information, on ne dispose que <strong>de</strong> la donnée <strong><strong>de</strong>s</strong> plausibilités relatives<br />
<strong><strong>de</strong>s</strong> informations, modélisé par un pré-ordre qui indique si une information<br />
est plus plausible qu’une autre.<br />
On peut égalem<strong>en</strong>t m<strong>en</strong>tionner les fonctions <strong>de</strong> croyances (appelées aussi modèle<br />
<strong><strong>de</strong>s</strong> croyances transférables), qui sont une généralisation <strong><strong>de</strong>s</strong> probabilités : la distribution<br />
<strong>de</strong> poids ne s’effectue plus uniquem<strong>en</strong>t sur les élém<strong>en</strong>ts (événem<strong>en</strong>ts élém<strong>en</strong>taires)<br />
mais sur les sous-<strong>en</strong>sembles d’élém<strong>en</strong>ts. Ces fonctions <strong>de</strong> croyances peuv<strong>en</strong>t modéliser<br />
les différ<strong>en</strong>ts cadres sus-cités. Néanmoins ceux-ci prés<strong>en</strong>t<strong>en</strong>t suffisamm<strong>en</strong>t d’intérêt<br />
pour être étudiés séparém<strong>en</strong>t.<br />
7
Même si l’approche bayési<strong>en</strong>ne (probabiliste) est l’approche dominante <strong>en</strong> intellig<strong>en</strong>ce<br />
artificielle (<strong>en</strong> particulier aux Etats-Unis), pour <strong><strong>de</strong>s</strong> raisons assez naturelles, <strong>en</strong><br />
particulier sa facilité <strong>de</strong> mise <strong>en</strong> oeuvre, elle ne constitue pas l’approche universelle<br />
car elle manque d’expressivité. Elle ne permet pas <strong>en</strong> particulier <strong>de</strong> représ<strong>en</strong>ter l’ignorance.<br />
Il est possible <strong>de</strong> représ<strong>en</strong>ter une incertitu<strong>de</strong> (quantifiée), par exemple que la<br />
probabilité <strong>de</strong> A est <strong>de</strong> 50%, celle <strong>de</strong> B <strong>de</strong> 30% et celle <strong>de</strong> C <strong>de</strong> 20%. Mais il est impossible<br />
<strong>de</strong> représ<strong>en</strong>ter (avec une distribution <strong>de</strong> probabilité unique) l’ignorance <strong>en</strong>tre<br />
A, B et C, c’est-<strong>à</strong>-dire que l’on sait que soit A, soit B, soit C va se réaliser mais que<br />
l’on ne connait pas les probabilités correspondantes. Les Bayési<strong>en</strong>s utilis<strong>en</strong>t souv<strong>en</strong>t<br />
dans ce cas l’hypothèse laplaci<strong>en</strong>ne, c’est-<strong>à</strong>-dire qu’ils suppos<strong>en</strong>t une équiprobabilité<br />
<strong>en</strong>tre les différ<strong>en</strong>ts événem<strong>en</strong>ts, donc qu’ils associ<strong>en</strong>t <strong>à</strong> ce cas la distribution <strong>de</strong> probabilité<br />
: A avec 33%, B avec 33% et C avec 33%. Ce qui ne représ<strong>en</strong>te clairem<strong>en</strong>t pas<br />
la même information. Or, l’ignorance se représ<strong>en</strong>te très facilem<strong>en</strong>t avec les approches<br />
plus qualitatives. Celles-ci, étudiées principalem<strong>en</strong>t <strong>en</strong> Europe, permett<strong>en</strong>t égalem<strong>en</strong>t<br />
<strong>de</strong> raisonner avec <strong><strong>de</strong>s</strong> informations plus pauvres, c’est-<strong>à</strong>-dire dans <strong><strong>de</strong>s</strong> cas où l’on dispose<br />
<strong>de</strong> certaines informations sur la plausibilité <strong><strong>de</strong>s</strong> informations, mais pas suffisamm<strong>en</strong>t<br />
pour définir une distribution <strong>de</strong> probabilité. De plus, elles sont plus proches <strong><strong>de</strong>s</strong><br />
mécanismes utilisés par les humains, car il est rare qu’un humain raisonne avec une<br />
information numérique aussi précise qu’une distribution <strong>de</strong> probabilité.<br />
Nous nous plaçons dans les cadres les plus qualitatifs, c’est-<strong>à</strong>-dire que dans la plupart<br />
<strong>de</strong> nos travaux l’incertitu<strong>de</strong> sera modélisée par <strong><strong>de</strong>s</strong> pré-ordres.<br />
1.3. Résolution <strong>de</strong> conflits logiques<br />
La majeure partie <strong>de</strong> nos travaux peut se regrouper sous la problématique du raisonnem<strong>en</strong>t<br />
<strong>à</strong> partir d’un <strong>en</strong>semble <strong>de</strong> formules logiques incohér<strong>en</strong>t. Suivant les hypothèses<br />
que l’on peut faire sur cet <strong>en</strong>semble <strong>de</strong> formules, on obti<strong>en</strong>t <strong><strong>de</strong>s</strong> cadres différ<strong>en</strong>ts, qui<br />
nécessit<strong>en</strong>t <strong><strong>de</strong>s</strong> métho<strong><strong>de</strong>s</strong> <strong>de</strong> raisonnem<strong>en</strong>t spécifiques :<br />
• Incohér<strong>en</strong>ce : on ne dispose d’aucune information supplém<strong>en</strong>taire. Il est donc<br />
nécessaire d’obt<strong>en</strong>ir <strong><strong>de</strong>s</strong> conclusions cohér<strong>en</strong>tes (raisonnables) <strong>à</strong> partir d’un <strong>en</strong>semble<br />
d’information incohér<strong>en</strong>t.<br />
• Révision : une <strong><strong>de</strong>s</strong> formules est plus importante que les autres. Il faut donc gar<strong>de</strong>r<br />
cette formule, tout <strong>en</strong> éliminant les incohér<strong>en</strong>ces.<br />
• Fusion : les formules provi<strong>en</strong>n<strong>en</strong>t <strong>de</strong> sources différ<strong>en</strong>tes. Il faut alors définir une<br />
base cohér<strong>en</strong>te <strong>à</strong> partir <strong>de</strong> ces informations, <strong>en</strong> pr<strong>en</strong>ant <strong>en</strong> compte la localisation<br />
<strong>de</strong> ces informations (avec <strong><strong>de</strong>s</strong> argum<strong>en</strong>ts majoritaires par exemple).<br />
• Négociation : les formules provi<strong>en</strong>n<strong>en</strong>t <strong>de</strong> sources différ<strong>en</strong>tes, comme pour la<br />
fusion, mais les sources gar<strong>de</strong>nt la maîtrise <strong><strong>de</strong>s</strong> modifications <strong>de</strong> leurs formules.<br />
Il faut donc t<strong>en</strong>ir compte <strong>de</strong> possibles interactions (coalitions, etc.).<br />
Nous nous sommes principalem<strong>en</strong>t intéressés aux trois premiers points. La négociation<br />
est un sujet que nous avons abordé plus récemm<strong>en</strong>t. Les résultats obt<strong>en</strong>us sur<br />
ce sujet sont plus préliminaires, mais ce sujet constitue le point que nous comptons<br />
développer <strong>en</strong> priorité <strong>à</strong> l’av<strong>en</strong>ir. Nous allons structurer notre prés<strong>en</strong>tation autour <strong>de</strong><br />
ces quatre points, <strong>en</strong> insistant sur nos projets concernant le quatrième, qui constituera<br />
donc une partie <strong><strong>de</strong>s</strong> perspectives et <strong>de</strong> notre projet <strong>de</strong> recherche.<br />
8
1.4. Plan<br />
Ce docum<strong>en</strong>t est constitué <strong>de</strong> <strong>de</strong>ux parties. La première prés<strong>en</strong>te une synthèse <strong>de</strong><br />
nos travaux <strong>de</strong> recherche. La secon<strong>de</strong> partie complète la première et regroupe une sélection<br />
<strong>de</strong> publications qui nous semble significative et représ<strong>en</strong>tative <strong>de</strong> notre démarche<br />
sci<strong>en</strong>tifique.<br />
La partie principale <strong>de</strong> ce docum<strong>en</strong>t est la synthèse <strong>de</strong> nos travaux <strong>de</strong> recherche.<br />
Nous prés<strong>en</strong>terons <strong>de</strong> manière structurée une gran<strong>de</strong> partie <strong>de</strong> nos travaux <strong>de</strong> recherche,<br />
mais nous t<strong>en</strong>terons égalem<strong>en</strong>t d’insister sur la démarche adoptée et sur les buts poursuivis.<br />
Afin <strong>de</strong> placer nos travaux dans leur contexte, nous prés<strong>en</strong>terons un bref état <strong>de</strong><br />
l’art pour chaque thème abordé. Il ne faut toutefois pas considérer ce docum<strong>en</strong>t comme<br />
une monographie, car la prés<strong>en</strong>tation <strong>de</strong> l’état <strong>de</strong> l’art et <strong><strong>de</strong>s</strong> travaux concernant chaque<br />
thème est biaisée vers nos travaux. Nous t<strong>en</strong>terons tout <strong>de</strong> même <strong>à</strong> chaque fois <strong>de</strong> m<strong>en</strong>tionner<br />
les travaux principaux, afin que ce docum<strong>en</strong>t puisse servir <strong>de</strong> point d’<strong>en</strong>trée <strong>à</strong><br />
ces différ<strong>en</strong>ts domaines.<br />
Dans le prochain chapitre, nous exposons notre démarche sci<strong>en</strong>tifique générale,<br />
c’est-<strong>à</strong>-dire la démarche que nous adoptons pour l’<strong>en</strong>semble <strong>de</strong> nos travaux. Elle constitue<br />
la principale justification <strong>de</strong> l’approche axiomatique que nous privilégions dans la<br />
majeure partie <strong>de</strong> nos travaux.<br />
Les chapitres suivants prés<strong>en</strong>t<strong>en</strong>t nos travaux concernant les quatre thèmes prés<strong>en</strong>tés<br />
<strong>à</strong> la section précé<strong>de</strong>nte : raisonnem<strong>en</strong>t <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce, révision <strong>de</strong><br />
croyances, fusion <strong>de</strong> croyances, et négociation.<br />
Le chapitre 3 prés<strong>en</strong>te le problème du raisonnem<strong>en</strong>t <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce. Le<br />
point <strong>de</strong> départ est un <strong>en</strong>semble <strong>de</strong> formules logiques qui est incohér<strong>en</strong>t. Le problème<br />
principal est alors celui <strong>de</strong> l’infér<strong>en</strong>ce <strong>à</strong> partir <strong>de</strong> cet <strong>en</strong>semble incohér<strong>en</strong>t. Une autre<br />
question importante est égalem<strong>en</strong>t celle <strong>de</strong> la mesure <strong>de</strong> l’incohér<strong>en</strong>ce cont<strong>en</strong>ue dans<br />
cet <strong>en</strong>semble.<br />
Le chapitre 4 concerne la révision <strong>de</strong> croyances. Dans ce cadre une nouvelle formule<br />
doit être ajoutée <strong>à</strong> la base <strong>de</strong> croyances et cette formule est typiquem<strong>en</strong>t incohér<strong>en</strong>te<br />
avec les formules <strong>de</strong> la base. Le cadre <strong>de</strong> base <strong>de</strong> la révision <strong>de</strong> croyances, le<br />
cadre AGM [AGM85, Gär88, KM91b], est unanimem<strong>en</strong>t accepté. Mais ce cadre ne<br />
modélise qu’une seule étape <strong>de</strong> révision. On a donc besoin <strong>de</strong> contraindre le comportem<strong>en</strong>t<br />
<strong><strong>de</strong>s</strong> opérateurs lors <strong><strong>de</strong>s</strong> itérations. Cela conduit au problème <strong>de</strong> la révision itérée<br />
<strong>de</strong> croyances.<br />
Le chapitre 5 s’intéresse <strong>à</strong> la fusion <strong>de</strong> croyances. On dispose dans ce cas d’un <strong>en</strong>semble<br />
<strong>de</strong> sources (ag<strong>en</strong>ts) qui fourniss<strong>en</strong>t <strong><strong>de</strong>s</strong> formules propositionnelles dont l’union<br />
est incohér<strong>en</strong>te. Il faut alors définir <strong><strong>de</strong>s</strong> opérateurs afin d’associer <strong>à</strong> ces profils <strong>de</strong> bases<br />
<strong>de</strong> croyances une information cohér<strong>en</strong>te synthétisant au mieux les croyances du groupe<br />
<strong>de</strong> sources.<br />
Le chapitre 6 abor<strong>de</strong> la problématique <strong>de</strong> la négociation. Nous nous intéressons <strong>à</strong><br />
une définition abstraite <strong>de</strong> la négociation, que nous avons nommée conciliation. L’idée<br />
est <strong>de</strong> ne pas s’ « <strong>en</strong>combrer » <strong>de</strong> détails pratiques tels que le protocole <strong>de</strong> communication<br />
utilisé, etc., mais d’abor<strong>de</strong>r le problème sous son aspect fonctionnel : un opérateur<br />
<strong>de</strong> négociation (conciliation) est une fonction qui pr<strong>en</strong>d <strong>en</strong> <strong>en</strong>trée un profil <strong>de</strong> bases <strong>de</strong><br />
croyances et qui fournit <strong>en</strong> sortie un nouveau profil <strong>de</strong> croyances où le conflit a disparu,<br />
9
ou tout au moins a diminué.<br />
Le chapitre 7 détaille quelques perspectives <strong>de</strong> ce travail.<br />
La secon<strong>de</strong> partie du docum<strong>en</strong>t prés<strong>en</strong>te une sélection <strong>de</strong> huit articles. Réaliser cette<br />
sélection n’a pas été chose aisée, mais la règle finalem<strong>en</strong>t choisie a été la suivante : nous<br />
avons sélectionné <strong>de</strong>ux articles pour chaque thème abordé dans la première partie du<br />
mémoire (raisonnem<strong>en</strong>t <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce, révision, fusion, négociation), qui<br />
illustr<strong>en</strong>t le mieux la démarche sci<strong>en</strong>tifique discutée au chapitre 2 et notre approche<br />
axiomatique <strong>de</strong> ces problèmes.<br />
1.5. Notations<br />
Nous considérons un langage propositionnel L = {α, β, . . .}, défini <strong>à</strong> partir d’un<br />
<strong>en</strong>semble fini <strong>de</strong> variables propositionnelles P = {a, b, c, . . .}, et <strong><strong>de</strong>s</strong> connecteurs<br />
usuels.<br />
Une interprétation ω est une fonction <strong>de</strong> P dans {0, 1}. L’<strong>en</strong>semble <strong><strong>de</strong>s</strong> interprétations<br />
est noté W. Une interprétation ω est un modèle d’une formule α ∈ L si et seulem<strong>en</strong>t<br />
si elle la r<strong>en</strong>d vraie. mod(α) représ<strong>en</strong>te l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> modèles <strong>de</strong> la formule α,<br />
i.e. mod(α) = {ω ∈ W | ω |= α}.<br />
Une base K est un <strong>en</strong>semble fini 2 <strong>de</strong> formules, qui représ<strong>en</strong>te les croyances d’un<br />
ag<strong>en</strong>t. Soit K l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> bases. Une base K = {α 1 , . . . , α n } est cohér<strong>en</strong>te si la<br />
formule α = α 1 ∧ . . . ∧ α n est cohér<strong>en</strong>te. Une base K est une théorie si elle conti<strong>en</strong>t<br />
l’<strong>en</strong>semble <strong>de</strong> ses conséqu<strong>en</strong>ces logiques, i.e. K = Cn(K) = {α | K ⊢ α}. Une base<br />
(ou une formule) est dite complète si elle possè<strong>de</strong> un unique modèle.<br />
Un profil E = {K 1 , . . . , K n } est un multi-<strong>en</strong>semble 3 <strong>de</strong> bases, qui représ<strong>en</strong>te<br />
les croyances d’un groupe d’ag<strong>en</strong>ts. Soit E l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> profils. On note ∧ E la<br />
conjonction <strong><strong>de</strong>s</strong> bases <strong>de</strong> E = {K 1 , . . . , K n }, i.e., ∧ E = K 1 ∧ . . . ∧ K n . Un profil E<br />
est cohér<strong>en</strong>t si et seulem<strong>en</strong>t si ∧ E est cohér<strong>en</strong>t. On note ⊔ l’union multi-<strong>en</strong>sembliste,<br />
et pour <strong><strong>de</strong>s</strong> profils singletons on note K ⊔ E au lieu <strong>de</strong> {K} ⊔ E. On note E n le<br />
profil où E est répété n fois, i.e. E n = E<br />
}<br />
⊔ .<br />
{{<br />
. . ⊔ E<br />
}<br />
. Deux profils sont équival<strong>en</strong>ts,<br />
noté E 1 ≡ E 2 , s’il existe une bijection f <strong>en</strong>tre E 1 et E 2 tel que pour tout K ∈ E 1<br />
f(K) ≡ K.<br />
Si ≤ est un pré-ordre sur A (i.e. une relation transitive et réflexive), alors < dénote<br />
l’ordre strict associé, défini par x < y si et seulem<strong>en</strong>t si x ≤ y et y ≰ x, et ≃ dénote<br />
la relation d’équival<strong>en</strong>ce associée, définie par x ≃ y si et seulem<strong>en</strong>t si x ≤ y et y ≤ x.<br />
Un pré-ordre est total si ∀x, y ∈ A, x ≤ y ou y ≤ x. Un pré-ordre qui n’est pas total<br />
est dit partiel. Soit un pré-ordre ≤ sur A, et B ⊆ A, alors min(B, ≤) = {x ∈ B |<br />
∄y ∈ B y < x}.<br />
Si X est un <strong>en</strong>semble, |X| représ<strong>en</strong>te le cardinal <strong>de</strong> l’<strong>en</strong>semble X. Nous utiliserons<br />
⊆ pour noter l’inclusion <strong>en</strong>sembliste et ⊂ pour noter l’inclusion stricte (i.e. A ⊂ B ssi<br />
A ⊆ B et B ⊈ A).<br />
n<br />
2. Sauf <strong>à</strong> la section 4.1 où l’on travaille avec <strong><strong>de</strong>s</strong> théories.<br />
3. Un multi-<strong>en</strong>semble est un <strong>en</strong>semble où un même élém<strong>en</strong>t peut apparaître plusieurs fois.<br />
10
2.1 Démarche<br />
2.2 Infér<strong>en</strong>ce non monotone<br />
2.3 Révision<br />
2.4 Vote<br />
2.5 Conclusion<br />
Chapitre II<br />
DÉMARCHE SCIENTIFIQUE<br />
On ne peut se passer d’une métho<strong>de</strong> pour se mettre <strong>en</strong> quête <strong>de</strong><br />
la vérité <strong><strong>de</strong>s</strong> choses.<br />
(Descartes)<br />
Afin <strong>de</strong> compr<strong>en</strong>dre et justifier notre approche, qui est principalem<strong>en</strong>t axée vers<br />
la caractérisation logique <strong><strong>de</strong>s</strong> processus <strong>de</strong> raisonnem<strong>en</strong>t, il peut être utile d’expliquer<br />
la démarche sci<strong>en</strong>tifique la motivant.<br />
2.1. Démarche<br />
Il nous semble que l’étu<strong>de</strong> et la compréh<strong>en</strong>sion <strong>de</strong> tout processus <strong>de</strong> raisonnem<strong>en</strong>t<br />
nécessite les étapes suivantes :<br />
1. Spécification du problème : cette première étape est habituellem<strong>en</strong>t assez directe,<br />
il s’agit <strong>de</strong> définir formellem<strong>en</strong>t le problème que l’on veut résoudre.<br />
2. Proposition <strong>de</strong> solutions : la <strong>de</strong>uxième étape consiste <strong>à</strong> rechercher <strong><strong>de</strong>s</strong> solutions<br />
ad hoc afin <strong>de</strong> résoudre le problème.<br />
3. Caractérisation logique : la troisième étape consiste <strong>en</strong> l’étu<strong>de</strong> <strong><strong>de</strong>s</strong> propriétés<br />
logiques <strong><strong>de</strong>s</strong> solutions au problème et <strong>en</strong> la recherche <strong>de</strong> caractérisations logiques<br />
<strong>de</strong> ces solutions.<br />
Souv<strong>en</strong>t, <strong>en</strong> intellig<strong>en</strong>ce artificielle, on se cont<strong>en</strong>te <strong>de</strong> la <strong>de</strong>uxième étape, et l’on<br />
ne voit pas forcém<strong>en</strong>t l’utilité <strong>de</strong> la troisième. Pourtant, cette troisième étape est indisp<strong>en</strong>sable<br />
si l’on veut prét<strong>en</strong>dre compr<strong>en</strong>dre et résoudre un problème donné. Elle est<br />
égalem<strong>en</strong>t indisp<strong>en</strong>sable pour pouvoir comparer les métho<strong><strong>de</strong>s</strong> ad hoc proposées lors <strong>de</strong><br />
la <strong>de</strong>uxième étape.<br />
11
En effet, proposer une solution ad hoc <strong>à</strong> un problème, même si celle-ci est difficile<br />
<strong>à</strong> trouver et <strong>à</strong> concevoir, n’est pas synonyme <strong>de</strong> compr<strong>en</strong>dre le problème. Considérons<br />
par exemple le problème <strong>de</strong> l’infér<strong>en</strong>ce non monotone, c’est-<strong>à</strong>-dire l’infér<strong>en</strong>ce <strong>à</strong><br />
partir d’un <strong>en</strong>semble d’informations simplem<strong>en</strong>t plausibles. Une solution ad hoc <strong>à</strong> ce<br />
problème est par exemple d’utiliser la logique <strong><strong>de</strong>s</strong> défauts. Cette solution permet <strong>de</strong><br />
définir une relation d’infér<strong>en</strong>ce non monotone. Mais ce n’est pas parce que l’on dispose<br />
<strong>de</strong> cette solution que l’on peut prét<strong>en</strong>dre avoir compris ce qu’est le problème <strong>de</strong><br />
l’infér<strong>en</strong>ce non monotone et <strong>en</strong>core moins l’avoir résolu.<br />
A titre <strong>de</strong> comparaison, pr<strong>en</strong>ons un autre problème qui est celui <strong>de</strong> faire voler <strong><strong>de</strong>s</strong><br />
objets. Une solution ad hoc <strong>à</strong> ce problème est d’utiliser un avion. Mais ce n’est pas<br />
parce que l’on a construit un avion que l’on a compris ce qu’est l’aérodynamique.<br />
Historiquem<strong>en</strong>t d’ailleurs, <strong><strong>de</strong>s</strong> avions ont volé bi<strong>en</strong> avant que l’on puisse démontrer<br />
pourquoi.<br />
Cela n’<strong>en</strong>lève ri<strong>en</strong> <strong>de</strong> l’importance et <strong>de</strong> la nécessité <strong>de</strong> la <strong>de</strong>uxième étape, car<br />
concevoir une relation d’infér<strong>en</strong>ce non monotone, comme concevoir un avion, sont<br />
<strong>de</strong>ux problèmes importants, <strong>de</strong>mandant <strong>de</strong> résoudre <strong><strong>de</strong>s</strong> questions difficiles. Mais on<br />
ne peut s’arrêter <strong>à</strong> cette étape. Pour le vol, il faut une théorie sci<strong>en</strong>tifique capable <strong>de</strong><br />
modéliser le phénomène. Et c’est exactem<strong>en</strong>t la même chose pour l’infér<strong>en</strong>ce non monotone,<br />
et pour l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> processus <strong>de</strong> raisonnem<strong>en</strong>t.<br />
Une difficulté supplém<strong>en</strong>taire que l’on a dans le cadre d’un problème d’intellig<strong>en</strong>ce<br />
artificielle est que la théorie que l’on veut mettre <strong>en</strong> place n’a pas pour objet d’étu<strong>de</strong> <strong><strong>de</strong>s</strong><br />
processus physiques, comme c’est le cas pour les sci<strong>en</strong>ces expérim<strong>en</strong>tales. Il n’est donc<br />
pas suffisant d’utiliser une approche hypothético-déductive classique, et <strong>de</strong> vali<strong>de</strong>r la<br />
théorie proposée <strong>en</strong> observant qu’elle se vérifie dans l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> expéri<strong>en</strong>ces.<br />
Lorsque ce que l’on veut modéliser est un processus cognitif, une possibilité est<br />
d’utiliser <strong><strong>de</strong>s</strong> expéri<strong>en</strong>ces <strong>de</strong> psychologie pour tester que la théorie proposée est conforme<br />
au comportem<strong>en</strong>t humain (voir par exemple [BBN04, BBN05, BNDP08]). Cela<br />
nécessite <strong>de</strong> considérer l’être humain comme modèle <strong>de</strong> rationalité. Or, il est assez<br />
facile <strong>de</strong> constater que l’homme est un bi<strong>en</strong> piètre ag<strong>en</strong>t rationnel. Cela est évi<strong>de</strong>nt<br />
dans nos interactions quotidi<strong>en</strong>nes, et cela est illustré dans <strong><strong>de</strong>s</strong> dizaines <strong>de</strong> travaux <strong>en</strong><br />
économie expérim<strong>en</strong>tale.<br />
La question est alors <strong>de</strong> savoir si l’intellig<strong>en</strong>ce artificielle consiste <strong>en</strong> la modélisation<br />
du comportem<strong>en</strong>t d’un être humain, ou <strong>en</strong> celui d’un ag<strong>en</strong>t rationnel idéal 1 . Il nous<br />
semble que les <strong>de</strong>ux problèmes <strong>de</strong> la modélisation du comportem<strong>en</strong>t humain et celui<br />
<strong>de</strong> la modélisation d’un ag<strong>en</strong>t rationnel idéal sont <strong>de</strong>ux questions intéressantes. Mais<br />
égalem<strong>en</strong>t qu’il est nécessaire <strong>de</strong> d’abord étudier le comportem<strong>en</strong>t d’un ag<strong>en</strong>t rationnel<br />
idéal, avant <strong>de</strong> passer au problème plus complexe <strong>de</strong> la modélisation du comportem<strong>en</strong>t<br />
d’un être humain.<br />
On peut d’ailleurs noter qu’<strong>en</strong> économie, et particulièrem<strong>en</strong>t <strong>en</strong> théorie <strong><strong>de</strong>s</strong> jeux,<br />
la même démarche a été adoptée, avec <strong><strong>de</strong>s</strong> travaux théoriques faisant une hypothèse <strong>de</strong><br />
rationalité (idéale) <strong><strong>de</strong>s</strong> ag<strong>en</strong>ts afin d’obt<strong>en</strong>ir <strong><strong>de</strong>s</strong> résultats intéressants, suivis <strong>de</strong> travaux<br />
exportant ces résultats dans le cadre <strong>de</strong> la rationalité limitée.<br />
1. Cette question a donné lieu <strong>à</strong> <strong><strong>de</strong>s</strong> discussions très intéressantes lors <strong><strong>de</strong>s</strong> journées d’Intellig<strong>en</strong>ces Artificielles<br />
Fondam<strong>en</strong>tales (IAF) <strong>de</strong> 2008.<br />
12
Si l’on ne considère pas l’être humain comme modèle <strong>de</strong> rationalité, cela ne permet<br />
donc pas <strong>de</strong> concevoir <strong><strong>de</strong>s</strong> expéri<strong>en</strong>ces afin <strong>de</strong> vali<strong>de</strong>r les théories proposées. En<br />
revanche, il est possible d’étudier <strong>en</strong> quoi les métho<strong><strong>de</strong>s</strong> ad hoc proposées lors <strong>de</strong> la<br />
<strong>de</strong>uxième étape sont <strong>en</strong> accord avec la théorie proposée lors <strong>de</strong> la troisième étape. On<br />
peut donc alors utiliser cette théorie pour justifier ces métho<strong><strong>de</strong>s</strong>.<br />
Cela ne signifie pas que la troisième étape est plus importante que la <strong>de</strong>uxième,<br />
car il est <strong>de</strong> toute façon nécessaire <strong>de</strong> disposer <strong>de</strong> métho<strong><strong>de</strong>s</strong> pratiques <strong>de</strong> résolution<br />
du problème <strong>en</strong> question. Et cela permet égalem<strong>en</strong>t d’obt<strong>en</strong>ir les premières intuitions<br />
afin <strong>de</strong> trouver <strong><strong>de</strong>s</strong> propriétés caractérisant ces métho<strong><strong>de</strong>s</strong>. Il faut donc voir ces étapes<br />
comme complém<strong>en</strong>taires l’une <strong>de</strong> l’autre. Mais la troisième étape est au moins aussi<br />
importante pour ce qui est <strong>de</strong> la compréh<strong>en</strong>sion du problème <strong>en</strong> question. Et, afin <strong>de</strong><br />
résoudre <strong>de</strong> manière convaincante un problème donné, il est nécessaire <strong>de</strong> réaliser les<br />
trois étapes.<br />
Nous allons <strong>à</strong> prés<strong>en</strong>t illustrer l’intérêt <strong>de</strong> cette démarche sur quelques exemples significatifs.<br />
Et comme nous avons utilisé l’infér<strong>en</strong>ce non monotone comme illustration,<br />
nous comm<strong>en</strong>cerons par celle-ci.<br />
2.2. Infér<strong>en</strong>ce non monotone<br />
1. Spécification du problème : comm<strong>en</strong>t permettre l’infér<strong>en</strong>ce (<strong>de</strong> « s<strong>en</strong>s commun<br />
») <strong>à</strong> partir d’informations simplem<strong>en</strong>t plausibles ou <strong>de</strong> règles générales (du<br />
g<strong>en</strong>re « normalem<strong>en</strong>t les oiseaux vol<strong>en</strong>t »), dont les conclusions peuv<strong>en</strong>t être remises<br />
<strong>en</strong> cause si l’on ajoute <strong>de</strong> nouvelles informations (d’où la non monotonie) ?<br />
2. Proposition <strong>de</strong> solutions : <strong>de</strong> nombreuses métho<strong><strong>de</strong>s</strong> ont été proposées pour résoudre<br />
ce problème. On peut citer <strong>en</strong> particulier les métho<strong><strong>de</strong>s</strong> <strong>à</strong> base d’hypothèses<br />
<strong>de</strong> mon<strong>de</strong> clos, la logique <strong><strong>de</strong>s</strong> défauts, la circonscription, etc.<br />
3. Caractérisation logique : Gabbay a été le premier <strong>à</strong> suggérer que <strong>de</strong> ne caractériser<br />
ces logiques que par le fait qu’elles ne satisfaisai<strong>en</strong>t pas la propriété <strong>de</strong> monotonie<br />
<strong>de</strong> la logique classique n’était pas suffisant. Il a donc suggéré <strong>de</strong> t<strong>en</strong>ter<br />
d’étudier les propriétés communes et souhaitables <strong>de</strong> ces différ<strong>en</strong>tes métho<strong><strong>de</strong>s</strong><br />
[Gab85]. Cela a été développé <strong>en</strong> particulier par Makinson [Mak94], puis par<br />
Kraus, Lehmann et Magidor [KLM90, LM92]. On a pu montrer que l’<strong>en</strong>semble<br />
<strong><strong>de</strong>s</strong> propriétés minimales que l’on peut att<strong>en</strong>dre d’une relation d’infér<strong>en</strong>ce non<br />
monotone (le système P <strong>de</strong> [KLM90]) était équival<strong>en</strong>t <strong>à</strong> une sémantique très naturelle<br />
: la sémantique préfér<strong>en</strong>tielle [Sho87].<br />
Cette caractérisation est indisp<strong>en</strong>sable non seulem<strong>en</strong>t pour compr<strong>en</strong>dre ce qu’est<br />
une logique non monotone, mais égalem<strong>en</strong>t pour comparer les métho<strong><strong>de</strong>s</strong> ad hoc proposées.<br />
Cela permet d’étudier quelles sont les propriétés satisfaites par ces métho<strong><strong>de</strong>s</strong>,<br />
au lieu <strong>de</strong> les comparer <strong>en</strong> échangeant <strong><strong>de</strong>s</strong> exemples qu’une métho<strong>de</strong> résout alors que<br />
l’autre non. Ceci ne permet pas d’établir <strong>de</strong> conclusions d’une portée suffisante, mais a<br />
été longtemps l’usage dans les articles traitant <strong>de</strong> non monotonie.<br />
13
2.3. Révision<br />
1. Spécification du problème : comm<strong>en</strong>t incorporer aux croyances d’un ag<strong>en</strong>t<br />
une nouvelle information qui remet <strong>en</strong> question une partie <strong>de</strong> ses croyances actuelles<br />
?<br />
2. Proposition <strong>de</strong> solutions : c’est un problème qui se pose dès que l’on doit gérer<br />
les croyances d’un ag<strong>en</strong>t, ou une base <strong>de</strong> données. Plusieurs métho<strong><strong>de</strong>s</strong> dédiées<br />
ont donc été proposées [FUV83, Dal88, Bor85, KW85, Win88].<br />
3. Caractérisation logique : Alchourrón, Gär<strong>de</strong>nfors and Makinson ont proposé<br />
<strong><strong>de</strong>s</strong> propriétés logiques pour les opérateurs <strong>de</strong> révision, ainsi qu’une caractérisation<br />
logique montrant que les opérateurs satisfaisant ces propriétés correspon<strong>de</strong>nt<br />
<strong>à</strong> <strong><strong>de</strong>s</strong> métho<strong><strong>de</strong>s</strong> <strong>de</strong> révision très naturelles [AGM85, Gär88]. Depuis, <strong>de</strong> nombreux<br />
théorèmes <strong>de</strong> représ<strong>en</strong>tation ont été énoncés, illustrant le rôle fondam<strong>en</strong>tal<br />
<strong>de</strong> cette caractérisation logique.<br />
2.4. Vote<br />
Nous voudrions <strong>à</strong> prés<strong>en</strong>t souligner que cette approche n’est pas utile juste dans le<br />
cadre du raisonnem<strong>en</strong>t et <strong>de</strong> l’intellig<strong>en</strong>ce artificielle, mais qu’elle s’est déj<strong>à</strong> montrée<br />
fécon<strong>de</strong> dans d’autres champs disciplinaires, <strong>en</strong> particulier <strong>en</strong> économie.<br />
1. Spécification du problème : comm<strong>en</strong>t définir une bonne métho<strong>de</strong> <strong>de</strong> vote ?<br />
2. Proposition <strong>de</strong> solutions : <strong>de</strong> très nombreux systèmes <strong>de</strong> vote ont été proposés,<br />
et ont généré beaucoup <strong>de</strong> débats afin <strong>de</strong> déterminer quels étai<strong>en</strong>t les meilleurs<br />
d’<strong>en</strong>tre eux. On peut faire remonter ces discussions au moins jusqu’au XVIII e<br />
siècle avec les échanges <strong>en</strong>tre Condorcet [Con85] et Borda [Bor81].<br />
3. Caractérisation logique : un cap a été franchi lorsque Arrow a décidé, plutôt<br />
que d’étudier un système <strong>de</strong> vote particulier, d’étudier les propriétés que l’on<br />
peut att<strong>en</strong>dre <strong>de</strong> ces systèmes. Il a ainsi montré un théorème d’impossibilité prouvant<br />
qu’il n’existe aucune « bonne » métho<strong>de</strong> <strong>de</strong> vote [Arr63]. Cela a permis <strong>de</strong><br />
franchir un palier dans la compréh<strong>en</strong>sion <strong><strong>de</strong>s</strong> métho<strong><strong>de</strong>s</strong> <strong>de</strong> vote et a marqué un<br />
tournant dans la discipline (et a valu un prix Nobel <strong>à</strong> Arrow <strong>en</strong> 1972).<br />
Nous nous limitons <strong>à</strong> cet exemple <strong>de</strong> choix social, mais les approches axiomatiques<br />
ont égalem<strong>en</strong>t donné <strong><strong>de</strong>s</strong> résultats intéressants <strong>en</strong> théorie <strong><strong>de</strong>s</strong> jeux.<br />
2.5. Conclusion<br />
Il nous semble donc qu’il est nécessaire, afin <strong>de</strong> prét<strong>en</strong>dre compr<strong>en</strong>dre un processus<br />
<strong>de</strong> raisonnem<strong>en</strong>t, <strong>de</strong> réaliser ces trois étapes : spécification, résolution (proposition <strong>de</strong><br />
solutions), caractérisation.<br />
L’ordre que nous avons utilisé pour prés<strong>en</strong>ter la <strong>de</strong>uxième étape (résolution) et la<br />
troisième (caractérisation) est l’ordre usuel, car il y a souv<strong>en</strong>t une solution ad hoc<br />
<strong>de</strong> proposée avant que l’on arrive <strong>à</strong> caractériser ces solutions. Mais fréquemm<strong>en</strong>t ces<br />
14
<strong>de</strong>ux étapes sont m<strong>en</strong>ées <strong>en</strong> parallèle, et <strong><strong>de</strong>s</strong> échanges <strong>en</strong>tres ces <strong>de</strong>ux étapes sont très<br />
utiles, afin <strong>de</strong> s’assurer que la modélisation réalisée lors <strong>de</strong> la troisième étape capture<br />
bi<strong>en</strong> l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> solutions définies lors <strong>de</strong> la <strong>de</strong>uxième étape, et <strong>de</strong> l’améliorer si ce<br />
n’est pas le cas ; et afin d’utiliser la troisième étape pour trouver <strong>de</strong> meilleures solutions<br />
ad hoc ou pour améliorer les propositions existantes. Cela illustre la complém<strong>en</strong>tarité<br />
<strong>de</strong> ces <strong>de</strong>ux étapes.<br />
Notre approche <strong><strong>de</strong>s</strong> différ<strong>en</strong>ts problèmes concerne principalem<strong>en</strong>t la troisième étape,<br />
c’est-<strong>à</strong>-dire l’étu<strong>de</strong> <strong><strong>de</strong>s</strong> propriétés logiques <strong><strong>de</strong>s</strong> processus <strong>en</strong> question et leur caractérisation<br />
logique.<br />
Ainsi, lorsque nous proposons <strong>de</strong> nouvelles métho<strong><strong>de</strong>s</strong> ad hoc pour un problème<br />
particulier, <strong>en</strong> particulier pour <strong><strong>de</strong>s</strong> problèmes où ces métho<strong><strong>de</strong>s</strong> ne sont pas nombreuses,<br />
nous considérons ces travaux comme un pas vers la troisième étape et la caractérisation<br />
logique.<br />
15
3.1 Infér<strong>en</strong>ce <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce<br />
3.1.1 Affaiblir les hypothèses<br />
3.1.2 Affaiblir la logique<br />
3.2 Mesure <strong>de</strong> l’incohér<strong>en</strong>ce<br />
3.2.1 Mesures basées sur les formules<br />
3.2.2 Mesures basées sur les variables<br />
3.2.3 Valeurs d’incohér<strong>en</strong>ce<br />
Chapitre III<br />
RAISONNEMENT EN PRÉSENCE D’INCOHÉRENCE<br />
Autant que savoir, douter me plaît.<br />
(Dante)<br />
Lorsque l’on doit raisonner <strong>à</strong> partir d’un <strong>en</strong>semble <strong>de</strong> formules incohér<strong>en</strong>t, on ne<br />
peut pas utiliser la logique classique, qui trivialise. Il faut donc trouver <strong><strong>de</strong>s</strong> mécanismes<br />
dédiés. Le problème principal est celui <strong>de</strong> l’infér<strong>en</strong>ce, afin <strong>de</strong> pouvoir<br />
exploiter la base malgré ses incohér<strong>en</strong>ces. Un autre problème important est celui <strong>de</strong> la<br />
mesure <strong>de</strong> l’incohér<strong>en</strong>ce. Il peut <strong>en</strong> effet être utile d’évaluer <strong>à</strong> quel point une base est<br />
incohér<strong>en</strong>te. De même, savoir quelles sont les formules qui apport<strong>en</strong>t beaucoup ou peu<br />
d’incohér<strong>en</strong>ce peut être très utile pour <strong>de</strong> nombreuses tâches.<br />
3.1. Infér<strong>en</strong>ce <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce<br />
Il s’agit ici <strong>de</strong> définir <strong><strong>de</strong>s</strong> relations qui permett<strong>en</strong>t d’obt<strong>en</strong>ir <strong><strong>de</strong>s</strong> conclusions non<br />
triviales <strong>à</strong> partir d’un <strong>en</strong>semble incohér<strong>en</strong>t <strong>de</strong> formules, ce qui n’est pas possible avec<br />
la logique (propositionnelle) classique, qui trivialise <strong>en</strong> inférant toutes les formules du<br />
langage.<br />
Pour éviter la trivialisation, il faut affaiblir la puissance infér<strong>en</strong>tielle <strong>de</strong> la logique<br />
classique. Il n’y a que <strong>de</strong>ux possibilités :<br />
• affaiblir l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> hypothèses (<strong>en</strong>semble <strong>de</strong> formules).<br />
• affaiblir la logique (la relation d’infér<strong>en</strong>ce).<br />
On peut situer les relations d’infér<strong>en</strong>ce <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce par rapport aux<br />
autres logiques (classiques, non monotones) <strong>en</strong> utilisant la définition <strong>de</strong> Tarski [Tar36,<br />
TCW83] d’une infér<strong>en</strong>ce <strong>en</strong> terme d’inclusion <strong>de</strong> modèles.<br />
17
Ainsi l’infér<strong>en</strong>ce <strong>en</strong> logique classique A |= B peut être définie comme l’inclusion<br />
<strong><strong>de</strong>s</strong> modèles <strong>de</strong> A dans les modèles <strong>de</strong> B. Cette infér<strong>en</strong>ce est donc vérifiée si <strong>à</strong> chaque<br />
fois que A est vrai (i.e. pour chaque interprétation qui satisfait A) B l’est égalem<strong>en</strong>t :<br />
A |= B ssi mod(A) ⊆ mod(B)<br />
On peut définir l’infér<strong>en</strong>ce non monotone <strong>de</strong> manière similaire. La principale différ<strong>en</strong>ce<br />
est que, au lieu <strong>de</strong> <strong>de</strong>man<strong>de</strong>r que tous les modèles <strong>de</strong> A soi<strong>en</strong>t <strong><strong>de</strong>s</strong> modèles <strong>de</strong><br />
B, on ne s’intéresse qu’aux modèles les plus typiques (c’est le s<strong>en</strong>s <strong>de</strong> la sémantique<br />
préfér<strong>en</strong>tielle <strong>de</strong> [Sho87]) ; soit γ(X) une fonction <strong>de</strong> sélection qui retourne un sous<strong>en</strong>semble<br />
<strong>de</strong> X :<br />
A |∼ B ssi γ(mod(A)) ⊆ mod(B)<br />
Le problème lorsque l’on veut définir une relation d’infér<strong>en</strong>ce <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce<br />
est que A n’a pas <strong>de</strong> modèle classique. Il faut donc trouver un moy<strong>en</strong> d’associer<br />
un <strong>en</strong>semble non vi<strong>de</strong> <strong>de</strong> modèles <strong>à</strong> A. Une première solution est d’affaiblir<br />
les hypothèses, c’est-<strong>à</strong>-dire d’utiliser une procédure afin <strong>de</strong> ne sélectionner qu’un sous<br />
<strong>en</strong>semble 1 <strong><strong>de</strong>s</strong> formules <strong>de</strong> A :<br />
A |= H B ssi mod(γ(A)) ⊆ mod(B)<br />
Une <strong>de</strong>uxième solution est d’affaiblir la relation d’infér<strong>en</strong>ce. En effet, si A n’a pas<br />
<strong>de</strong> modèle classique, une possibilité est d’utiliser <strong><strong>de</strong>s</strong> interprétations (multi-valuées par<br />
exemple) qui garantiss<strong>en</strong>t que A aura toujours <strong><strong>de</strong>s</strong> modèles. Cela aura pour conséqu<strong>en</strong>ce<br />
d’affaiblir la logique, car cela ne permettra pas d’avoir autant <strong>de</strong> conséqu<strong>en</strong>ces.<br />
Mais cela permettra dans tous les cas d’avoir une relation d’infér<strong>en</strong>ce non triviale. On<br />
utilise donc exactem<strong>en</strong>t la définition générale <strong>de</strong> Tarski, mais avec une notion <strong>de</strong> modèle<br />
différ<strong>en</strong>te <strong>de</strong> celle <strong>de</strong> la logique classique :<br />
A |= M B ssi mod M (A) ⊆ mod M (B)<br />
Cette <strong><strong>de</strong>s</strong>cription <strong><strong>de</strong>s</strong> logiques classiques, non monotones, et sous incohér<strong>en</strong>ce est<br />
intéressante pour bi<strong>en</strong> compr<strong>en</strong>dre les différ<strong>en</strong>tes problématiques <strong>en</strong> prés<strong>en</strong>ce. Cela<br />
montre <strong>en</strong> particulier que l’infér<strong>en</strong>ce non monotone est une infér<strong>en</strong>ce supra-classique :<br />
le problème est que l’on a trop <strong>de</strong> modèles pour A et que l’on ne veut raisonner qu’<strong>à</strong><br />
partir d’un sous-<strong>en</strong>semble <strong>de</strong> ces modèles. Et, qu’au contraire, l’infér<strong>en</strong>ce sous incohér<strong>en</strong>ce<br />
est une infér<strong>en</strong>ce infra-classique : le problème est que l’on n’a pas assez <strong>de</strong><br />
modèles pour A.<br />
Mais cette <strong><strong>de</strong>s</strong>cription est un peu simpliste pour r<strong>en</strong>dre compte <strong>de</strong> l’<strong>en</strong>semble <strong><strong>de</strong>s</strong><br />
relations d’infér<strong>en</strong>ce sous incohér<strong>en</strong>ce. En particulier, beaucoup <strong><strong>de</strong>s</strong> solutions basées<br />
sur l’affaiblissem<strong>en</strong>t <strong>de</strong> la logique sont <strong><strong>de</strong>s</strong> logiques paraconsistantes définies purem<strong>en</strong>t<br />
syntaxiquem<strong>en</strong>t, <strong>en</strong> changeant la signification <strong><strong>de</strong>s</strong> connecteurs logiques, ou <strong>en</strong> contraignant<br />
les métho<strong><strong>de</strong>s</strong> <strong>de</strong> preuves, ce qui ne permet pas <strong>de</strong> les définir sémantiquem<strong>en</strong>t <strong>en</strong><br />
terme <strong>de</strong> modèles.<br />
Détaillons <strong>à</strong> prés<strong>en</strong>t les <strong>de</strong>ux solutions.<br />
1. Plus exactem<strong>en</strong>t il s’agit habituellem<strong>en</strong>t d’un <strong>en</strong>semble <strong>de</strong> sous-<strong>en</strong>sembles.<br />
18
3.1.1. Affaiblir les hypothèses<br />
L’affaiblissem<strong>en</strong>t <strong><strong>de</strong>s</strong> hypothèses conduit aux approches basées sur les <strong>en</strong>sembles<br />
maximaux cohér<strong>en</strong>ts. Puisqu’il n’est pas possible <strong>de</strong> gar<strong>de</strong>r l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> formules,<br />
l’idée est <strong>de</strong> définir <strong><strong>de</strong>s</strong> infér<strong>en</strong>ces <strong>à</strong> partir <strong>de</strong> sous-<strong>en</strong>sembles <strong>de</strong> ces formules.<br />
Lorsque toutes les formules ont la même importance, on peut définir les relations<br />
d’infér<strong>en</strong>ce suivantes [RM70, BDP97] (voir [Bre89, BDP98] lorsque certaines<br />
formules sont plus importantes/prioritaires que d’autres). Soit une base <strong>de</strong> croyances<br />
K = {ϕ 1 , . . . , ϕ n } :<br />
Définition 1 K ′ est un sous-<strong>en</strong>semble maximal cohér<strong>en</strong>t (ou maxcons) <strong>de</strong> K s’il<br />
satisfait :<br />
• K ′ ⊆ K<br />
• K ′ ⊥<br />
• Si K ′ ⊂ K ′′ ⊆ K, alors K ′′ ⊢ ⊥<br />
Notons MAXCONS(K) l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> maxcons <strong>de</strong> K.<br />
Habituellem<strong>en</strong>t une base incohér<strong>en</strong>te admet plusieurs sous-<strong>en</strong>sembles maximaux<br />
cohér<strong>en</strong>ts. Il faut alors choisir une stratégie afin <strong>de</strong> définir les conséqu<strong>en</strong>ces <strong>de</strong> K <strong>à</strong><br />
partir <strong><strong>de</strong>s</strong> conséqu<strong>en</strong>ces classiques <strong><strong>de</strong>s</strong> maxcons. De nombreuses relations d’infér<strong>en</strong>ces<br />
sont alors définissables dans ce cadre. Les principales sont :<br />
Définition 2 • L’infér<strong>en</strong>ce universelle [RM70] (ou MC-infér<strong>en</strong>ce[BDP97]) où<br />
α est une conséqu<strong>en</strong>ce <strong>de</strong> K si elle est conséqu<strong>en</strong>ce <strong>de</strong> tous les maxcons :<br />
K ⊢ MC α ssi ∀K ′ ∈ MAXCONS(K) K ′ ⊢ α<br />
• L’infér<strong>en</strong>ce exist<strong>en</strong>tielle [RM70] où α est une conséqu<strong>en</strong>ce <strong>de</strong> K si elle est<br />
conséqu<strong>en</strong>ce d’au moins un maxcons :<br />
K ⊢ ∃ α ssi ∃K ′ ∈ MAXCONS(K) K ′ ⊢ α<br />
• L’infér<strong>en</strong>ce argum<strong>en</strong>tative [BDP93] où α est une conséqu<strong>en</strong>ce <strong>de</strong> K si elle<br />
est conséqu<strong>en</strong>ce d’au moins un maxcons et si sa négation n’est conséqu<strong>en</strong>ce<br />
d’aucun maxcons :<br />
K ⊢ A α ssi ∃K ′ ∈ MAXCONS(K) K ′ ⊢ α et ∄K ′ ∈ MAXCONS(K) K ′ ⊢ ¬α<br />
• L’infér<strong>en</strong>ce (universelle) <strong>à</strong> base <strong>de</strong> cardinalité [BDP97, BKMS92] (ou L-infér<strong>en</strong>ce<br />
[BDP97]) où α est une conséqu<strong>en</strong>ce <strong>de</strong> K si elle conséqu<strong>en</strong>ce <strong>de</strong> tous<br />
les plus grands (pour la cardinalité) maxcons, soit MAXCONS card (K) = {K c |<br />
K c ∈ MAXCONS(K) et ∀K ′ ∈ MAXCONS(K) |K c | ≥ |K ′ |} :<br />
K ⊢ L α ssi ∀K ′ ∈ MAXCONS card (K) K ′ ⊢ α<br />
• L’infér<strong>en</strong>ce libre 2 [BDP92] où α est une conséqu<strong>en</strong>ce <strong>de</strong> K si elle est conséqu<strong>en</strong>ce<br />
<strong>de</strong> l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> formules qui apparti<strong>en</strong>n<strong>en</strong>t <strong>à</strong> tous les maxcons 3 :<br />
K ⊢ F ree α ssi {ϕ | ∀K ′ ∈ MAXCONS(K) ϕ ∈ K ′ } ⊢ α<br />
2. Qui correspond <strong>à</strong> l’approche WIDTIO (« Wh<strong>en</strong> In Doubt Throw It Out ») [Win90].<br />
3. Dans [BDP92] cette relation est définie <strong>à</strong> partir <strong><strong>de</strong>s</strong> formules n’appart<strong>en</strong>ant <strong>à</strong> aucun <strong>en</strong>semble minimal<br />
incohér<strong>en</strong>t, ce qui est équival<strong>en</strong>t.<br />
19
⊢ L<br />
⊢ ∃ ⊢<br />
⊢ Free ⊢ MC<br />
⊢ A<br />
FIGURE 3.1 – Relations d’infér<strong>en</strong>ce <strong>à</strong> base <strong>de</strong> maxcons<br />
Comme on peut s’y att<strong>en</strong>dre d’après leur définition, les relations prés<strong>en</strong>tées sont<br />
liées logiquem<strong>en</strong>t, comme représ<strong>en</strong>té <strong>à</strong> la figure 3.1 [BDP97]. En particulier, elles<br />
ét<strong>en</strong><strong>de</strong>nt toutes la logique classique, dans le s<strong>en</strong>s où dans le cas où la base K est cohér<strong>en</strong>te,<br />
elles donn<strong>en</strong>t toutes exactem<strong>en</strong>t les mêmes infér<strong>en</strong>ces que la logique classique,<br />
et dans le cas où la base K est incohér<strong>en</strong>te elles permett<strong>en</strong>t tout <strong>de</strong> même d’inférer<br />
<strong><strong>de</strong>s</strong> conséqu<strong>en</strong>ces non triviales. C’est une propriété d’autant plus intéressante que<br />
dans l’approche consistant <strong>à</strong> affaiblir la logique (i.e. les logiques paraconsistantes) que<br />
nous abor<strong>de</strong>rons dans la section suivante, la plupart <strong>de</strong> ces logiques ne donn<strong>en</strong>t pas les<br />
mêmes conséqu<strong>en</strong>ces que la logique classique lorsque la base K est cohér<strong>en</strong>te.<br />
La relation la plus délicate <strong>à</strong> manipuler est l’infér<strong>en</strong>ce exist<strong>en</strong>tielle qui permet<br />
d’inférer <strong>à</strong> la fois une formule α et sa négation ¬α. Cela ne conduit pas <strong>à</strong> une trivialisation<br />
(par explosion) comme dans le cadre <strong>de</strong> la logique classique, mais il n’est<br />
alors par exemple pas possible <strong>de</strong> pr<strong>en</strong>dre la fermeture classique <strong>de</strong> l’<strong>en</strong>semble <strong><strong>de</strong>s</strong><br />
conséqu<strong>en</strong>ces sans risquer l’incohér<strong>en</strong>ce.<br />
Beaucoup d’autres relations d’infér<strong>en</strong>ces peuv<strong>en</strong>t être imaginées, comme une infér<strong>en</strong>ce<br />
argum<strong>en</strong>tative <strong>à</strong> base <strong>de</strong> cardinalité par exemple.<br />
Des travaux plus réc<strong>en</strong>ts affaibliss<strong>en</strong>t les hypothèses d’une autre manière qu’<strong>en</strong><br />
sélectionnant <strong><strong>de</strong>s</strong> maxcons. On peut par exemple m<strong>en</strong>tionner l’affaiblissem<strong>en</strong>t par oubli<br />
<strong>de</strong> variables [LM02], où l’utilisation <strong>de</strong> nos opérateurs <strong>de</strong> fusion (voir section 5.2.1)<br />
afin <strong>de</strong> définir une relation d’infér<strong>en</strong>ce qui affaiblit les formules <strong>en</strong> sélectionnant les<br />
modèles les plus proches [Ari08].<br />
Dans ces approches, une hypothèse sous-jac<strong>en</strong>te est que <strong><strong>de</strong>s</strong> formules différ<strong>en</strong>tes<br />
sont liées par un li<strong>en</strong> moins fort qu’une conjonction logique. Dans le cas où elle est<br />
cohér<strong>en</strong>te, une base <strong>de</strong> croyances B = {ϕ 1 , . . . , ϕ n }, où les ϕ i sont <strong><strong>de</strong>s</strong> formules<br />
propositionnelles, est équival<strong>en</strong>te <strong>à</strong> la base B ′ = {ϕ 1 ∧ . . . ∧ ϕ n }. En revanche, dans le<br />
cas où cette conjonction est incohér<strong>en</strong>te, ces <strong>de</strong>ux bases ont typiquem<strong>en</strong>t <strong><strong>de</strong>s</strong> <strong>en</strong>sembles<br />
<strong>de</strong> conséqu<strong>en</strong>ces distincts. Cela illustre le fait que la virgule utilisée dans la base B est<br />
un connecteur particulier, différ<strong>en</strong>t <strong>de</strong> la conjonction. Nous avons étudié une logique<br />
comportant un tel connecteur « virgule ». Ce cadre permet <strong>de</strong> fournir une sémantique <strong>à</strong><br />
ce connecteur et <strong>de</strong> généraliser un certain nombre d’approches permettant <strong>de</strong> raisonner<br />
<strong>en</strong> prés<strong>en</strong>ce d’informations contradictoires [28, 66].<br />
3.1.2. Affaiblir la logique<br />
L’autre possibilité lorsque l’on veut raisonner non trivialem<strong>en</strong>t <strong>à</strong> partir d’une base<br />
classiquem<strong>en</strong>t incohér<strong>en</strong>te est <strong>de</strong> gar<strong>de</strong>r l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> formules mais d’affaiblir la re-<br />
20
lation d’infér<strong>en</strong>ce. Cela conduit aux logiques paraconsistantes. Il y a <strong>de</strong>ux possibilités :<br />
• Soit on fait <strong>en</strong> sorte <strong>de</strong> bloquer la trivialisation <strong>en</strong> affaiblissant les connecteurs<br />
logiques.<br />
• Soit on fait <strong>en</strong> sorte <strong>de</strong> localiser les incohér<strong>en</strong>ces <strong>en</strong> utilisant plus que les <strong>de</strong>ux<br />
valeurs <strong>de</strong> vérité (vrai/faux) usuelles.<br />
Affaiblissem<strong>en</strong>t <strong><strong>de</strong>s</strong> connecteurs logiques<br />
Cette voie regroupe l’ess<strong>en</strong>tiel <strong><strong>de</strong>s</strong> travaux sur les logiques paraconsistantes. L’idée<br />
est ici <strong>de</strong> faire <strong>en</strong> sorte que l’on ne puisse dériver l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> formules <strong>à</strong> partir <strong>de</strong><br />
la base incohér<strong>en</strong>te (principe appelé ex falso quodlibet sequitur ou explosion). C’est<strong>à</strong>-dire<br />
qu’on peut définir les logiques paraconsistantes comme les logiques ⊢ P qui<br />
satisfont la propriété suivante :<br />
∃K ∃α ∃β K ⊢ P α et K ⊢ P ¬α et K P β<br />
Nous ne ferons pas <strong>de</strong> rappel <strong>de</strong> ces nombreux travaux, voir [Hun96, Béz00, CM01]<br />
pour une introduction sur ce sujet. La caractéristique commune <strong>de</strong> ces logiques est<br />
la nécessité d’affaiblir les connecteurs logiques. En particulier l’affaiblissem<strong>en</strong>t <strong>de</strong> la<br />
négation conduit par exemple aux C-systèmes <strong>de</strong> da Costa [CM01]. Le problème avec<br />
ce g<strong>en</strong>re <strong>de</strong> solutions est que l’on s’éloigne <strong>de</strong> la logique classique : <strong>en</strong> général, ces<br />
logiques ne permett<strong>en</strong>t pas les mêmes infér<strong>en</strong>ces qu’avec la logique classique lorsque<br />
la base K est cohér<strong>en</strong>te (on peut néanmoins citer l’approche originale <strong>de</strong> Besnard et<br />
Schaub [BS96] consistant <strong>à</strong> découpler chaque variable <strong>de</strong> sa négation puis d’ajouter<br />
<strong><strong>de</strong>s</strong> règles par défaut pour t<strong>en</strong>ter <strong>en</strong>suite <strong>de</strong> recoupler un maximum <strong>de</strong> ces variables<br />
avec leur négation, qui n’a pas ce défaut).<br />
Une autre possibilité est <strong>de</strong> contraindre les preuves, afin <strong>de</strong> bloquer l’explosion. Par<br />
exemple la logique quasi-classique [BH95] interdit l’utilisation <strong><strong>de</strong>s</strong> règles comme la<br />
résolution après l’utilisation <strong>de</strong> règles comme l’introduction <strong>de</strong> disjonction. L’intérêt<br />
<strong>de</strong> cette logique est qu’elle permet (quasim<strong>en</strong>t) les mêmes infér<strong>en</strong>ces que la logique<br />
classique si la base est cohér<strong>en</strong>te.<br />
Dans ce cadre, nous avons défini [3, 45, 67] une nouvelle logique, nommée logique<br />
quasi-possibiliste, qui généralise la logique possibiliste et la logique quasi-classique<br />
[BH95], permettant <strong>de</strong> tirer parti <strong><strong>de</strong>s</strong> avantages <strong><strong>de</strong>s</strong> <strong>de</strong>ux logiques, qui sont toutes les<br />
<strong>de</strong>ux proches <strong>de</strong> la logique propositionnelle classique. Cette logique peut résoudre les<br />
conflits apparaissant au même niveau <strong>de</strong> certitu<strong>de</strong> (comme la logique quasi-classique),<br />
mais elle peut égalem<strong>en</strong>t pr<strong>en</strong>dre <strong>en</strong> compte la stratification <strong>de</strong> la base pour introduire<br />
<strong>de</strong> la gradualité dans l’analyse <strong><strong>de</strong>s</strong> conflits (comme <strong>en</strong> logique possibiliste). Nous avons<br />
proposé égalem<strong>en</strong>t <strong><strong>de</strong>s</strong> mesures <strong>de</strong> conflits associées <strong>à</strong> la logique définie.<br />
Utilisation <strong>de</strong> logiques multi-valuées<br />
L’utilisation <strong>de</strong> logiques multi-valuées permet <strong>de</strong> circonscrire les incohér<strong>en</strong>ces aux<br />
variables concernées et d’éviter le ex falso quodlibet sequitur. Cette approche est plus<br />
fine que celles basées sur la sélection <strong>de</strong> sous-<strong>en</strong>sembles maximaux cohér<strong>en</strong>ts <strong>de</strong> formules<br />
puisqu’avec ces approches ce sont les formules qui sont prises comme unité <strong>de</strong><br />
base, alors qu’<strong>en</strong> utilisant une logique multi-valuée ce sont les variables. Cela permet<br />
21
donc égalem<strong>en</strong>t <strong>de</strong> pr<strong>en</strong>dre <strong>en</strong> compte <strong><strong>de</strong>s</strong> informations prov<strong>en</strong>ant <strong>de</strong> formules incohér<strong>en</strong>tes.<br />
La logique la plus typique est la logique LP <strong>de</strong> Priest [Pri91]. C’est une logique trivaluée,<br />
avec <strong>en</strong> plus <strong><strong>de</strong>s</strong> valeurs <strong>de</strong> vérité Vrai (T) et Faux (F), une valeur B signifiant<br />
incohér<strong>en</strong>t (« <strong>à</strong> la fois vrai et faux » ).<br />
Définition 3 • Une interprétation ω <strong>de</strong> LP est une fonction qui associe <strong>à</strong> chaque<br />
variable propositionnelle une <strong><strong>de</strong>s</strong> valeurs <strong>de</strong> vérité F, B, T. Soit 3 P l’<strong>en</strong>semble<br />
<strong><strong>de</strong>s</strong> interprétations <strong>de</strong> LP .<br />
On ordonne les valeurs <strong>de</strong> vérité comme suit : F < t B < t T.<br />
Les interprétations sont ét<strong>en</strong>dues aux formules comme suit :<br />
– ω(⊤) = T, ω(⊥) = F<br />
– ω(¬α) = B ssi ω(α) = B<br />
ω(¬α) = T ssi ω(α) = F<br />
– ω(α ∧ β) = min ≤t (ω(α), ω(β))<br />
– ω(α ∨ β) = max ≤t (ω(α), ω(β))<br />
• L’<strong>en</strong>semble <strong><strong>de</strong>s</strong> modèles 4 d’une formule ϕ est :<br />
Mod LP (ϕ) = {ω ∈ 3 P | ω(ϕ) ∈ {T, B}}<br />
• La relation d’infér<strong>en</strong>ce LP est alors définie par :<br />
K |= LP ϕ ssi Mod LP (K) ⊆ Mod LP (ϕ)<br />
Le problème est que la relation d’infér<strong>en</strong>ce LP ne permet pas d’inférer beaucoup<br />
<strong>de</strong> conséqu<strong>en</strong>ces. Il est plus intéressant d’utiliser une variante <strong>de</strong> cette logique avec<br />
minimisation, la logique LP m [Pri91] :<br />
Définition 4 • Soit ω! l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> variables « incohér<strong>en</strong>tes » d’une interprétation<br />
ω, i.e.<br />
ω! = {x ∈ P | ω(x) = B}<br />
Alors les modèles minimaux d’une formules sont les « plus classiques » (i.e. les<br />
modèles avec le plus grand nombre <strong>de</strong> variables propositionnelles (pour l’inclusion<br />
<strong>en</strong>sembliste) affectées <strong>à</strong> T ou F) :<br />
min(Mod LP (ϕ)) = {ω ∈ Mod LP (ϕ) | ∄ω ′ ∈ Mod LP (ϕ) t.q. ω ′ ! ⊂ ω!}<br />
La relation d’infér<strong>en</strong>ce LP m est alors définie par :<br />
K |= LPm ϕ ssi min(Mod LP (K)) ⊆ Mod LP (ϕ)<br />
La logique LP m permet égalem<strong>en</strong>t d’obt<strong>en</strong>ir les mêmes conséqu<strong>en</strong>ces que la logique<br />
classique lorsque la base est cohér<strong>en</strong>te.<br />
4. Lorsque l’on travaille avec une logique multi-valuée, il faut définir un <strong>en</strong>semble <strong>de</strong> valeurs <strong>de</strong> vérité<br />
désignées, c’est-<strong>à</strong>-dire les valeurs <strong>de</strong> vérité qui définiss<strong>en</strong>t les modèles d’une formule. Dans le cas <strong>de</strong> la<br />
logique LP il s’agit <strong>de</strong> {T, B}.<br />
22
Dans [22] nous avons exploré l’utilisation <strong>de</strong> logiques tri-valuées pour le raisonnem<strong>en</strong>t<br />
paraconsistant. Nous avons défini et étudié plusieurs formes d’infér<strong>en</strong>ces trivaluées,<br />
et nous les avons comparées du point <strong>de</strong> vue <strong><strong>de</strong>s</strong> propriétés logiques, <strong>de</strong> la<br />
pru<strong>de</strong>nce et <strong>de</strong> la complexité algorithmique. L’idée est d’obt<strong>en</strong>ir plus <strong>de</strong> conséqu<strong>en</strong>ces<br />
qu’avec la logique trivaluée <strong>de</strong> base LP. Nous avons proposé 3 mécanismes différ<strong>en</strong>ts :<br />
le premier est une infér<strong>en</strong>ce argum<strong>en</strong>tative où une formule est inférée si elle est conséqu<strong>en</strong>ce<br />
<strong>de</strong> la logique sous-jac<strong>en</strong>te et que sa négation ne l’est pas. Le second est basé<br />
sur un principe <strong>de</strong> minimisation <strong>de</strong> l’incertitu<strong>de</strong> : seules les formules dont la valeur <strong>de</strong><br />
vérité est T sont inférées (pas celles évaluées <strong>à</strong> B). Le troisième définit les formules<br />
inférées comme les formules « au moins aussi vraies » que la base. En combinant ces<br />
mécanismes <strong>en</strong>tre eux, et avec les infér<strong>en</strong>ces basiques et celles basées sur une infér<strong>en</strong>ce<br />
préfér<strong>en</strong>tielle, cela a donné 9 nouvelles relations d’infér<strong>en</strong>ces différ<strong>en</strong>tes.<br />
Une fois que l’on a accepté le principe <strong>de</strong> travailler avec plus <strong>de</strong> <strong>de</strong>ux valeurs <strong>de</strong><br />
vérité, une question importante est <strong>de</strong> savoir combi<strong>en</strong> <strong>de</strong> valeurs <strong>de</strong> vérité sont nécessaires/utiles.<br />
Ginsberg a étudié la famille <strong><strong>de</strong>s</strong> logiques multi-valuées <strong>à</strong> base <strong>de</strong><br />
bi-treillis [Gin88]. Les valeurs <strong>de</strong> vérité sont ordonnées selon <strong>de</strong>ux ordres : l’ordre sur<br />
la vérité (plus ou moins vrai ou faux), et l’ordre sur la connaissance (plus ou moins<br />
d’information ou d’ignorance). Deux cas particuliers sont les logiques FOUR, qui est<br />
la logique multi-valuée <strong>de</strong> base pour le raisonnem<strong>en</strong>t paraconsitant proposée par Belnap<br />
[Bel77b, Bel77a], (B représ<strong>en</strong>te intuitivem<strong>en</strong>t la valeur incohér<strong>en</strong>te « vrai et faux »<br />
et U la valeur inconnue « ni vrai ni faux ») et la logique SEVEN qui correspond <strong>à</strong><br />
la logique <strong><strong>de</strong>s</strong> défauts supernormaux (dF représ<strong>en</strong>te la valeur « faux par défaut »,<br />
dT représ<strong>en</strong>te la valeur « vrai par défaut », dB représ<strong>en</strong>te la valeur « vrai et faux par<br />
défaut »).<br />
k<br />
B<br />
•<br />
k<br />
B<br />
•<br />
F•<br />
•T<br />
F•<br />
• dF<br />
dB<br />
•<br />
•dT<br />
•T<br />
•<br />
U<br />
t<br />
•<br />
U<br />
t<br />
FIGURE 3.2 – (FOUR, ≤ t , ≤ k )<br />
FIGURE 3.3 – (SEVEN, ≤ t , ≤ k )<br />
On peut utiliser <strong><strong>de</strong>s</strong> logiques avec autant <strong>de</strong> valeurs <strong>de</strong> vérité que l’on veut. Mais<br />
Arieli et Avron ont montré dans un très bon article [AA98] que 4 valeurs (donc les<br />
logiques issues <strong>de</strong> la logique <strong>de</strong> Belnap [Bel77b, Bel77a]) étai<strong>en</strong>t suffisantes, dans<br />
le s<strong>en</strong>s où toute relation d’infér<strong>en</strong>ce basée sur une logique multi-valuée <strong>à</strong> base <strong>de</strong> bitreillis<br />
peut être définie dans une logique <strong>à</strong> 4 valeurs.<br />
Nous avons égalem<strong>en</strong>t étudié [10] la définition <strong>de</strong> relations d’infér<strong>en</strong>ce basées sur<br />
une interprétation bipolaire <strong><strong>de</strong>s</strong> logiques multi-valuées <strong>à</strong> base <strong>de</strong> treillis. La motivation<br />
<strong>de</strong> ce travail était d’étudier les conséqu<strong>en</strong>ces d’une interprétation bipolaire <strong><strong>de</strong>s</strong> valeurs<br />
23
<strong>de</strong> vérité, c’est-<strong>à</strong>-dire qu’au lieu <strong>de</strong> considérer l’ordre <strong>de</strong> vérité et l’ordre <strong>de</strong> connaissance<br />
usuels, on utilise <strong>de</strong>ux ordres où l’on décorelle le vrai et le faux, on a donc un<br />
ordre du vrai (verum) et un ordre du faux (falsum). On peut représ<strong>en</strong>ter cela graphiquem<strong>en</strong>t<br />
par une rotation <strong><strong>de</strong>s</strong> valeurs <strong>de</strong> vérité comme illustré dans les figures 3.4 et 3.5<br />
sur la logique FOUR. Utiliser alors ces nouveaux pré-ordres pour définir <strong><strong>de</strong>s</strong> relations<br />
d’infér<strong>en</strong>ces préfér<strong>en</strong>tielles (<strong>à</strong> base <strong>de</strong> minimisation) permet <strong>de</strong> définir <strong>de</strong> nouvelles<br />
relations d’infér<strong>en</strong>ce.<br />
k<br />
B<br />
•<br />
v<br />
T•<br />
•B<br />
F•<br />
•T<br />
•<br />
U<br />
t<br />
U•<br />
•F<br />
f<br />
FIGURE 3.4 – (FOUR, ≤ t , ≤ k )<br />
FIGURE 3.5 – (FOUR, ≤ v , ≤ f )<br />
3.2. Mesure <strong>de</strong> l’incohér<strong>en</strong>ce<br />
Il est usuel <strong>en</strong> logique classique d’utiliser une mesure binaire <strong>de</strong> la contradiction :<br />
une base est soit cohér<strong>en</strong>te, soit contradictoire. Cette dichotomie est évi<strong>de</strong>nte lorsque le<br />
seul outil déductif est l’infér<strong>en</strong>ce classique, puisque une base contradictoire n’est alors<br />
d’aucune utilité. Mais nous disposons d’un nombre important <strong>de</strong> logiques développées<br />
pour permettre <strong><strong>de</strong>s</strong> infér<strong>en</strong>ces non triviales <strong>à</strong> partir <strong>de</strong> bases contradictoires. Sous cet<br />
angle, cette dichotomie n’est pas suffisante pour donner une mesure <strong>de</strong> la contradiction<br />
d’une base <strong>de</strong> croyance. Une mesure plus fine est nécessaire.<br />
Nous avons écrit un chapitre d’état <strong>de</strong> l’art [15] sur les mesures d’information et<br />
les mesures <strong>de</strong> contradiction <strong><strong>de</strong>s</strong> bases logiques, comme par exemples les mesures<br />
proposées par Hunter [Hun02], Knight [Kni02, Kni03], Lozinskii [Loz94] ou par nous<br />
[26]. Il y a différ<strong>en</strong>tes définitions pour ces mesures, ce qui est naturel dans la mesure où<br />
il n’existe pas <strong>de</strong> logique paraconsistante unique et incontestée mais plutôt un <strong>en</strong>semble<br />
<strong>de</strong> logiques ayant chacune <strong><strong>de</strong>s</strong> avantages et <strong><strong>de</strong>s</strong> inconvéni<strong>en</strong>ts. Il est donc normal que<br />
plusieurs définitions soi<strong>en</strong>t possibles pour ces mesures <strong>de</strong> contradiction. C’est <strong>à</strong> notre<br />
connaissance le premier article récapitulatif sur ces mesures <strong>de</strong> contradiction et d’information<br />
(pour <strong><strong>de</strong>s</strong> bases incohér<strong>en</strong>tes).<br />
Nous allons nous focaliser dans la suite sur les mesures <strong>de</strong> contradiction, mais les<br />
mesures d’information pour <strong><strong>de</strong>s</strong> bases incohér<strong>en</strong>tes (que nous avons traitées égalem<strong>en</strong>t<br />
dans [15]) sont égalem<strong>en</strong>t intéressantes. L’idée est que si l’on veut importer les idées<br />
<strong>de</strong> Shannon sur la mesure <strong>de</strong> l’information dans <strong><strong>de</strong>s</strong> cadres logiques, la mesure <strong>de</strong> l’information<br />
revi<strong>en</strong>t plus ou moins <strong>à</strong> compter le nombre <strong>de</strong> modèles. Or une base incohér<strong>en</strong>te,<br />
qui n’a aucun modèle, ne conti<strong>en</strong>t, suivant cette définition, aucune information.<br />
24
Cela n’est pas raisonnable dès que l’on arrive <strong>à</strong> distinguer <strong><strong>de</strong>s</strong> bases incohér<strong>en</strong>tes (ce<br />
qui est le cas avec toute logique pour le raisonnem<strong>en</strong>t <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce). Il<br />
faut donc généraliser cette notion dans ce cadre. Voir [15] pour un aperçu <strong><strong>de</strong>s</strong> travaux<br />
<strong>de</strong> [Loz94, Kni03] et [26].<br />
On peut classer les mesures <strong>de</strong> contradiction dans <strong>de</strong>ux classes, suivant que l’on<br />
pr<strong>en</strong>d comme « unité <strong>de</strong> mesure » les formules ou les variables. Nous allons prés<strong>en</strong>ter<br />
brièvem<strong>en</strong>t ces <strong>de</strong>ux classes.<br />
3.2.1. Mesures basées sur les formules<br />
L’idée <strong>de</strong> ces mesures est que plus l’incohér<strong>en</strong>ce <strong>de</strong>man<strong>de</strong> <strong>de</strong> formules <strong>de</strong> la base,<br />
moins cette incohér<strong>en</strong>ce est importante. Cette intuition peut-être justifiée par <strong><strong>de</strong>s</strong> exemples<br />
tels que le paradoxe <strong>de</strong> la loterie.<br />
Exemple 1 Il y a un certain nombre <strong>de</strong> billets <strong>de</strong> loterie dont l’un est le ticket gagnant.<br />
Représ<strong>en</strong>tons par w i le fait que le ticket i est le ticket gagnant. On représ<strong>en</strong>te alors<br />
la phrase précé<strong>de</strong>nte par w 1 ∨ . . . ∨ w n . De plus pour chaque ticket i on suppose <strong>de</strong><br />
manière pessimiste (ou simplem<strong>en</strong>t probabilistiquem<strong>en</strong>t réaliste si le nombre <strong>de</strong> tickets<br />
est important) que ce ticket n’est pas le ticket gagnant, donc que ¬w i . On a donc la<br />
base K Ln suivante :<br />
K Ln = {¬w 1 , . . . , ¬w n , w 1 ∨ . . . ∨ w n }<br />
Clairem<strong>en</strong>t, s’il n’y a que <strong>de</strong>ux ou trois tickets pour la loterie, cette base est hautem<strong>en</strong>t<br />
incohér<strong>en</strong>te. Mais s’il y a <strong><strong>de</strong>s</strong> millions <strong>de</strong> tickets, il n’y a intuitivem<strong>en</strong>t pratiquem<strong>en</strong>t<br />
pas <strong>de</strong> conflit dans la base.<br />
Cette idée est le point <strong>de</strong> départ <strong>de</strong> la mesure proposée par Knight [Kni01, Kni03,<br />
Kni02].<br />
Définition 5 Une fonction <strong>de</strong> probabilité sur L est une fonction P : P → [0, 1] t.q. :<br />
• si ⊢ α, alors P (α) = 1<br />
• si ⊢ ¬(α ∧ β), alors P (α ∨ β) = P (α) + P (β)<br />
Voir [Par94] pour plus <strong>de</strong> détails sur cette définition. Dans le cas fini, cette définition<br />
donne une distribution <strong>de</strong> probabilité sur les interprétations, et la probabilité d’une<br />
formule est alors la somme <strong><strong>de</strong>s</strong> probabilités <strong>de</strong> ses modèles.<br />
La mesure d’incohér<strong>en</strong>ce proposée par Knight [Kni01] est définie par :<br />
Définition 6 Soit une base <strong>de</strong> croyances K.<br />
• K est η−cohér<strong>en</strong>t (0 ≤ η ≤ 1) si il existe une fonction <strong>de</strong> probabilité P telle<br />
que P (α) ≥ η pour tout α ∈ K.<br />
• K est maximalem<strong>en</strong>t η−cohér<strong>en</strong>t si η est maximal (i.e. si γ > η alors K n’est<br />
pas γ−consistant).<br />
La notion <strong>de</strong> η-cohér<strong>en</strong>ce maximale est alors une mesure d’incohér<strong>en</strong>ce <strong>de</strong> la base.<br />
On peut vérifier rapi<strong>de</strong>m<strong>en</strong>t que cette définition formalise l’idée que plus on a besoin<br />
25
<strong>de</strong> formules pour générer l’incohér<strong>en</strong>ce, moins celle-ci est problématique. On peut vérifier<br />
qu’une base est maximalem<strong>en</strong>t 0-cohér<strong>en</strong>te si et seulem<strong>en</strong>t si elle conti<strong>en</strong>t une<br />
formule contradictoire, et qu’elle est maximalem<strong>en</strong>t 1-cohér<strong>en</strong>te si et seulem<strong>en</strong>t si elle<br />
est cohér<strong>en</strong>te. Voyons égalem<strong>en</strong>t ce que cela donne sur quelques exemples.<br />
Exemple 2 Soit K 1 = {a, b, ¬a ∨ ¬b}.<br />
K 1 est maximalem<strong>en</strong>t 2 3 −cohér<strong>en</strong>t.<br />
Soit K 2 = {a ∧ b, ¬a ∧ ¬b, a ∧ ¬b}.<br />
K 2 est maximalem<strong>en</strong>t 1 3−cohér<strong>en</strong>t, et chacune <strong>de</strong> ses sous-bases <strong>de</strong> cardinalité 2<br />
est maximalem<strong>en</strong>t 1 2 −cohér<strong>en</strong>t.<br />
Pour un <strong>en</strong>semble minimal incohér<strong>en</strong>t <strong>de</strong> formules, calculer cette mesure est facile :<br />
Définition 7 Un <strong>en</strong>semble minimal incohér<strong>en</strong>t 5 M <strong>de</strong> K est défini par :<br />
• M ⊆ K<br />
• M ⊢ ⊥<br />
• Si M ′ ⊂ M alors M ′ ⊥<br />
Soit MI(K) l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> <strong>en</strong>sembles minimalem<strong>en</strong>t incohér<strong>en</strong>ts <strong>de</strong> K.<br />
Théorème 1 ([Kni01]) Si K ′ ∈ MI(K), alors K ′ est maximalem<strong>en</strong>t |K′ |−1<br />
|K ′ |<br />
−consistant.<br />
En général, cette mesure est plus compliquée <strong>à</strong> calculer, mais il est possible d’utiliser<br />
la métho<strong>de</strong> du simplexe pour le calcul [Kni01].<br />
3.2.2. Mesures basées sur les variables<br />
Une autre métho<strong>de</strong> pour évaluer l’incohér<strong>en</strong>ce d’un <strong>en</strong>semble <strong>de</strong> formules est <strong>de</strong><br />
regar<strong>de</strong>r quelle est la proportion du langage concernée par l’incohér<strong>en</strong>ce. Il n’est donc<br />
pas possible d’utiliser la logique classique <strong>à</strong> cette fin puisque l’incohér<strong>en</strong>ce contamine<br />
l’<strong>en</strong>semble <strong>de</strong> la base (et du langage). Mais si l’on compare les <strong>de</strong>ux bases<br />
K 1 = {a, ¬a, b ∧ c, d} et K 2 = {a, ¬a, b ∧ ¬c, c ∧ ¬b, d, ¬d}, on remarque que dans<br />
K 1 l’incohér<strong>en</strong>ce concerne principalem<strong>en</strong>t la variable a, alors que dans K 2 toutes les<br />
variables sont incluses dans un conflit. C’est ce g<strong>en</strong>re <strong>de</strong> distinctions que ces approches<br />
permett<strong>en</strong>t.<br />
Une métho<strong>de</strong> afin <strong>de</strong> circonscrire l’incohér<strong>en</strong>ce aux variables directem<strong>en</strong>t concernées<br />
est d’utiliser <strong><strong>de</strong>s</strong> logiques multi-valuées, avec au moins une troisième « valeur <strong>de</strong><br />
vérité » indiquant qu’il y a un conflit sur la valeur <strong>de</strong> vérité (vrai ou faux) <strong>de</strong> la variable.<br />
Nous n’avons pas ici la place <strong>de</strong> détailler l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> mesures qui ont été proposées<br />
[Gra78, Hun02, GH06] [26], voir [15] pour plus <strong>de</strong> détails sur ces approches.<br />
Nous ne donnerons donc qu’un exemple illustratif : le <strong>de</strong>gré d’incohér<strong>en</strong>ce proposé<br />
par Grant [Gra78].<br />
Soit X une interprétation quadri-valuée (T, F, B, U), où B signifie intuitivem<strong>en</strong>t « <strong>à</strong><br />
la fois vrai et faux » , et U signifie « ni vrai ni faux » , on définit <strong>de</strong>ux fonctions :<br />
5. Ces <strong>en</strong>sembles sont égalem<strong>en</strong>t appelés MUS (pour Minimal Unsatisfiable Subformula) dans la littérature<br />
sur SAT [GMP06, GMP09].<br />
26
• CCount(X) = {a ∈ P | X(a) = T ou X(a) = F}<br />
• ICount(X) = {a ∈ P | X(a) = B}<br />
Le <strong>de</strong>gré d’incohér<strong>en</strong>ce 6 est un rapport <strong>en</strong>tre les conflits cont<strong>en</strong>us dans la base<br />
(ICount) et l’information cont<strong>en</strong>ue dans la base (CCount) :<br />
Définition 8 Le <strong>de</strong>gré d’incohér<strong>en</strong>ce d’une interprétation quadri-valuée X est :<br />
Inc G (X) =<br />
CCount(X)<br />
CCount(X) + 2 ∗ ICount(X)<br />
On peut alors définir le <strong>de</strong>gré d’incohér<strong>en</strong>ce comme le <strong>de</strong>gré d’incohér<strong>en</strong>ce maximum<br />
<strong>de</strong> ses modèles 7 :<br />
Inc G (K) = max<br />
X|= 4K Inc G(X)<br />
Le problème avec cette mesure est qu’elle ne donne pas <strong><strong>de</strong>s</strong> résultats très satisfaisants,<br />
parce qu’elle considère trop <strong>de</strong> modèles. Comme nous l’avons dit dans la section<br />
3.1.2, les logiques multi-valuées sans minimisation admett<strong>en</strong>t <strong>de</strong> trop nombreux<br />
modèles. Pour améliorer le comportem<strong>en</strong>t <strong>de</strong> cette mesure il est nécessaire d’utiliser<br />
<strong><strong>de</strong>s</strong> logiques multi-valuées avec minimisation. En utilisant la logique quasi-classique<br />
[BH95] par exemple, cela conduit <strong>à</strong> la définition <strong>de</strong> la mesure <strong>de</strong> coher<strong>en</strong>ce proposée<br />
par Hunter [Hun02].<br />
Le problème que nous voyons dans ce g<strong>en</strong>re <strong>de</strong> mesures est qu’elles mélang<strong>en</strong>t<br />
quantité <strong>de</strong> contradiction et quantité d’information, puisque ICount mesure les conflits<br />
(nombre <strong>de</strong> variables « incohér<strong>en</strong>tes ») et CCount mesure l’information (nombre <strong>de</strong><br />
variables sur lesquelles on a une information). Ces mesures sont donc <strong><strong>de</strong>s</strong> mesures<br />
composites, définies <strong>à</strong> partir d’une mesure <strong>de</strong> contradiction et d’une mesure d’information.<br />
Elles peuv<strong>en</strong>t prés<strong>en</strong>ter <strong>de</strong> l’intérêt, mais nous p<strong>en</strong>sons qu’elles ne form<strong>en</strong>t pas <strong>de</strong><br />
bonnes mesures <strong>en</strong> tant que mesures <strong>de</strong> contradiction puisqu’elles ne se cont<strong>en</strong>t<strong>en</strong>t pas<br />
<strong>de</strong> mesurer le conflit.<br />
Nous avons proposé [26, 46] une approche <strong>à</strong> base <strong>de</strong> tests pour définir dans un<br />
cadre uniforme <strong><strong>de</strong>s</strong> mesures d’information et <strong>de</strong> contradiction <strong>de</strong> bases <strong>de</strong> croyances.<br />
Intuitivem<strong>en</strong>t, dans cette approche, le <strong>de</strong>gré <strong>de</strong> contradiction d’une base est le coût d’un<br />
plan <strong>de</strong> test préféré pour purifier la base alors que le <strong>de</strong>gré d’information d’une base<br />
est le coût d’un plan <strong>de</strong> test préféré pour déterminer quel est le mon<strong>de</strong> réel. Plusieurs<br />
critères <strong>de</strong> préfér<strong>en</strong>ce peuv<strong>en</strong>t être utilisés. Les propriétés <strong><strong>de</strong>s</strong> mesures d’incohér<strong>en</strong>ce<br />
<strong>à</strong> base <strong>de</strong> plans <strong>de</strong> tests introduites ont été analysées et elles ont été comparées avec<br />
diverses mesures proposées jusque-l<strong>à</strong> dans la littérature (par Hunter [Hun02], Knight<br />
[Kni02, Kni03], Lozinskii [Loz94] notamm<strong>en</strong>t). Un point fort <strong>de</strong> l’approche proposée<br />
est que le cadre développé est suffisamm<strong>en</strong>t général pour que diverses approches pour<br />
l’infér<strong>en</strong>ce <strong>en</strong> prés<strong>en</strong>ce <strong>de</strong> contradictions puiss<strong>en</strong>t être prises <strong>en</strong> compte. Voyons un<br />
exemple <strong>de</strong> ces plans <strong>de</strong> tests dans le cadre <strong>de</strong> la logique LP m .<br />
6. On <strong>de</strong>vrait plutôt parler <strong>de</strong> <strong>de</strong>gré <strong>de</strong> cohér<strong>en</strong>ce puisque la valeur maximale est obt<strong>en</strong>ue pour une base<br />
cohér<strong>en</strong>te.<br />
7. Soit |= 4 la relation <strong>de</strong> satisfaction <strong>de</strong> la logique <strong>de</strong> Belnap [Bel77b, Bel77a]<br />
27
a∧(¬a∨b)∧(¬b∨c)∧(¬c∨¬a)<br />
a ¬a<br />
a∧b∧(¬b∨c)∧¬c<br />
c ¬c<br />
¬a∧(¬b∨c)<br />
a∧b∧c<br />
a∧b∧¬b∧c<br />
b ¬b<br />
a∧b∧¬c<br />
a∧¬b∧¬c<br />
FIGURE 3.6 – Degré <strong>de</strong> contradiction dans LP m<br />
Exemple 3 Soit la base K = {a∧(¬a∨b)∧(¬b∨c)∧(¬c∨¬a)}. La figure 3.6 prés<strong>en</strong>te<br />
un plan <strong>de</strong> coût minimal (dans le contexte atomique standard 8 ) qui purifie (i.e. permet<br />
<strong>de</strong> supprimer tous les conflits <strong>de</strong> la base) la base K. Tous les autres plans <strong>de</strong> test ont<br />
un coût plus élevé. Le <strong>de</strong>gré <strong>de</strong> contradiction <strong>de</strong> K est <strong>de</strong> 3 (c’est la profon<strong>de</strong>ur 9 du<br />
plan <strong>de</strong> tests <strong>de</strong> la figure 3.6).<br />
3.2.3. Valeurs d’incohér<strong>en</strong>ce<br />
Jusqu’ici nous avons discuté <strong>de</strong> mesures d’incohér<strong>en</strong>ce, c’est-<strong>à</strong>-dire <strong>de</strong> fonctions<br />
qui permett<strong>en</strong>t <strong>de</strong> mesurer <strong>à</strong> quel point une base est incohér<strong>en</strong>te, et donc <strong>de</strong> comparer<br />
la quantité d’incohér<strong>en</strong>ce <strong>de</strong> plusieurs bases.<br />
Il est intéressant <strong>de</strong> disposer <strong>de</strong> ce type <strong>de</strong> mesures par exemple pour s’<strong>en</strong> servir afin<br />
<strong>de</strong> définir <strong><strong>de</strong>s</strong> métho<strong><strong>de</strong>s</strong> <strong>de</strong> raisonnem<strong>en</strong>t diverses, où il peut être utile <strong>de</strong> disposer d’un<br />
critère afin <strong>de</strong> choisir, parmi un <strong>en</strong>semble <strong>de</strong> bases, la base la moins contradictoire.<br />
Une mesure bi<strong>en</strong> plus utile serait, au lieu <strong>de</strong> quantifier l’incohér<strong>en</strong>ce au niveau, global,<br />
<strong>de</strong> la base, <strong>de</strong> pouvoir évaluer <strong>à</strong> quel point chaque formule <strong>de</strong> la base est conflictuelle.<br />
C’est-<strong>à</strong>-dire d’évaluer la responsabilité <strong>de</strong> chaque formule dans les incohér<strong>en</strong>ces<br />
<strong>de</strong> la base. Cela permettrait d’utiliser ces mesures par exemple pour i<strong>de</strong>ntifier les formules<br />
les plus conflictuelles, afin <strong>de</strong> résoudre les conflits : on peut par exemple définir<br />
une infér<strong>en</strong>ce <strong>à</strong> base <strong>de</strong> maximaux cohér<strong>en</strong>ts où la maximalité est calculée <strong>à</strong> partir <strong>de</strong><br />
cette mesure, ou <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> révision gardant <strong>en</strong> priorité les formules les moins<br />
conflictuelles, etc.<br />
Pour distinguer les <strong>de</strong>ux types <strong>de</strong> mesures, nous nommons mesures d’incohér<strong>en</strong>ces<br />
les mesures au niveau <strong><strong>de</strong>s</strong> bases, et valeurs d’incohér<strong>en</strong>ces les mesures au niveau<br />
<strong><strong>de</strong>s</strong> formules.<br />
De manière assez naturelle, si l’on dispose d’une valeur d’incohér<strong>en</strong>ce, permettant<br />
<strong>de</strong> définir le conflit <strong>de</strong> chaque formule, on peut utiliser cette valeur pour définir une<br />
mesure d’incohér<strong>en</strong>ce, <strong>en</strong> agrégeant les valeurs d’incohér<strong>en</strong>ce <strong><strong>de</strong>s</strong> formules <strong>de</strong> la base<br />
(<strong>en</strong> pr<strong>en</strong>ant leur maximum par exemple).<br />
8. C’est-<strong>à</strong>-dire que l’on considère que les seuls tests disponibles consist<strong>en</strong>t <strong>à</strong> tester la valeur d’une variable<br />
et que tous ces tests ont le même coût.<br />
9. Comme tous les tests ont le même coup, on compare les plans <strong>de</strong> tests par la profon<strong>de</strong>ur maximale <strong>de</strong><br />
leurs branches.<br />
28
Définir <strong><strong>de</strong>s</strong> mesures d’incohér<strong>en</strong>ce <strong>à</strong> partir <strong>de</strong> valeurs d’incohér<strong>en</strong>ces prés<strong>en</strong>te un<br />
avantage supplém<strong>en</strong>taire :<br />
L’idée vi<strong>en</strong>t du constat que les mesures existantes se partag<strong>en</strong>t <strong>en</strong> <strong>de</strong>ux classes,<br />
ayant chacune ses inconvéni<strong>en</strong>ts. La première classe regroupe les mesures qui ti<strong>en</strong>n<strong>en</strong>t<br />
compte du nombre <strong>de</strong> formules interv<strong>en</strong>ant dans la production <strong>de</strong> l’incohér<strong>en</strong>ce (section<br />
3.2.1). Cela permet <strong>de</strong> pr<strong>en</strong>dre <strong>en</strong> compte la répartition <strong><strong>de</strong>s</strong> conflits <strong>en</strong>tre les différ<strong>en</strong>tes<br />
formules <strong>de</strong> la base, mais ne permet pas d’inspecter plus finem<strong>en</strong>t les conflits<br />
dans les formules. La <strong>de</strong>uxième classe permet d’inspecter plus finem<strong>en</strong>t les conflits au<br />
niveau <strong><strong>de</strong>s</strong> variables propositionnelles, plutôt qu’au niveau <strong><strong>de</strong>s</strong> formules, mais cela se<br />
paye par une incapacité <strong>à</strong> pr<strong>en</strong>dre <strong>en</strong> compte la répartition du conflit <strong>en</strong>tre les différ<strong>en</strong>tes<br />
formules <strong>de</strong> la base (section 3.2.2). Jusqu’ici il semblait difficile <strong>de</strong> pr<strong>en</strong>dre <strong>en</strong><br />
compte ces <strong>de</strong>ux facteurs, et aucune <strong><strong>de</strong>s</strong> mesures proposées n’y parv<strong>en</strong>ait.<br />
Nous avons défini [32] <strong>de</strong> nouvelles mesures d’incohér<strong>en</strong>ce basées sur la valeur <strong>de</strong><br />
Shapley (une notion issue <strong>de</strong> la théorie <strong><strong>de</strong>s</strong> jeux). Nous avons utilisé une idée simple :<br />
utiliser une mesure <strong>de</strong> la <strong>de</strong>uxième classe (section 3.2.2), permettant un exam<strong>en</strong> fin<br />
<strong>de</strong> l’incohér<strong>en</strong>ce au niveau <strong><strong>de</strong>s</strong> variables propositionnelles, et l’utiliser pour définir un<br />
jeu sous forme coalitionnelle. On utilise <strong>en</strong>suite la valeur <strong>de</strong> Shapley sur le jeu ainsi<br />
défini afin <strong>de</strong> redistribuer cette incohér<strong>en</strong>ce sur les différ<strong>en</strong>tes formules responsables<br />
du conflit. Les <strong>de</strong>ux dim<strong>en</strong>sions sont donc bi<strong>en</strong> prises <strong>en</strong> compte dans la détermination<br />
<strong>de</strong> la valeur d’incohér<strong>en</strong>ce <strong>de</strong> la formule.<br />
Nous donnons <strong>à</strong> prés<strong>en</strong>t quelques exemples afin d’illustrer le fait que les valeurs<br />
que l’on obti<strong>en</strong>t sur <strong><strong>de</strong>s</strong> cas simples sont très intuitives. L’exemple ci-<strong><strong>de</strong>s</strong>sous est celui<br />
<strong>de</strong> la valeur d’incohér<strong>en</strong>ce <strong>de</strong> Shapley la plus simple S Id . La mesure d’incohér<strong>en</strong>ce<br />
associée ŜI d<br />
est simplem<strong>en</strong>t calculée <strong>à</strong> partir du maximum <strong><strong>de</strong>s</strong> valeurs d’incohér<strong>en</strong>ce<br />
<strong><strong>de</strong>s</strong> formules <strong>de</strong> la base.<br />
Exemple 4 • K 1 = {a, ¬a, b}.<br />
La valeur d’incohér<strong>en</strong>ce est S Id (K 1 ) = ( 1 2 , 1 2<br />
, 0). Et la mesure d’incohér<strong>en</strong>ce<br />
<strong>de</strong> la base est ŜI d<br />
(K 1 ) = 1 2 .<br />
b est une formule libre, elle n’est <strong>en</strong>gagée dans aucun conflit, elle a donc la<br />
valeur 0 (c’est une <strong><strong>de</strong>s</strong> propriétés <strong>de</strong> ces valeurs d’incohér<strong>en</strong>ce). Les <strong>de</strong>ux autres<br />
formules (a et ¬a) sont aussi responsables l’une que l’autre du conflit, elles se<br />
partag<strong>en</strong>t donc la responsabilité du conflit, leur valeur d’incohér<strong>en</strong>ce est donc<br />
1<br />
2 .<br />
• K 2 = {a, b, b ∧ c, ¬b ∧ d}.<br />
La valeur est S Id (K 2 ) = (0, 1 6 , 1 6 , 4 6<br />
). Et la mesure d’incohér<strong>en</strong>ce <strong>de</strong> la base est<br />
Ŝ Id (K 2 ) = 2 3 .<br />
La <strong>de</strong>rnière formule est clairem<strong>en</strong>t celle qui apporte le plus d’incohér<strong>en</strong>ce <strong>à</strong> la<br />
base puisque <strong>en</strong>lever simplem<strong>en</strong>t cette formule restaure la cohér<strong>en</strong>ce <strong>de</strong> la base.<br />
• K 4 = {a ∧ ¬a, b, ¬b, c}.<br />
La valeur est S Id (K 4 ) = ( 4 6 , 1 6 , 1 6<br />
, 0). Et la mesure d’incohér<strong>en</strong>ce <strong>de</strong> la base est<br />
Ŝ Id (K 4 ) = 2 3 .<br />
La première formule est une contradiction, il faut donc absolum<strong>en</strong>t la retirer<br />
<strong>de</strong> la base si on veut supprimer l’incohér<strong>en</strong>ce. C’est donc cette formule qui est<br />
la plus conflictuelle. Ensuite il faut <strong>en</strong>lever soit b soit ¬b, ce <strong>de</strong>ux formules ont<br />
29
donc une responsabilité dans les conflits <strong>de</strong> la base. Ce qui n’est pas le cas <strong>de</strong> la<br />
<strong>de</strong>rnière formule.<br />
Nous ne détaillons pas plus cette approche ici, puisque l’article correspondant est<br />
inclus dans la <strong>de</strong>uxième partie du docum<strong>en</strong>t (chapitre 9).<br />
Nous avons égalem<strong>en</strong>t travaillé <strong>à</strong> la définition <strong>de</strong> valeurs d’incohér<strong>en</strong>ces basées<br />
sur le calcul <strong><strong>de</strong>s</strong> <strong>en</strong>sembles minimaux incohér<strong>en</strong>ts [36]. Cela se justifie par le fait<br />
que ces <strong>en</strong>sembles représ<strong>en</strong>t<strong>en</strong>t l’ess<strong>en</strong>ce <strong><strong>de</strong>s</strong> conflits <strong>de</strong> la base. Or, nous avons montré<br />
qu’une <strong>de</strong> ces valeurs d’incohér<strong>en</strong>ce était égalem<strong>en</strong>t une valeur d’incohér<strong>en</strong>ce <strong>de</strong><br />
Shapley (celle <strong>de</strong> l’exemple ci-<strong><strong>de</strong>s</strong>sus). Ce résultat nous a permis <strong>de</strong> donner une caractérisation<br />
logique complète <strong>de</strong> cette mesure. C’est <strong>à</strong> notre connaissance la première<br />
caractérisation logique d’une mesure d’incohér<strong>en</strong>ce.<br />
Nous ne détaillons pas plus cette approche ici, puisque l’article correspondant est<br />
inclus dans la <strong>de</strong>uxième partie du docum<strong>en</strong>t (chapitre 10).<br />
30
4.1 Le cadre AGM<br />
4.2 Révision itérée<br />
4.3 Itération dans le cadre AGM<br />
4.4 Révision itérée et états épistémiques<br />
4.5 Opérateurs d’amélioration<br />
Chapitre IV<br />
RÉVISION<br />
Pour atteindre la connaissance,<br />
ajoute <strong><strong>de</strong>s</strong> choses chaque jour.<br />
Pour atteindre la sagesse,<br />
retire <strong><strong>de</strong>s</strong> choses chaque jour.<br />
(Lao Tzu, Tao-te Ching, ch. 48)<br />
Le problème posé par la révision <strong>de</strong> croyances peut être résumé par la question<br />
suivante : étant donnée une base <strong>de</strong> croyances représ<strong>en</strong>tant les croyances d’un<br />
ag<strong>en</strong>t <strong>à</strong> propos du mon<strong>de</strong> et une nouvelle information <strong>à</strong> propos <strong>de</strong> ce mon<strong>de</strong>,<br />
pouvant remettre <strong>en</strong> cause une partie <strong>de</strong> ses croyances, quels changem<strong>en</strong>ts cette nouvelle<br />
information va-t-elle produire dans la base <strong>de</strong> croyances ?<br />
Alchourrón, Gär<strong>de</strong>nfors et Makinson (AGM) ont proposé un <strong>en</strong>semble <strong>de</strong> postulats<br />
qu’un opérateur <strong>de</strong> révision « raisonnable » doit satisfaire [AGM85].<br />
Une critique <strong>en</strong>vers le cadre AGM est qu’il ne permet pas d’assurer un bon comportem<strong>en</strong>t<br />
lors <strong>de</strong> l’itération du processus <strong>de</strong> révision. De nombreuses solutions ont été<br />
proposées [DP97, Bou93, Leh95, Wil94] mais aucune n’est totalem<strong>en</strong>t satisfaisante.<br />
4.1. Le cadre AGM<br />
Dans cette section, nous considérons <strong><strong>de</strong>s</strong> théories, c’est-<strong>à</strong>-dire <strong><strong>de</strong>s</strong> bases K closes<br />
pour la déduction logique (i.e. Cn(K) = K).<br />
Si l’on considère une formule α, pour un ag<strong>en</strong>t avec les croyances K, cette formule<br />
α ne peut avoir que 3 statuts épistémiques différ<strong>en</strong>ts :<br />
• Soit α ∈ K, c’est-<strong>à</strong>-dire que l’ag<strong>en</strong>t croit que α est vraie. On dit que α est<br />
acceptée par l’ag<strong>en</strong>t.<br />
• Soit ¬α ∈ K, c’est-<strong>à</strong>-dire que l’ag<strong>en</strong>t croit que α est fausse. On dit que α est<br />
31
ejetée par l’ag<strong>en</strong>t.<br />
• Soit α ∉ K et ¬α ∉ K, on dit que α est indéterminée (conting<strong>en</strong>te) pour l’ag<strong>en</strong>t.<br />
Les opérateurs <strong>de</strong> changem<strong>en</strong>t <strong>de</strong> croyances peuv<strong>en</strong>t alors être définis comme <strong><strong>de</strong>s</strong><br />
transitions <strong>en</strong>tre ces différ<strong>en</strong>ts statuts, comme illustré <strong>à</strong> la figure 4.1.<br />
Indéterminée<br />
Acceptée<br />
Expansion<br />
Contraction<br />
Révision<br />
Révision<br />
Expansion<br />
Contraction<br />
Refusée<br />
FIGURE 4.1 – Transitions <strong>en</strong>tre statuts épistémiques<br />
Lorsque la formule passe d’un statut indéterminée <strong>à</strong> acceptée (ou symétriquem<strong>en</strong>t<br />
refusée), cette transition est nommée expansion, car on ajoute simplem<strong>en</strong>t <strong>de</strong> l’information.<br />
La transition inverse (<strong>de</strong> acceptée/refusée <strong>à</strong> indéterminée) est appelée contraction,<br />
car on souhaite <strong>en</strong>lever une information <strong><strong>de</strong>s</strong> croyances <strong>de</strong> l’ag<strong>en</strong>t. Et lorsque l’on<br />
passe directem<strong>en</strong>t du statut acceptée <strong>à</strong> refusée (ou symétriquem<strong>en</strong>t <strong>de</strong> refusée <strong>à</strong> acceptée),<br />
la transition est nommée révision. Dans ce cas, on change d’avis sur la véracité<br />
d’une information.<br />
Pour ces trois types d’opérateurs <strong>de</strong> changem<strong>en</strong>t <strong>de</strong> croyances, on souhaite évi<strong>de</strong>mm<strong>en</strong>t<br />
avoir <strong><strong>de</strong>s</strong> propriétés <strong>de</strong> rationalité, c’est-<strong>à</strong>-dire que les opérateurs doiv<strong>en</strong>t respecter<br />
un certain nombre <strong>de</strong> propriétés garantissant qu’ils réalis<strong>en</strong>t le changem<strong>en</strong>t att<strong>en</strong>du.<br />
Cette approche axiomatique est réellem<strong>en</strong>t fondatrice <strong>de</strong> la théorie <strong>de</strong> la révision <strong>de</strong><br />
croyances AGM. L’idée est, au lieu <strong>de</strong> définir <strong><strong>de</strong>s</strong> opérateurs particuliers, d’énumérer<br />
un <strong>en</strong>semble <strong>de</strong> propriétés que tout opérateur <strong>de</strong> changem<strong>en</strong>t raisonnable <strong>de</strong>vrait satisfaire.<br />
Puis on regar<strong>de</strong> s’il existe <strong><strong>de</strong>s</strong> opérateurs satisfaisant l’<strong>en</strong>semble <strong>de</strong> ces propriétés<br />
et on essaye év<strong>en</strong>tuellem<strong>en</strong>t <strong>de</strong> les caractériser, c’est-<strong>à</strong>-dire <strong>de</strong> montrer que <strong><strong>de</strong>s</strong> opérateurs<br />
satisfaisant un <strong>en</strong>semble <strong>de</strong> propriétés donné s’exprim<strong>en</strong>t tous sous une certaine<br />
forme. C’est ce qu’on appellera <strong><strong>de</strong>s</strong> théorèmes <strong>de</strong> représ<strong>en</strong>tation (on peut égalem<strong>en</strong>t<br />
parler <strong>de</strong> caractérisation logique).<br />
En ce qui concerne les opérateurs <strong>de</strong> changem<strong>en</strong>t <strong>de</strong> croyances, les propriétés att<strong>en</strong>dues<br />
sont assez faciles <strong>à</strong> exprimer intuitivem<strong>en</strong>t, par 3 principes :<br />
• Principe <strong>de</strong> succès (appelé aussi primauté <strong>de</strong> la nouvelle information) : le<br />
changem<strong>en</strong>t doit réussir, c’est-<strong>à</strong>-dire qu’après l’opération, l’information doit avoir<br />
le statut voulu.<br />
• Principe <strong>de</strong> cohér<strong>en</strong>ce : on veut que la base résultante <strong>de</strong> l’opération soit une<br />
base cohér<strong>en</strong>te (on veut donc éviter la trivialisation).<br />
• Principe <strong>de</strong> changem<strong>en</strong>t minimal : on veut modifier les croyances <strong>de</strong> l’ag<strong>en</strong>t<br />
le moins possible, pour assurer qu’on n’élimine aucune information <strong>de</strong> manière<br />
inconsidérée, et qu’on n’ajoute aucune information non souhaitée.<br />
32
Alchourrón, Gär<strong>de</strong>nfors et Makinson ont proposé une formalisation logique <strong>de</strong> ces<br />
propriétés [AGM85, Gär88]. Voyons donc <strong>à</strong> prés<strong>en</strong>t comm<strong>en</strong>t formaliser ces principes<br />
pour chacun <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> changem<strong>en</strong>t.<br />
Expansion<br />
4.1.1. Postulats AGM<br />
Un opérateur d’expansion + est une fonction <strong>de</strong> K × L vers K qui vérifie les<br />
propriétés suivantes :<br />
(K+1) K + α est une théorie<br />
(K+2) α ∈ K + α<br />
(K+3) K ⊆ K + α<br />
(K+4) Si α ∈ K, alors K + α = K<br />
(K+5) Si K ′ ⊆ K, alors K ′ + α ⊆ K + α<br />
(K+6) K + α est la plus petite base satisfaisant (K+1)-(K+5)<br />
(clôture)<br />
(succès)<br />
(inclusion)<br />
(vacuité)<br />
(monotonie)<br />
(minimalité)<br />
L’explication intuitive <strong>de</strong> ces axiomes est la suivante : (K+1) assure que le résultat<br />
<strong>de</strong> l’expansion est bi<strong>en</strong> une théorie. (K+2) dit que la nouvelle information doit être<br />
vraie dans la nouvelle base <strong>de</strong> croyance. La motivation du nom expansion peut être<br />
expliquée par (K+3) qui certifie que l’on gar<strong>de</strong> toutes les informations <strong>de</strong> l’anci<strong>en</strong>ne<br />
base. (K+4) dit que si la nouvelle information apparti<strong>en</strong>t déj<strong>à</strong> <strong>à</strong> la base <strong>de</strong> croyance<br />
alors il n’y a ri<strong>en</strong> <strong>à</strong> faire pour l’accepter. Le postulat (K+5) exprime la monotonie <strong>de</strong><br />
l’expansion. Et le <strong>de</strong>rnier postulat (K+6) exprime la minimalité du changem<strong>en</strong>t, c’est<strong>à</strong>-dire<br />
qu’il s’assure que la nouvelle base <strong>de</strong> croyance ne conti<strong>en</strong>t pas <strong>de</strong> croyance non<br />
justifiée par l’ajout <strong>de</strong> la nouvelle information.<br />
Il n’y a qu’un seul opérateur d’expansion :<br />
Théorème 2 ([Gär88]) L’opérateur d’expansion + satisfait les postulats (K+1)-(K+6)<br />
ssi K + A = Cn(K ∪ A).<br />
Contraction<br />
Un opérateur <strong>de</strong> contraction ÷ est une fonction <strong>de</strong> K × L vers K qui vérifie les<br />
propriétés suivantes :<br />
(K÷1) K ÷ α est une théorie<br />
(K÷2) K ÷ α ⊆ K<br />
(K÷3) Si α /∈ K, alors K ÷ α = K<br />
(K÷4) Si α, alors α /∈ K ÷ α<br />
(K÷5) Si α ∈ K, alors K ⊆ (K ÷ α) + α<br />
(K÷6) Si ⊢ α ↔ β, alors K ÷ α = K ÷ β<br />
(K÷7) (K ÷ α) ∩ (K ÷ β) ⊆ K ÷ (α ∧ β)<br />
(K÷8) Si α /∈ K ÷ (α ∧ β), alors K ÷ (α ∧ β) ⊆ K ÷ α<br />
33<br />
(clôture)<br />
(inclusion)<br />
(vacuité)<br />
(succès)<br />
(restauration)<br />
(préservation)<br />
(intersection)<br />
(conjonction)
(K÷1) assure que le résultat <strong>de</strong> la contraction est bi<strong>en</strong> une théorie. (K÷2) garantit<br />
que lors <strong>de</strong> la contraction aucune nouvelle information n’est ajoutée <strong>à</strong> la base <strong>de</strong><br />
croyance. (K÷3) dit que si l’information α n’est pas acceptée par K, il n’y a ri<strong>en</strong> <strong>à</strong> faire<br />
pour retirer α <strong>de</strong> K. Le postulat (K÷4) assure le succès <strong>de</strong> la contraction, c’est-<strong>à</strong>-dire<br />
que si α n’est pas une tautologie, alors la contraction réussit. Le postulat (K÷5) assure<br />
que la contraction <strong>de</strong> K par α suivie <strong>de</strong> l’expansion par α redonne la théorie K comme<br />
résultat (l’inclusion inverse <strong>de</strong> (K÷5) étant une conséqu<strong>en</strong>ce <strong>de</strong> (K÷1)-(K÷4)). (K÷6)<br />
dit que le résultat <strong>de</strong> la contraction ne dép<strong>en</strong>d pas <strong>de</strong> la syntaxe <strong>de</strong> l’information. Ces<br />
six postulats sont les postulats <strong>de</strong> base pour les opérateurs <strong>de</strong> contraction. (K÷7) et<br />
(K÷8) sont appelés postulats supplém<strong>en</strong>taires 1 . (K÷7) dit que si une information est<br />
<strong>à</strong> la fois dans la contraction par α et dans la contraction par β alors elle doit être dans<br />
la contraction par la conjonction α ∧ β. (K÷8) exprime la minimalité du changem<strong>en</strong>t<br />
pour la conjonction.<br />
Révision<br />
Un opérateur <strong>de</strong> révision ∗ est une fonction <strong>de</strong> K × L vers K qui vérifie les propriétés<br />
suivantes :<br />
(K*1) K ∗ α est une théorie<br />
(K*2) α ∈ K ∗ α<br />
(K*3) K ∗ α ⊆ K + α<br />
(K*4) Si ¬α /∈ K, alors K + α ⊆ K ∗ α<br />
(K*5) K ∗ α = K ⊥ ssi ⊢ ¬α<br />
(K*6) Si ⊢ α ↔ β, alors K ∗ α = K ∗ β<br />
(K*7) K ∗ (α ∧ β) ⊆ (K ∗ α) + β<br />
(K*8) Si ¬β /∈ K ∗ α, alors (K ∗ α) + β ⊆ K ∗ (α ∧ β)<br />
(clôture)<br />
(succès)<br />
(inclusion)<br />
(vacuité)<br />
(cohér<strong>en</strong>ce)<br />
(ext<strong>en</strong>sionalité)<br />
(inclusion conjonctive)<br />
(vacuité conjonctive)<br />
L’interprétation <strong>de</strong> ces postulats est la suivante : (K*1) s’assure que le résultat <strong>de</strong><br />
la révision est bi<strong>en</strong> une théorie. (K*2) dit que la nouvelle information est vraie dans la<br />
nouvelle base <strong>de</strong> croyance. Le postulat (K*3) implique que la révision par la nouvelle<br />
information ne peut pas ajouter <strong>de</strong> croyance qui ne soit une conséqu<strong>en</strong>ce <strong>de</strong> la nouvelle<br />
information et <strong>de</strong> la base <strong>de</strong> croyance. Et les postulats (K*3) et (K*4) <strong>en</strong>semble signifi<strong>en</strong>t<br />
que, lorsque la nouvelle information n’est pas contradictoire avec l’anci<strong>en</strong>ne base<br />
<strong>de</strong> croyance, alors la révision <strong>de</strong> la base <strong>de</strong> croyance se résume <strong>à</strong> l’expansion <strong>de</strong> cette<br />
base. (K*5) exprime le fait que la seule façon d’arriver <strong>à</strong> une base incohér<strong>en</strong>te par une<br />
révision est <strong>de</strong> réviser par une information contradictoire. (K*6) dit que le résultat <strong>de</strong><br />
la révision ne dép<strong>en</strong>d pas <strong>de</strong> la syntaxe <strong>de</strong> la nouvelle information. Ces six postulats<br />
sont les postulats <strong>de</strong> base pour les opérateurs <strong>de</strong> révision, les <strong>de</strong>ux postulats (K*7) et<br />
(K*8) ont été appelés postulats supplém<strong>en</strong>taires et exprim<strong>en</strong>t le bon comportem<strong>en</strong>t <strong><strong>de</strong>s</strong><br />
opérateurs <strong>de</strong> révision <strong>en</strong> terme <strong>de</strong> minimalité <strong>de</strong> changem<strong>en</strong>t. Ils assur<strong>en</strong>t que la révision<br />
par une conjonction <strong>de</strong> <strong>de</strong>ux informations revi<strong>en</strong>t <strong>à</strong> une révision par la première<br />
1. De fait, certains travaux étudi<strong>en</strong>t <strong><strong>de</strong>s</strong> opérateurs ne satisfaisant que les postulats <strong>de</strong> base. Notre opinion<br />
est que ces postulats supplém<strong>en</strong>taires sont au moins aussi importants pour un bon opérateur que les postulats<br />
<strong>de</strong> base.<br />
34
information et une expansion par la secon<strong>de</strong> dès que cela est possible (i.e. dès que la<br />
secon<strong>de</strong> information ne contredit aucune croyance issue <strong>de</strong> la première révision).<br />
Révision <strong>en</strong> logique propositionnelle<br />
Les propriétés que nous v<strong>en</strong>ons <strong>de</strong> prés<strong>en</strong>ter sont valables pour n’importe quelle<br />
logique 2 . On peut les exprimer plus simplem<strong>en</strong>t dans le cadre <strong>de</strong> la logique propositionnelle.<br />
C’est ce que nous allons voir ici.<br />
Soi<strong>en</strong>t <strong>de</strong>ux formules propositionnelles ϕ et µ. L’opérateur ◦ est un opérateur <strong>de</strong><br />
révision s’il vérifie les postulats suivants :<br />
(R1) ϕ ◦ µ ⊢ µ<br />
(R2) Si ϕ ∧ µ est cohér<strong>en</strong>t alors ϕ ◦ µ ≡ ϕ ∧ µ<br />
(R3) Si µ est cohér<strong>en</strong>t alors ϕ ◦ µ est cohér<strong>en</strong>t<br />
(R4) Si ϕ 1 ≡ ϕ 2 et µ 1 ≡ µ 2 alors ϕ 1 ◦ µ 1 ≡ ϕ 2 ◦ µ 2<br />
(R5) (ϕ ◦ µ) ∧ ψ ⊢ ϕ ◦ (µ ∧ ψ)<br />
(R6) Si (ϕ ◦ µ) ∧ ψ est cohér<strong>en</strong>t alors ϕ ◦ (µ ∧ ψ) ⊢ (ϕ ◦ µ) ∧ ψ<br />
Soit un opérateur <strong>de</strong> révision ∗ sur <strong><strong>de</strong>s</strong> théories et ◦ un opérateur <strong>de</strong> révision sur <strong><strong>de</strong>s</strong><br />
bases <strong>de</strong> croyances propositionnelles. On dit que l’opérateur ∗ correspond <strong>à</strong> l’opérateur<br />
◦ si quand K = Cn(ϕ), alors K ∗ α = Cn(ϕ ◦ α).<br />
Théorème 3 ([KM91b]) Soit un opérateur <strong>de</strong> révision ∗ et son opérateur ◦ correspondant.<br />
Alors ∗ satisfait les postulats (K ∗ 1) − (K ∗ 8) si et seulem<strong>en</strong>t si ◦ vérifie les<br />
postulats (R1) − (R6).<br />
I<strong>de</strong>ntités<br />
Comme le suggère la figure 4.1, il est possible <strong>de</strong> décomposer la révision, qui est<br />
une transition du statut accepté au statut refusé, <strong>en</strong> une contraction, qui effectuera la<br />
transition du statut accepté vers le statut indéterminé, suivie d’une expansion, qui effectuera<br />
la transition du statut indéterminé au statut refusé. C’est ce que nous dit l’i<strong>de</strong>ntité<br />
<strong>de</strong> Levi :<br />
• K ∗ α = (K ÷ ¬α) + α<br />
(I<strong>de</strong>ntité <strong>de</strong> Levi)<br />
Théorème 4 ([Gär88]) Si l’opérateur <strong>de</strong> contraction ÷ satisfait (K÷1)-(K÷4) et<br />
(K÷6) et l’opérateur d’expansion + satisfait (K+1)-(K+6), alors l’opérateur <strong>de</strong> révision<br />
∗ défini par l’i<strong>de</strong>ntité <strong>de</strong> Levi satisfait (K*1)-(K*6). De plus, si (K÷7) est satisfait,<br />
alors (K*7) est satisfait pour la révision ainsi définie, et si (K÷8) est satisfait, alors<br />
(K*8) est satisfait pour la révision ainsi définie.<br />
Remarque 1 Le postulat recovery (K÷5) n’est pas nécessaire pour ce résultat.<br />
On peut égalem<strong>en</strong>t définir la contraction <strong>à</strong> partir <strong>de</strong> l’opérateur <strong>de</strong> révision, grâce <strong>à</strong><br />
l’i<strong>de</strong>ntité <strong>de</strong> Harper :<br />
2. Plus exactem<strong>en</strong>t, n’importe quelle relation <strong>de</strong> conséqu<strong>en</strong>ce au s<strong>en</strong>s <strong>de</strong> Tarski.<br />
35
• K ÷ α = K ∩ (K ∗ ¬α)<br />
(I<strong>de</strong>ntité <strong>de</strong> Harper)<br />
Pour compr<strong>en</strong>dre l’idée <strong>de</strong> cette i<strong>de</strong>ntité, supposons que α soit pour le mom<strong>en</strong>t<br />
acceptée par K. Le résultat <strong>de</strong> la contraction <strong>de</strong> K par α est l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> formules<br />
qui sont vraies indép<strong>en</strong>damm<strong>en</strong>t <strong>de</strong> l’acceptation <strong>de</strong> α, c’est-<strong>à</strong>-dire les formules qui<br />
sont vraies <strong>à</strong> la fois lorsque α est acceptée (K) et lorsque α est refusée (K ∗ ¬α).<br />
Théorème 5 ([Gär88]) Si l’opérateur <strong>de</strong> révision ∗ satisfait (K*1)-(K*6), alors l’opérateur<br />
<strong>de</strong> contraction ÷ défini par l’i<strong>de</strong>ntité <strong>de</strong> Harper satisfait (K÷1)-(K÷6). De plus,<br />
si (K*7) est satisfait, alors (K÷7) est satisfait pour la contraction ainsi définie, et si<br />
(K*8) est satisfait, alors (K÷8) est satisfait pour la contraction ainsi définie.<br />
Ces <strong>de</strong>ux i<strong>de</strong>ntités montr<strong>en</strong>t un li<strong>en</strong> très étroit <strong>en</strong>tre opérateurs <strong>de</strong> révision et opérateurs<br />
<strong>de</strong> contraction, qui peuv<strong>en</strong>t tous <strong>de</strong>ux être définis <strong>à</strong> partir <strong>de</strong> l’autre. On n’a donc<br />
pas réellem<strong>en</strong>t <strong>à</strong> étudier les <strong>de</strong>ux types d’opérateurs, mais il suffit d’<strong>en</strong> étudier un,<br />
puis d’utiliser les i<strong>de</strong>ntités pour définir l’autre. Le choix <strong>de</strong> l’opérateur <strong>de</strong> base est<br />
principalem<strong>en</strong>t une question <strong>de</strong> goût. Habituellem<strong>en</strong>t les logici<strong>en</strong>s et les philosophes<br />
pr<strong>en</strong>n<strong>en</strong>t les opérateurs <strong>de</strong> contraction comme opérateurs <strong>de</strong> base, puisque l’i<strong>de</strong>ntité<br />
<strong>de</strong> Levi prés<strong>en</strong>te la révision comme une composition d’une contraction suivie d’une<br />
expansion. Et la plupart <strong><strong>de</strong>s</strong> chercheurs <strong>en</strong> intellig<strong>en</strong>ce artificielle choisiss<strong>en</strong>t plutôt les<br />
opérateurs <strong>de</strong> révision comme opérateurs <strong>de</strong> base, puisque c’est l’opération dont on a<br />
le plus besoin dans les systèmes <strong>à</strong> bases <strong>de</strong> connaissances.<br />
4.1.2. Théorèmes <strong>de</strong> représ<strong>en</strong>tation<br />
Maint<strong>en</strong>ant que nous avons défini ce que nous att<strong>en</strong>dions <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> changem<strong>en</strong>t<br />
<strong>de</strong> croyances, il est temps <strong>de</strong> donner <strong><strong>de</strong>s</strong> moy<strong>en</strong>s pratiques <strong>de</strong> définir ces opérateurs.<br />
C’est ce que vont nous fournir les théorèmes <strong>de</strong> représ<strong>en</strong>tation.<br />
Nous ne prés<strong>en</strong>tons dans la suite que trois théorèmes <strong>de</strong> représ<strong>en</strong>tation : celui utilisant<br />
les intersections partielles, celui utilisant les <strong>en</strong>racinem<strong>en</strong>ts épistémiques, et celui<br />
utilisant les assignem<strong>en</strong>ts fidèles. Il existe d’autres théorèmes <strong>de</strong> représ<strong>en</strong>tation (voir<br />
[AM85, FH96a] par exemple).<br />
Contraction par intersection partielle<br />
L’idée <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> contraction par intersection est <strong>de</strong> gar<strong>de</strong>r le maximum <strong>de</strong><br />
formules <strong>de</strong> l’anci<strong>en</strong>ne base. On va gar<strong>de</strong>r l’<strong>en</strong>semble <strong>de</strong> tous les sous-<strong>en</strong>sembles <strong>de</strong><br />
la base qui n’impliqu<strong>en</strong>t pas l’information que l’on veut retirer. Et l’infér<strong>en</strong>ce <strong>à</strong> partir<br />
<strong>de</strong> cet <strong>en</strong>semble sera l’infér<strong>en</strong>ce sceptique, c’est-<strong>à</strong>-dire que la nouvelle base <strong>de</strong> l’ag<strong>en</strong>t<br />
sera constituée <strong>de</strong> l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> formules que l’on pourra inférer <strong>de</strong> tous ces sous<strong>en</strong>sembles.<br />
Définition 9 Soi<strong>en</strong>t une théorie K et une proposition α. L’<strong>en</strong>semble <strong><strong>de</strong>s</strong> sous-théories<br />
maximales <strong>de</strong> K n’impliquant pas α, noté K⊥α, est l’<strong>en</strong>semble <strong>de</strong> tous les K ′ qui<br />
vérifi<strong>en</strong>t :<br />
• K ′ ⊆ K<br />
• K ′ α<br />
36
• Si K ′ ⊂ K ′′ ⊆ K alors K ′′ ⊢ α<br />
Définition 10 La fonction <strong>de</strong> contraction par intersection totale (full meet contraction)<br />
÷ f est définie comme<br />
{ ⋂ (K⊥α) si K⊥α n’est pas vi<strong>de</strong>, et<br />
K ÷ f α =<br />
K sinon<br />
Le problème avec cette définition est que le résultat est beaucoup trop fort, puisque<br />
si l’on effectue une contraction (par intersection totale) par α, le résultat est l’<strong>en</strong>semble<br />
<strong><strong>de</strong>s</strong> formules <strong>de</strong> K qui sont conséqu<strong>en</strong>ces logiques <strong>de</strong> ¬α. Cela a <strong>en</strong> particulier comme<br />
conséqu<strong>en</strong>ce que :<br />
Théorème 6 ([AM82]) Si une fonction <strong>de</strong> révision ∗ est définie <strong>à</strong> partir d’une fonction<br />
<strong>de</strong> contraction par intersection totale au moy<strong>en</strong> <strong>de</strong> l’i<strong>de</strong>ntité <strong>de</strong> Levi, alors pour chaque<br />
proposition α telle que ¬α ∈ K, on a K ∗ α = Cn(α).<br />
C’est-<strong>à</strong>-dire que l’on oublie toute information <strong>à</strong> propos <strong><strong>de</strong>s</strong> anci<strong>en</strong>nes croyances <strong>de</strong><br />
l’ag<strong>en</strong>t, ce qui n’est pas très souhaitable. Le problème est qu’<strong>en</strong> gardant l’<strong>en</strong>semble <strong>de</strong><br />
toutes les sous-théories maximales <strong>de</strong> K n’impliquant pas α, on a retiré trop d’information.<br />
L’idée est alors <strong>de</strong> n’<strong>en</strong> gar<strong>de</strong>r que certaines (les « meilleures », les « plus typiques<br />
», etc.).<br />
Définition 11 Soit une théorie K, une fonction <strong>de</strong> sélection γ est une fonction qui<br />
associe <strong>à</strong> chaque proposition α l’<strong>en</strong>semble γ(K⊥α), qui est un sous-<strong>en</strong>semble non<br />
vi<strong>de</strong> <strong>de</strong> K⊥α si celui-ci n’est pas vi<strong>de</strong> et γ(K⊥α) = {K} sinon.<br />
Définition 12 Une fonction <strong>de</strong> contraction par intersection partielle (partial meet<br />
contraction) ÷ est définie comme<br />
K ÷ α = ⋂ γ(K⊥α)<br />
Enonçons <strong>à</strong> prés<strong>en</strong>t le théorème <strong>de</strong> représ<strong>en</strong>tation, qui indique que tout opérateur<br />
<strong>de</strong> contraction par intersection partielle satisfait les propriétés logiques att<strong>en</strong>dues pour<br />
la contraction, et inversem<strong>en</strong>t que tout opérateur satisfaisant ces propriétés logiques<br />
peut être défini par un opérateur <strong>de</strong> contraction par intersection partielle.<br />
Théorème 7 ([AGM85]) Soit un opérateur ÷, ÷ est une fonction <strong>de</strong> contraction par<br />
intersection partielle si et seulem<strong>en</strong>t si ÷ satisfait les postulats (K÷1)-(K÷6).<br />
Il manque pour le mom<strong>en</strong>t <strong>de</strong>ux propriétés, que l’on va obt<strong>en</strong>ir <strong>en</strong> contraignant un<br />
peu la fonction <strong>de</strong> sélection.<br />
Définition 13 Une fonction <strong>de</strong> sélection γ est relationnelle si et seulem<strong>en</strong>t si pour tout<br />
K il existe une relation ≤ sur K × K telle que<br />
γ(K⊥α) = {K ′ ∈ K⊥α | K ′ ≤ K ′′ , ∀ K ′′ ∈ K⊥α}<br />
Si ≤ est une relation transitive alors γ est dite relationnelle transitive.<br />
37
Théorème 8 ([AGM85]) Soit un opérateur ÷, ÷ est une fonction <strong>de</strong> contraction par<br />
intersection partielle relationnelle transitive (transitively relational partial meet contraction<br />
function) si et seulem<strong>en</strong>t si ÷ satisfait les postulats (K÷1)-(K÷8).<br />
Avec ce résultat on voit <strong>en</strong> quoi les postulats supplém<strong>en</strong>taires (K÷7)-(K÷8) parl<strong>en</strong>t<br />
<strong>de</strong> minimalité du changem<strong>en</strong>t. En effet, l’ajout <strong>de</strong> ces postulats implique l’exist<strong>en</strong>ce<br />
d’une relation <strong>de</strong> plausibilité ≤ guidant le processus <strong>de</strong> sélection <strong><strong>de</strong>s</strong> sous-théories.<br />
Contraction par Enracinem<strong>en</strong>ts Epistémiques<br />
L’idée est ici d’ordonner les formules <strong>de</strong> la base <strong>de</strong> croyances <strong><strong>de</strong>s</strong> plus importantes<br />
(fiables/crédibles) au moins importantes. Et lorsque l’on doit réaliser une contraction,<br />
pr<strong>en</strong>dre cet ordre <strong>en</strong> compte pour n’éliminer que les formules les moins importantes.<br />
Soi<strong>en</strong>t <strong>de</strong>ux formules α et β, la notation α ≤ β signifie "β est au moins aussi <strong>en</strong>racinée<br />
(plausible / importante) que α". ≤ est un <strong>en</strong>racinem<strong>en</strong>t épistémique (epistemic<br />
<strong>en</strong>tr<strong>en</strong>chm<strong>en</strong>t) s’il satisfait les propriétés suivantes [Gär88] :<br />
(EE1) Si α ≤ β et β ≤ γ, alors α ≤ γ<br />
(EE2) Si α ⊢ β, alors α ≤ β<br />
(EE3) α ≤ α ∧ β ou β ≤ α ∧ β<br />
(EE4) Si K ≠ K ⊥ , α /∈ K ssi ∀β α ≤ β<br />
(EE5) Si β ≤ α ∀β, alors ⊢ α<br />
(transitivité)<br />
(domination)<br />
(conjonction)<br />
(minimalité)<br />
(maximalité)<br />
Théorème 9 ([Gär88]) Une fonction <strong>de</strong> contraction ÷ satisfait (K ÷ 1) − (K ÷ 8) si<br />
et seulem<strong>en</strong>t si il existe ≤ satisfaisant (EE1)-(EE5), où β ≤ α ssi β /∈ K ÷ α ∧ β ou<br />
⊢ α ∧ β.<br />
Intuitivem<strong>en</strong>t ce théorème dit que si β n’est pas conséqu<strong>en</strong>ce du résultat <strong>de</strong> la<br />
contraction par la conjonction α ∧ β c’est qu’il n’était pas plus <strong>en</strong>raciné que α.<br />
Révision par assignem<strong>en</strong>ts fidèles - Systèmes <strong>de</strong> sphères<br />
L’idée est d’ordonner les interprétations <strong>de</strong> la plus plausible <strong>à</strong> la moins plausible.<br />
Donnons d’abord la définition générale <strong>en</strong> terme <strong>de</strong> systèmes <strong>de</strong> sphères donnée par<br />
Grove [Gro88].<br />
Définition 14 • On appelle mon<strong>de</strong> possible un sous-<strong>en</strong>semble maximal cohér<strong>en</strong>t<br />
du langage et on note M L l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> mon<strong><strong>de</strong>s</strong> possibles du langage L.<br />
• Soit une base <strong>de</strong> croyances K. Si K = K ⊥ alors [K] = ∅, sinon [K] = {M ∈<br />
M L | K ⊆ M}.<br />
• Soit un <strong>en</strong>semble S ∈ M L , l’<strong>en</strong>semble K S = ⋂ {M | M ∈ S}.<br />
Définition 15 Un système <strong>de</strong> sphères c<strong>en</strong>tré sur [K] [Gro88] est une collection <strong>de</strong><br />
sous-<strong>en</strong>sembles S <strong>de</strong> M L qui vérifi<strong>en</strong>t les conditions suivantes :<br />
(S1) Si S, S ′ ∈ S, alors S ⊆ S ′ ou S ′ ⊆ S<br />
(S2) [K] ∈ S<br />
38
(S3) Si S ∈ S, alors [K] ⊆ S<br />
(S4) M L ∈ S<br />
(S5) Si α est une formule et si [α] intersecte une sphère <strong>de</strong> S, alors il existe une<br />
sphère minimale qui intersecte [α] (on note C(α) = [α] ∩ S α ).<br />
Théorème 10 ([Gro88]) Soit une base <strong>de</strong> croyances K. Il existe un système <strong>de</strong> sphères<br />
S c<strong>en</strong>tré sur [K] tel que pour toute formule α, K ∗ α = K C(α) si et seulem<strong>en</strong>t si ∗ est<br />
un opérateur <strong>de</strong> révision satisfaisant (K ∗ 1) − (K ∗ 8).<br />
[α]<br />
. .. M L<br />
S 3<br />
S 2<br />
C(α) S 1<br />
[K]<br />
FIGURE 4.2 – Révision par α d’un système <strong>de</strong> sphères c<strong>en</strong>tré sur [K]<br />
Graphiquem<strong>en</strong>t on peut représ<strong>en</strong>ter cela comme sur la figure 4.2 : les mon<strong><strong>de</strong>s</strong> possibles<br />
qui satisfont la base ([K]), sont les mon<strong><strong>de</strong>s</strong> les plus plausibles. Puis on ordonne<br />
tous les autres mon<strong><strong>de</strong>s</strong> suivant leur plausibilité (cela donne les sphères S 1 , S 2 , . . .).<br />
Et lorsque l’on révise par une nouvelle information α, le résultat est l’<strong>en</strong>semble <strong><strong>de</strong>s</strong><br />
mon<strong><strong>de</strong>s</strong> possibles <strong>de</strong> cette nouvelle information qui sont les plus plausibles vis <strong>à</strong> vis <strong>de</strong><br />
K (i.e. du système <strong>de</strong> sphères c<strong>en</strong>tré sur [K]).<br />
Lorsque l’on travaille <strong>en</strong> logique propositionnelle finie ces systèmes <strong>de</strong> sphères<br />
peuv<strong>en</strong>t être représ<strong>en</strong>tés par <strong><strong>de</strong>s</strong> pré-ordres totaux, ce qui conduit aux assignem<strong>en</strong>ts<br />
fidèles <strong>de</strong> Katsuno et M<strong>en</strong><strong>de</strong>lzon :<br />
Définition 16 Un assignem<strong>en</strong>t fidèle (faithful assignm<strong>en</strong>t) [KM91b] est une fonction<br />
qui associe <strong>à</strong> chaque base <strong>de</strong> croyances K un pré-ordre ≤ K sur les interprétations tel<br />
que :<br />
• Si ω |= K et ω ′ |= K, alors ω ≃ K ω ′<br />
• Si ω |= K et ω ′ ̸|= K, alors ω < K ω ′<br />
• Si K 1 ≡ K 2 , alors ≤ K1 =≤ K2<br />
Théorème 11 ([KM91b]) Un opérateur <strong>de</strong> révision ◦ satisfait les postulats (R1)-(R6)<br />
si et seulem<strong>en</strong>t si il existe un assignem<strong>en</strong>t fidèle qui associe <strong>à</strong> chaque base <strong>de</strong> croyances<br />
K un pré-ordre total ≤ K tel que<br />
mod(K ◦ µ) = min(mod(µ), ≤ K )<br />
39
On a donc le même mécanisme, mais le système <strong>de</strong> sphères est plus simplem<strong>en</strong>t<br />
représ<strong>en</strong>té par un pré-ordre total. Même si la représ<strong>en</strong>tation sous forme <strong>de</strong> systèmes <strong>de</strong><br />
sphères est souv<strong>en</strong>t utilisée pour décrire le comportem<strong>en</strong>t <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> révision<br />
[Rot09], nous trouvons plus pratique d’utiliser la représ<strong>en</strong>tation sous forme <strong>de</strong> préordres<br />
totaux décrite ci-<strong><strong>de</strong>s</strong>sous. Non seulem<strong>en</strong>t cela évite <strong><strong>de</strong>s</strong> erreurs d’interprétation<br />
car la représ<strong>en</strong>tation sous forme <strong>de</strong> système <strong>de</strong> sphères, qui est planaire, peut faire<br />
croire <strong>à</strong> une spatialité qui n’existe pas. Mais surtout cela est beaucoup plus intuitif pour<br />
décrire le comportem<strong>en</strong>t <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> révision itérée. Nous utiliserons donc la<br />
représ<strong>en</strong>tation <strong>de</strong> la figure 4.3.<br />
L 3<br />
α<br />
L 2<br />
L 1<br />
L 0<br />
≤ K<br />
FIGURE 4.3 – Révision <strong>de</strong> K par α<br />
Les interprétations sont réparties sur différ<strong>en</strong>ts niveaux, <strong>de</strong>ux interprétations <strong>à</strong> un<br />
même niveau sont aussi plausibles l’une que l’autre (i.e. ω ≃ K ω ′ ) et une interprétation<br />
ω apparaissant <strong>à</strong> un niveau inférieur <strong>à</strong> une autre ω ′ est strictem<strong>en</strong>t plus plausible 3 (i.e.<br />
ω < K ω ′ ). Bi<strong>en</strong> <strong>en</strong>t<strong>en</strong>du les interprétations apparaissant au niveau le plus bas sont les<br />
modèles <strong>de</strong> la base <strong>de</strong> croyances K.<br />
La traduction <strong>en</strong>tre systèmes <strong>de</strong> sphère et le pré-ordre associé <strong>à</strong> K par l’assignem<strong>en</strong>t<br />
fidèle est directe. Chaque niveau correspond <strong>à</strong> une sphère (L 0 <strong>à</strong> [K], et chaque L i <strong>à</strong><br />
S i − S i−1 ).<br />
Lorsque l’on révise par une nouvelle information α, le résultat est constitué <strong>de</strong><br />
l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> modèles <strong>de</strong> α les plus plausibles suivant le pré-ordre <strong>de</strong> plausibilité ≤ K<br />
associé <strong>à</strong> K par l’assignem<strong>en</strong>t fidèle, soi<strong>en</strong>t les interprétations situées sur le segm<strong>en</strong>t<br />
blanc sur la figure 4.3.<br />
4.1.3. Li<strong>en</strong>s avec les logiques non monotones<br />
La théorie <strong>de</strong> la révision AGM a <strong><strong>de</strong>s</strong> li<strong>en</strong>s forts avec les logiques non monotones,<br />
telles que définies par Makinson [Mak94], et Kraus, Lehmann, Magidor [KLM90,<br />
LM92], ce qui a fait dire <strong>à</strong> Gär<strong>de</strong>nfors [Gär90] : « Belief revision and nonmonotonic<br />
logic are two si<strong><strong>de</strong>s</strong> of the same coin. »<br />
3. Contrairem<strong>en</strong>t <strong>à</strong> la plupart <strong><strong>de</strong>s</strong> domaines où les meilleures options sont les plus gran<strong><strong>de</strong>s</strong>, <strong>en</strong> révision<br />
<strong>de</strong> croyances les meilleures options sont souv<strong>en</strong>t les plus petites (et donc on minimise au lieu <strong>de</strong> maximiser).<br />
Cela est peut-être une conséqu<strong>en</strong>ce <strong>de</strong> l’obsession d’obt<strong>en</strong>ir un changem<strong>en</strong>t minimal.<br />
40
Théorème 12 ([MG89, FL94]) • Soi<strong>en</strong>t un opérateur <strong>de</strong> révision ∗ et une base<br />
<strong>de</strong> croyances K. Si on définit la relation |∼ K comme suit :<br />
α |∼ K ∗ β ssi β ∈ K ∗ α<br />
Alors |∼ ∗<br />
K<br />
est une relation rationnelle qui satisfait la règle suivante, nommée<br />
préservation <strong>de</strong> la cohér<strong>en</strong>ce :<br />
Si α |∼ K ∗ ⊥, alors α ⊢ ⊥<br />
• Soit |∼ une relation rationnelle qui préserve la cohér<strong>en</strong>ce, il existe une base K<br />
et un opérateur ∗ tels que |∼ = |∼ K ∗ .<br />
On peut égalem<strong>en</strong>t montrer un li<strong>en</strong> étroit <strong>en</strong>tre la logique possibiliste et les <strong>en</strong>racinem<strong>en</strong>ts<br />
épistémiques, et donc la contraction et la révision : soit une relation ≥ c sur les<br />
formules, α ≥ c β signifie « α est au moins aussi certain que β ». ≥ c est une relation<br />
<strong>de</strong> nécessité qualitative si elle vérifie les propriétés suivantes [Dub86, DP91] :<br />
(D1) α ≥ c α<br />
(D2) α ≥ c β ou β ≥ c α<br />
(D3) Si α ≥ c β et β ≥ c γ, alors α ≥ c γ<br />
(D4) ⊤ > c ⊥<br />
(D5) ⊤ ≥ c α<br />
(D6) Si α ≥ c β, alors α ∧ γ ≥ c β ∧ γ<br />
(réflexivité)<br />
(totalité)<br />
(transitivité)<br />
(non trivialité)<br />
(certitu<strong>de</strong> <strong>de</strong> la tautologie)<br />
(stabilité conjonctive)<br />
Cette relation <strong>de</strong> nécessité qualitative <strong>en</strong>tre formules est <strong>à</strong> la base <strong>de</strong> la logique<br />
possibiliste.<br />
Théorème 13 ([DP91]) L’<strong>en</strong>semble d’axiomes (D1), (D2), (D3), (D5) et (D6) est équival<strong>en</strong>t<br />
<strong>à</strong> (EE1)-(EE4).<br />
Comme on peut le voir avec ce théorème, la différ<strong>en</strong>ce majeure <strong>en</strong>tre les <strong>en</strong>racinem<strong>en</strong>ts<br />
épistémiques et les relations <strong>de</strong> nécessité qualitative (<strong>à</strong> la base <strong>de</strong> la logique<br />
possibiliste), est que les relations <strong>de</strong> nécessité autoris<strong>en</strong>t l’exist<strong>en</strong>ce <strong>de</strong> formules aussi<br />
prioritaires que les tautologies, et qui ne pourront donc pas être remises <strong>en</strong> question<br />
par <strong><strong>de</strong>s</strong> révisions, ce qui n’est pas possible dans le cas <strong><strong>de</strong>s</strong> <strong>en</strong>racinem<strong>en</strong>ts épistémiques<br />
(comparer (D5) et (EE5)). On peut donc voir ces formules comme <strong><strong>de</strong>s</strong> contraintes<br />
d’intégrité, ou <strong><strong>de</strong>s</strong> connaissances (par oppositions aux croyances « usuelles » qui, elles,<br />
peuv<strong>en</strong>t être remises <strong>en</strong> doute).<br />
4.2. Révision itérée<br />
Il est assez facile <strong>de</strong> montrer que la caractérisation AGM n’est pas suffisante pour<br />
modéliser l’itération du processus <strong>de</strong> révision. Des propriétés supplém<strong>en</strong>taires sont nécessaires<br />
afin <strong>de</strong> contraindre le comportem<strong>en</strong>t <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> révision itérée. Il est<br />
égalem<strong>en</strong>t nécessaire <strong>de</strong> passer <strong>à</strong> une représ<strong>en</strong>tation <strong><strong>de</strong>s</strong> croyances plus riche qu’une<br />
41
simple base <strong>de</strong> croyances. On utilise donc ce que l’on appelle <strong><strong>de</strong>s</strong> états épistémiques<br />
Ψ. A chaque état épistémique Ψ correspond une base, notée Bel(Ψ), représ<strong>en</strong>tant les<br />
croyances actuelles <strong>de</strong> l’ag<strong>en</strong>t. Ψ <strong>en</strong>co<strong>de</strong> égalem<strong>en</strong>t d’autres informations sur la plausibilité<br />
<strong><strong>de</strong>s</strong> formules que l’ag<strong>en</strong>t ne croit pas actuellem<strong>en</strong>t. Pour simplifier les notations<br />
dans la suite, lorsqu’un état épistémique sera utilisé dans une formule logique, il s’agira<br />
<strong>de</strong> la base <strong>de</strong> croyances associée, ainsi on écrira par exemple Ψ ⊢ µ, Ψ ∧ µ, ω |= Ψ au<br />
lieu respectivem<strong>en</strong>t <strong>de</strong> Bel(Ψ) ⊢ µ, Bel(Ψ) ∧ µ, ω |= Bel(Ψ)<br />
On peut alors facilem<strong>en</strong>t réécrire les postulats <strong>de</strong> Katsuno et M<strong>en</strong><strong>de</strong>lzon dans le<br />
cadre <strong><strong>de</strong>s</strong> états épistémiques [DP97] :<br />
(R*1) Ψ ◦ µ ⊢ µ<br />
(R*2) Si Ψ ∧ µ est cohér<strong>en</strong>t, alors Ψ ◦ µ ≡ Ψ ∧ µ<br />
(R*3) Si µ est cohér<strong>en</strong>t, alors Ψ ◦ µ est cohér<strong>en</strong>t<br />
(R*4) Si Ψ 1 =Ψ 2 et µ 1 ≡µ 2 , alors Ψ 1 ◦ µ 1 ≡ Ψ 2 ◦ µ 2<br />
(R*5) (Ψ ◦ µ) ∧ ϕ ⊢ Ψ ◦ (µ ∧ ϕ)<br />
(R*6) Si (Ψ ◦ µ) ∧ ϕ est cohér<strong>en</strong>t, alors Ψ ◦ (µ ∧ ϕ) ⊢ (Ψ ◦ µ) ∧ ϕ<br />
On peut alors ajouter d’autres postulats afin <strong>de</strong> contraindre le comportem<strong>en</strong>t <strong><strong>de</strong>s</strong><br />
opérateurs lors d’itérations (remarquons que ces postulats, contrairem<strong>en</strong>t aux précé<strong>de</strong>nts,<br />
font apparaître <strong>de</strong>ux opérations <strong>de</strong> révision successives).<br />
(C1) Si α ⊢ µ, alors (Ψ ◦ µ) ◦ α ≡ Ψ ◦ α<br />
(C2) Si α ⊢ ¬µ, alors (Ψ ◦ µ) ◦ α ≡ Ψ ◦ α<br />
(C3) Si Ψ ◦ α ⊢ µ, alors (Ψ ◦ µ) ◦ α ⊢ µ<br />
(C4) Si Ψ ◦ α ¬µ, alors (Ψ ◦ µ) ◦ α ¬µ<br />
L’explication <strong><strong>de</strong>s</strong> postulats est la suivante : (C1) dit que si <strong>de</strong>ux informations sont<br />
incorporées successivem<strong>en</strong>t et si la <strong>de</strong>uxième implique la première, alors incorporer<br />
seulem<strong>en</strong>t la secon<strong>de</strong> donne la même base. (C2) dit que lorsque <strong>de</strong>ux informations<br />
contradictoires arriv<strong>en</strong>t, la secon<strong>de</strong> seule donnerait le même base. (C3) dit qu’une information<br />
doit être gardée si l’on effectue une révision par une information qui, étant<br />
donnée la base, implique la première. (C4) dit qu’aucune information ne peut contribuer<br />
<strong>à</strong> son propre rejet.<br />
Dans [DP94] les postulats (C1)-(C4) ont d’abord été donnés comme complém<strong>en</strong>t<br />
aux postulats usuels (R1)-(R6) [KM91b]. Freund et Lehmann [FL94] ont montré que<br />
(C2) est incompatible avec les postulats AGM. De plus Lehmann [Leh95] a montré<br />
que les postulats (C1) et (R1)-(R6) impliqu<strong>en</strong>t (C3) et (C4). Dans [DP97], Darwiche<br />
et Pearl ont reformulé leurs postulats (et les postulats AGM) <strong>en</strong> termes d’états épistémiques<br />
((R*1)-(R*6)) et ont <strong>de</strong> ce fait <strong>en</strong>levé cette contradiction et ces redondances.<br />
On peut alors montrer les théorèmes <strong>de</strong> représ<strong>en</strong>tation suivants 4 , dont le premier<br />
est une simple généralisation du théorème <strong>de</strong> Katsuno et M<strong>en</strong><strong>de</strong>lzon :<br />
4. Les assignem<strong>en</strong>ts fidèles sont généralisés pour les états épistémiques, ils <strong>de</strong>vi<strong>en</strong>n<strong>en</strong>t donc <strong><strong>de</strong>s</strong> fonctions<br />
<strong>de</strong> l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> états épistémiques vers les pré-ordres, avec : 1. Si ω |= Ψ et ω ′ |= Ψ, alors ω ≃ Ψ ω ′<br />
2. Si ω |= Ψ et ω ′ ̸|= Ψ, alors ω < Ψ ω ′ 3. Si Ψ 1 = Ψ 2 , alors ≤ Ψ1 =≤ Ψ2<br />
42
Théorème 14 ([DP97]) Un opérateur <strong>de</strong> révision ◦ satisfait les postulats (R*1)-(R*6)<br />
si et seulem<strong>en</strong>t si il existe un assignem<strong>en</strong>t fidèle qui associe <strong>à</strong> tout état épistémique Ψ<br />
un pré-ordre total sur les interprétations ≤ Ψ tel que :<br />
mod(Ψ ◦ µ) = min(mod(µ), ≤ Ψ )<br />
Le second est plus intéressant car il ajoute les contraintes concernant l’itération :<br />
Théorème 15 ([DP97]) Soit un opérateur <strong>de</strong> révision qui vérifie (R*1)-(R*6). L’opérateur<br />
vérifie (C1)-(C4) si et seulem<strong>en</strong>t si l’opérateur et l’assignem<strong>en</strong>t fidèle correspondant<br />
vérifi<strong>en</strong>t :<br />
(CR1) Si ω |= µ et ω ′ |= µ, alors ω ≤ Ψ ω ′ ssi ω ≤ Ψ◦µ ω ′<br />
(CR2) Si ω |= ¬µ et ω ′ |= ¬µ, alors ω ≤ Ψ ω ′ ssi ω ≤ Ψ◦µ ω ′<br />
(CR3) Si ω |= µ et ω ′ |= ¬µ, alors ω < Ψ ω ′ seulem<strong>en</strong>t si ω < Ψ◦µ ω ′<br />
(CR4) Si ω |= µ et ω ′ |= ¬µ, alors ω ≤ Ψ ω ′ seulem<strong>en</strong>t si ω ≤ Ψ◦µ ω ′<br />
Ce théorème <strong>de</strong> représ<strong>en</strong>tation est important car il signifie que l’on peut considérer<br />
les opérateurs <strong>de</strong> révision itérée comme <strong><strong>de</strong>s</strong> fonctions <strong>de</strong> transition <strong>en</strong>tre pré-ordres<br />
totaux (avec les contraintes données par (CR1-CR4)), et donc que l’on peut considérer<br />
les pré-ordres totaux comme la représ<strong>en</strong>tation canonique <strong><strong>de</strong>s</strong> états épistémiques,<br />
puisque le théorème <strong>de</strong> représ<strong>en</strong>tation exprime le fait que quelle que soit la représ<strong>en</strong>tation<br />
exacte <strong><strong>de</strong>s</strong> états épistémiques, il est possible <strong>de</strong> modéliser leur comportem<strong>en</strong>t au<br />
travers d’un assignem<strong>en</strong>t fidèle 5 .<br />
Boutilier propose [Bou93, Bou96] un opérateur <strong>de</strong> révision naturelle. Cet opérateur<br />
est un cas particulier d’opérateur <strong>de</strong> Darwiche et Pearl. Cet opérateur est le seul<br />
<strong>à</strong> satisfaire les propriétés (R*1)-(R*6) et la propriété <strong>de</strong> minimisation absolue (CB)<br />
suivante :<br />
(CB) Si Ψ ◦ ψ ⊢ ¬µ , alors (Ψ ◦ ψ) ◦ µ ≡ Ψ ◦ µ<br />
Il peut être considéré comme accomplissant un changem<strong>en</strong>t minimal dans le préordre<br />
associé aux états épistémiques. Malheureusem<strong>en</strong>t cette minimalisation du changem<strong>en</strong>t<br />
se paye par un mauvais comportem<strong>en</strong>t <strong>de</strong> l’opérateur, puisqu’il induit une<br />
conditionnalisation <strong><strong>de</strong>s</strong> informations successives, comme illustré sur l’exemple suivant<br />
[DP97] :<br />
Exemple 5 Je r<strong>en</strong>contre un étrange animal qui semble être un oiseau, je crois donc<br />
que cet animal est un oiseau. Cet animal se rapproche et je vois que cet animal est<br />
rouge, je p<strong>en</strong>se donc que cet animal est rouge. Un expert <strong>en</strong> animaux étranges passe<br />
par l<strong>à</strong> et m’appr<strong>en</strong>d qu’<strong>en</strong> fait cet animal n’est pas un oiseau mais un mammifère. Je<br />
révise donc mes croyances et p<strong>en</strong>se <strong>à</strong> prés<strong>en</strong>t que cet animal est un mammifère. Que<br />
dois-je croire <strong>à</strong> propos <strong>de</strong> la couleur <strong>de</strong> l’animal ?<br />
D’après l’opérateur <strong>de</strong> révision naturelle, on ne peut plus croire que l’animal est<br />
rouge (il suffit <strong>de</strong> pr<strong>en</strong>dre Bel(Ψ) = oiseau, µ = ¬oiseau et ψ = rouge). Bi<strong>en</strong> que<br />
la couleur et l’espèce <strong>de</strong> l’animal ne soi<strong>en</strong>t a priori absolum<strong>en</strong>t pas liées, l’ordre dans<br />
5. Voir [BLP05] pour une généralisation <strong>de</strong> ce cadre aux pré-ordres partiels.<br />
43
lequel on appr<strong>en</strong>d ces informations les « conditionne » <strong>en</strong> quelque sorte pour l’opérateur<br />
<strong>de</strong> révision naturelle, car la croyance « l’animal est rouge » dép<strong>en</strong>d <strong>de</strong> la croyance<br />
« l’animal est un oiseau », puisque remettre <strong>en</strong> cause cette <strong>de</strong>rnière invali<strong>de</strong> la première.<br />
Alors que si l’on avait appris la couleur <strong>de</strong> l’animal avant d’avoir les informations sur<br />
son espèce, on aurait maint<strong>en</strong>u cette information sur sa couleur.<br />
Il est un peu problématique que la caractérisation <strong>de</strong> Darwiche et Pearl autorise un<br />
tel opérateur. Il a donc été proposé <strong>de</strong> définir <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> révisions admissibles<br />
[BM06], afin <strong>de</strong> l’éliminer. Ces opérateurs sont définis par un postulat d’itération supplém<strong>en</strong>taire<br />
[BM06, JT07] :<br />
(P) Si Ψ ◦ α ⊬ ¬µ alors Ψ ◦ µ ◦ α ⊢ µ<br />
Définition 17 Un opérateur <strong>de</strong> révision est admissible [BM06] si il satisfait (R*1-<br />
R*6) 6 , (C1), (C2) et (P). 7<br />
Et le théorème <strong>de</strong> représ<strong>en</strong>tation correspondant est :<br />
Théorème 16 ([BM06, JT07]) Soit un opérateur <strong>de</strong> révision ◦ qui vérifie (R*1)-(R*6).<br />
L’opérateur ◦ vérifie (P) si et seulem<strong>en</strong>t si l’opérateur et l’assignem<strong>en</strong>t fidèle correspondant<br />
vérifi<strong>en</strong>t (CP) :<br />
(CP) Si ω |= µ et ω ′ |= ¬µ, alors ω ≤ Ψ ω ′ seulem<strong>en</strong>t si ω < Ψ◦µ ω ′<br />
Beaucoup d’opérateurs particuliers ont été proposés. On peut se référer <strong>à</strong> [Rot09]<br />
[13] pour une prés<strong>en</strong>tation <strong><strong>de</strong>s</strong> principales approches.<br />
4.3. Itération dans le cadre AGM<br />
Nous avons étudié quelles propriétés d’itération peuv<strong>en</strong>t être exprimées dans le<br />
cadre AGM usuel [25]. En effet, le saut représ<strong>en</strong>tationnel <strong><strong>de</strong>s</strong> bases <strong>de</strong> croyances vers<br />
les états épistémiques a été provoqué par les preuves d’incohér<strong>en</strong>ces <strong><strong>de</strong>s</strong> postulats <strong>de</strong><br />
Darwiche et Pearl avec le cadre AGM usuel. Mais il n’existait pas <strong>de</strong> travail examinant<br />
quelles sont les propriétés d’itération possibles dans le cadre AGM. Ces travaux<br />
constitu<strong>en</strong>t donc une justification a posteriori du saut représ<strong>en</strong>tationnel effectué par la<br />
communauté pour étudier la révision itérée.<br />
Nous avons <strong>en</strong> particulier mis <strong>en</strong> évi<strong>de</strong>nce les implications <strong><strong>de</strong>s</strong> axiomes <strong>de</strong> Darwiche<br />
et Pearl dans le cadre AGM usuel. Nous avons égalem<strong>en</strong>t proposé <strong><strong>de</strong>s</strong> propriétés<br />
très faibles pour l’itération et exploré leur conséqu<strong>en</strong>ces. Il se trouve que les résultats<br />
obt<strong>en</strong>us sont négatifs, donnant <strong><strong>de</strong>s</strong> résultats d’impossibilité <strong>de</strong> l’itération dans le cadre<br />
AGM usuel.<br />
Nous ne détaillons pas plus cette approche ici, puisque l’article correspondant est<br />
inclus dans la <strong>de</strong>uxième partie du docum<strong>en</strong>t (chapitre 12).<br />
6. Dans [BM06] les auteurs r<strong>en</strong>forc<strong>en</strong>t un peu le postulat (R*4).<br />
7. On peut noter que (C3) et (C4) sont conséqu<strong>en</strong>ces <strong>de</strong> ces postulats.<br />
44
4.4. Révision itérée et états épistémiques<br />
Toutes les propositions pour la révision itérée utilis<strong>en</strong>t donc plus ou moins formellem<strong>en</strong>t<br />
une notion d’état épistémique, c’est-<strong>à</strong>-dire qu’elles travaill<strong>en</strong>t sur <strong><strong>de</strong>s</strong> objets<br />
plus complexes qu’une simple base <strong>de</strong> croyances. Nous avons montré [1, 13] que ces<br />
propositions utilis<strong>en</strong>t le même type d’états épistémiques sous différ<strong>en</strong>tes sémantiques.<br />
Mais, même dans la proposition <strong>de</strong> Darwiche et Pearl [DP97], qui semble la plus aboutie,<br />
la distinction <strong>en</strong>tre la syntaxe et la sémantique <strong>de</strong> ces états épistémiques n’est pas<br />
nette, ce qui conduit <strong>à</strong> <strong><strong>de</strong>s</strong> confusions. Nous avons proposé une syntaxe pour les états<br />
épistémiques, définissant ceux-ci comme les états librem<strong>en</strong>t <strong>en</strong>g<strong>en</strong>drés <strong>à</strong> partir d’un état<br />
initial vierge et d’un constructeur. C’est-<strong>à</strong>-dire que, syntaxiquem<strong>en</strong>t, on peut considérer<br />
qu’un état épistémique est l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> informations reçues par l’ag<strong>en</strong>t (cette idée<br />
était déj<strong>à</strong> prés<strong>en</strong>te dans [Leh95], puisque ses postulats sont exprimés sur <strong><strong>de</strong>s</strong> suites <strong>de</strong><br />
formules). Nous avons <strong>en</strong>suite défini une notion <strong>de</strong> modèle sémantique associée <strong>à</strong> cette<br />
syntaxe et nous avons montré que cela permet <strong>de</strong> capturer le cadre AGM classique<br />
mais égalem<strong>en</strong>t les propositions pour la révision itérée [1, 13]. Nous avons égalem<strong>en</strong>t<br />
proposé <strong>de</strong> nouveaux opérateurs <strong>de</strong> révision itérée <strong>à</strong> mémoire qui se définiss<strong>en</strong>t par la<br />
suite d’informations reçues [21], puis <strong><strong>de</strong>s</strong> opérateurs dynamiques où la notion d’état<br />
épistémique est généralisée [44]. Nous avons <strong>en</strong>fin étudié comm<strong>en</strong>t définir <strong><strong>de</strong>s</strong> opérateurs<br />
<strong>de</strong> révision itérée où l’on ne révise plus par une simple formule, mais par un état<br />
épistémique [19].<br />
4.5. Opérateurs d’amélioration<br />
Les opérateurs <strong>de</strong> révision AGM [AGM85, Gär88] usuels satisfont le principe <strong>de</strong><br />
primauté <strong>de</strong> la nouvelle information 8 . C’est-<strong>à</strong>-dire que la nouvelle information est toujours<br />
acceptée (crue par l’ag<strong>en</strong>t) après la révision. Cela est une <strong><strong>de</strong>s</strong> hypothèses <strong>de</strong> base<br />
<strong>de</strong> la révision, parfaitem<strong>en</strong>t justifiée dans beaucoup <strong>de</strong> cas. Mais on pourrait souhaiter<br />
que l’impact <strong>de</strong> cette nouvelle information soit moins important sur les croyances <strong>de</strong><br />
l’ag<strong>en</strong>t. Il y a très peu <strong>de</strong> travaux dans cette optique, souv<strong>en</strong>t appelée révision non<br />
prioritaire (voir le numéro spécial <strong>de</strong> Theoria pour un bon aperçu [Han98]), et la<br />
plupart du temps cela revi<strong>en</strong>t <strong>à</strong> mettre la nouvelle information au même niveau que<br />
les croyances actuelles <strong>de</strong> l’ag<strong>en</strong>t, m<strong>en</strong>ant <strong>à</strong> <strong><strong>de</strong>s</strong> problèmes <strong>de</strong> fusion d’informations.<br />
Nous avons défini [35, 58, 73] une nouvelle famille d’opérateurs <strong>de</strong> changem<strong>en</strong>t, que<br />
nous avons appelés opérateurs d’amélioration (improvem<strong>en</strong>t), qui ont un comportem<strong>en</strong>t<br />
plus mesuré que les opérateurs <strong>de</strong> révision. Lorsque l’on incorpore une nouvelle<br />
information, cela améliore la crédibilité <strong>de</strong> cette information, mais l’ag<strong>en</strong>t ne l’accepte<br />
pas forcém<strong>en</strong>t. Il faudra itérer l’incorporation un certain nombre <strong>de</strong> fois pour assurer<br />
l’acceptation. Ce type d’opérateur <strong>de</strong> changem<strong>en</strong>t est parfaitem<strong>en</strong>t raisonnable. Les<br />
opérateurs <strong>de</strong> révision itérée usuels [DP97, BM06, JT07] sont un cas particulier <strong>de</strong><br />
notre cadre. Notre caractérisation est donc la plus générale pour modéliser le changem<strong>en</strong>t<br />
itéré. Techniquem<strong>en</strong>t la difficulté <strong>de</strong> la définition <strong>de</strong> ces opérateurs rési<strong>de</strong> dans le<br />
fait que leur caractérisation nécessite d’utiliser la définition <strong>de</strong> l’opérateur <strong>de</strong> révision<br />
8. Cela est formalisé par le postulat (R*1).<br />
45
associé (obt<strong>en</strong>u avec un nombre suffisant d’itérations i<strong>de</strong>ntiques). Il nous semble que<br />
c’est <strong>à</strong> cause <strong>de</strong> cette difficulté que ces opérateurs très naturels n’ont pas été caractérisés<br />
jusque l<strong>à</strong>.<br />
Nous ne détaillons pas plus cette approche ici, puisque l’article correspondant est<br />
inclus dans la <strong>de</strong>uxième partie du docum<strong>en</strong>t (chapitre 11).<br />
46
5.1 Caractérisation <strong>de</strong> la fusion<br />
5.2 Familles d’opérateurs <strong>de</strong> fusion<br />
5.3 Fusion <strong>de</strong> croyances versus fusion <strong>de</strong> buts<br />
5.4 Manipulation<br />
5.5 Fusion dans d’autres cadres<br />
Chapitre V<br />
FUSION<br />
Ce qui est contraire est utile et c’est <strong>de</strong> ce qui est <strong>en</strong> lutte que<br />
naît la plus belle harmonie ; tout se fait par discor<strong>de</strong>.<br />
(Héraclite)<br />
La majeure partie <strong>de</strong> nos travaux concerne la fusion <strong>de</strong> bases <strong>de</strong> croyances. Cette<br />
section sera donc un peu plus longue que les autres. Nous abor<strong>de</strong>rons tout d’abord<br />
la caractérisation logique <strong>de</strong> la fusion, puis les différ<strong>en</strong>tes familles d’opérateurs<br />
qui ont été proposées. Nous discutons <strong>en</strong>suite le problème <strong>de</strong> la manipulation <strong>de</strong><br />
la fusion. Enfin nous prés<strong>en</strong>tons <strong><strong>de</strong>s</strong> travaux <strong>de</strong> fusion d’informations plus structurées<br />
qu’<strong>en</strong> logique classique.<br />
5.1. Caractérisation <strong>de</strong> la fusion<br />
Nous avons proposé [17, 18, 2] une caractérisation logique <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion,<br />
qui est une généralisation du cadre AGM pour la révision [AGM85, Gär88] et qui pr<strong>en</strong>d<br />
<strong>en</strong> compte l’aspect agrégation <strong><strong>de</strong>s</strong> croyances.<br />
Nous avons défini <strong>de</strong>ux sous-classes d’opérateurs, les opérateurs majoritaires et les<br />
opérateurs d’arbitrage. Nous avons donné un théorème <strong>de</strong> représ<strong>en</strong>tation <strong>en</strong> termes <strong>de</strong><br />
pré-ordres sur les interprétations. Outre le fait que ce théorème fournit une sémantique<br />
<strong>à</strong> ces opérateurs et <strong>en</strong> donne une définition plus constructive, il permet égalem<strong>en</strong>t d’étudier<br />
plus finem<strong>en</strong>t [6] les rapports <strong>en</strong>tre la fusion, la théorie <strong>de</strong> la décision [Sav54] et<br />
la théorie du choix social [Arr63].<br />
Définition 18 △ : E × L ↦→ K est un opérateur <strong>de</strong> fusion contrainte si et seulem<strong>en</strong>t<br />
si il satisfait les propriétés suivantes :<br />
(IC0) △ µ (E) ⊢ µ<br />
(IC1) Si µ est cohér<strong>en</strong>t, alors △ µ (E) est cohér<strong>en</strong>t<br />
47
(IC2) Si E est cohér<strong>en</strong>t avec µ, alors △ µ (E) = ∧ E ∧ µ<br />
(IC3) Si E 1 ≡ E 2 et µ 1 ≡ µ 2 , alors △ µ1 (E 1 ) ≡ △ µ2 (E 2 )<br />
(IC4) Si K ⊢ µ et K ′ ⊢ µ, alors △ µ (K ⊔ K ′ ) ∧ K ⊥ ⇒ △ µ (K ⊔ K ′ ) ∧ K ′ ⊥<br />
(IC5) △ µ (E 1 ) ∧ △ µ (E 2 ) ⊢ △ µ (E 1 ⊔ E 2 )<br />
(IC6) Si △ µ (E 1 ) ∧ △ µ (E 2 ) est cohér<strong>en</strong>t, alors △ µ (E 1 ⊔ E 2 ) ⊢ △ µ (E 1 ) ∧ △ µ (E 2 )<br />
(IC7) △ µ1 (E) ∧ µ 2 ⊢ △ µ1∧µ 2<br />
(E)<br />
(IC8) Si △ µ1 (E) ∧ µ 2 est cohér<strong>en</strong>t, alors △ µ1∧µ 2<br />
(E) ⊢ △ µ1 (E) ∧ µ 2<br />
Une partie <strong>de</strong> ces propriétés avait déj<strong>à</strong> été proposée par Revesz [Rev97] pour caractériser<br />
ce qu’il appelle les opérateurs d’adéquation sémantique (mo<strong>de</strong>l fitting).<br />
La signification intuitive <strong>de</strong> ces propriétés est la suivante : (IC0) assure que le résultat<br />
<strong>de</strong> la fusion satisfait les contraintes d’intégrité. (IC1) dit que si les contraintes d’intégrité<br />
sont cohér<strong>en</strong>tes alors le résultat <strong>de</strong> la fusion est cohér<strong>en</strong>t, c’est-<strong>à</strong>-dire que l’on peut<br />
toujours extraire <strong><strong>de</strong>s</strong> croyances cohér<strong>en</strong>tes du groupe d’ag<strong>en</strong>ts. (IC2) <strong>de</strong>man<strong>de</strong> que,<br />
lorsque c’est possible, le résultat <strong>de</strong> la fusion soit simplem<strong>en</strong>t la conjonction <strong><strong>de</strong>s</strong> bases<br />
<strong>de</strong> croyances et <strong><strong>de</strong>s</strong> contraintes d’intégrité. Donc, lorsqu’il n’y a pas <strong>de</strong> conflit <strong>en</strong>tre<br />
les ag<strong>en</strong>ts et les contraintes, la fusion est simplem<strong>en</strong>t l’union <strong><strong>de</strong>s</strong> différ<strong>en</strong>tes croyances.<br />
(IC3) est le principe d’indép<strong>en</strong>dance <strong>de</strong> syntaxe, c’est-<strong>à</strong>-dire que le résultat <strong>de</strong> la fusion<br />
ne dép<strong>en</strong>d pas <strong>de</strong> la forme syntaxique <strong><strong>de</strong>s</strong> croyances mais simplem<strong>en</strong>t <strong><strong>de</strong>s</strong> opinions exprimées.<br />
(IC4) est la propriété d’équité. Elle assure que lorsque l’on fusionne l’opinion<br />
<strong>de</strong> <strong>de</strong>ux ag<strong>en</strong>ts, l’opérateur ne peut pas donner <strong>de</strong> préfér<strong>en</strong>ce <strong>à</strong> l’un d’eux. (IC5) exprime<br />
l’idée suivante : si un groupe E 1 se met d’accord sur un <strong>en</strong>semble d’alternatives<br />
qui conti<strong>en</strong>t l’alternative A, et si un autre groupe E 2 se met d’accord sur un autre <strong>en</strong>semble<br />
d’alternatives qui conti<strong>en</strong>t égalem<strong>en</strong>t A, alors si l’on joint les <strong>de</strong>ux groupes A<br />
fera <strong>en</strong>core partie <strong><strong>de</strong>s</strong> alternatives acceptables. Et (IC5) et (IC6) <strong>en</strong>semble, exprim<strong>en</strong>t le<br />
fait que, dès que l’on peut trouver <strong>de</strong>ux sous-groupes qui s’accor<strong>de</strong>nt sur au moins une<br />
alternative, alors le résultat <strong>de</strong> la fusion sera exactem<strong>en</strong>t l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> alternatives sur<br />
lesquelles ces <strong>de</strong>ux groupes s’accor<strong>de</strong>nt. (IC7) et (IC8) sont une généralisation directe<br />
<strong><strong>de</strong>s</strong> postulats (R5) et (R6) <strong>de</strong> la révision <strong>de</strong> croyances (voir section 4.1.1). Ils exprim<strong>en</strong>t<br />
<strong><strong>de</strong>s</strong> conditions sur les conjonctions <strong>de</strong> contraintes d’intégrité et s’assur<strong>en</strong>t <strong>de</strong> ce fait que<br />
la notion <strong>de</strong> proximité est bi<strong>en</strong> fondée. C’est-<strong>à</strong>-dire, par exemple, que si une alternative<br />
A est préférée parmi un <strong>en</strong>semble d’alternatives possibles et si on restreint le nombre<br />
d’alternatives possibles tout <strong>en</strong> gardant l’alternative A, celle-ci sera toujours préférée<br />
parmi les alternatives restantes. Cette propriété est assez usuelle dans les différ<strong>en</strong>tes<br />
théories du choix (décision, choix social, etc.).<br />
Nous allons <strong>à</strong> prés<strong>en</strong>t définir <strong>de</strong>ux sous classes d’opérateurs <strong>de</strong> fusion, les opérateurs<br />
<strong>de</strong> fusion majoritaires et les opérateurs d’arbitrage.<br />
Un opérateur <strong>de</strong> fusion majoritaire est un opérateur <strong>de</strong> fusion contrainte qui satisfait<br />
la propriété suivante :<br />
(Maj) ∃n △ µ (E 1 ⊔ E n 2 ) ⊢ △ µ (E 2 )<br />
Ce postulat exprime le fait que si une opinion a une large audi<strong>en</strong>ce, ce sera alors<br />
l’opinion du groupe. On peut remarquer que cette propriété est trés générale. Elle ne dit<br />
pas exactem<strong>en</strong>t le nombre <strong>de</strong> répétitions nécessaires d’un profil pour s’imposer (cela<br />
48
dép<strong>en</strong>d <strong>de</strong> l’opérateur), mais elle impose l’exist<strong>en</strong>ce d’un tel seuil. Les opérateurs <strong>de</strong><br />
fusion majoritaire t<strong>en</strong>t<strong>en</strong>t donc <strong>de</strong> satisfaire au mieux le groupe dans son <strong>en</strong>semble.<br />
D’un autre côté, les opérateurs d’arbitrage t<strong>en</strong>t<strong>en</strong>t <strong>de</strong> satisfaire chacun <strong><strong>de</strong>s</strong> élém<strong>en</strong>ts<br />
du groupe pris individuellem<strong>en</strong>t du mieux possible. Un opérateur d’arbitrage est un<br />
opérateur <strong>de</strong> fusion contrainte qui satisfait la propriété suivante :<br />
(Arb)<br />
△ µ1 (K 1 ) ≡ △ µ2 (K 2 )<br />
△ µ1↔¬µ 2<br />
({K 1 , K 2 }) ≡ (µ 1 ↔ ¬µ 2 )<br />
µ 1 µ 2<br />
µ 2 µ 1<br />
⎫⎪ ⎬<br />
⎪ ⎭<br />
⇒ △ µ1∨µ 2<br />
({K 1 , K 2 }) ≡ △ µ1 (K 1 )<br />
Ce postulat dit que si un <strong>en</strong>semble d’alternatives préférées sous un <strong>en</strong>semble <strong>de</strong><br />
contraintes d’intégrité µ 1 pour une base <strong>de</strong> croyances K 1 correspond <strong>à</strong> l’<strong>en</strong>semble <strong><strong>de</strong>s</strong><br />
alternatives préférées par la base K 2 sous les contraintes µ 2 , et si les alternatives qui<br />
n’apparti<strong>en</strong>n<strong>en</strong>t qu’<strong>à</strong> un <strong><strong>de</strong>s</strong> <strong>de</strong>ux <strong>en</strong>sembles <strong>de</strong> contraintes d’intégrité sont toutes aussi<br />
crédibles pour le groupe ({K 1 , K 2 }), alors les alternatives préférées pour le groupe<br />
parmi la disjonction <strong><strong>de</strong>s</strong> <strong>de</strong>ux <strong>en</strong>sembles <strong>de</strong> contraintes seront celles préférées par<br />
chacune <strong><strong>de</strong>s</strong> bases sous leurs contraintes respectives. Ce postulat est bi<strong>en</strong> plus intuitif<br />
lorsqu’il est exprimé sous la forme d’assignem<strong>en</strong>t syncrétique (voir condition 8). Il<br />
exprime le fait que ce sont les alternatives médianes qui sont favorisées.<br />
A prés<strong>en</strong>t que nous disposons d’une définition logique <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion<br />
contrainte, nous allons donner un théorème <strong>de</strong> représ<strong>en</strong>tation qui permet <strong>de</strong> définir<br />
ces opérateurs <strong>de</strong> manière bi<strong>en</strong> plus intuitive. Ce théorème montre qu’un opérateur <strong>de</strong><br />
fusion contrainte correspond <strong>à</strong> une famille <strong>de</strong> pré-ordres sur les interprétations.<br />
Définition 19 Un assignem<strong>en</strong>t syncrétique est une fonction qui associe <strong>à</strong> chaque profil<br />
E un pré-ordre ≤ E sur les interprétations telle que pour tous profils E, E 1 , E 2 et<br />
pour toutes bases K, K ′ les conditions suivantes sont satisfaites :<br />
1. Si ω |= ∧ E et ω ′ |= ∧ E, alors ω ≃ E ω ′<br />
2. Si ω |= ∧ E et ω ′ ̸|= ∧ E, alors ω < E ω ′<br />
3. Si E 1 ≡ E 2 , alors ≤ E1 =≤ E2<br />
4. ∀ω |= K ∃ω ′ |= K ′ ω ′ ≤ K⊔K ′ ω<br />
5. Si ω ≤ E1 ω ′ et ω ≤ E2 ω ′ , alors ω ≤ E1⊔E 2<br />
ω ′<br />
6. Si ω < E1 ω ′ et ω ≤ E2 ω ′ , alors ω < E1⊔E 2<br />
ω ′<br />
Un assignem<strong>en</strong>t syncrétique majoritaire est un assignem<strong>en</strong>t syncrétique qui satisfait<br />
la condition suivante :<br />
7. Si ω < E2 ω ′ , alors ∃n ω < E1⊔E n 2 ω′<br />
Un assignem<strong>en</strong>t syncrétique juste est un assignem<strong>en</strong>t syncrétique qui satisfait la<br />
condition suivante :<br />
8.<br />
ω < K1 ω ′<br />
ω < K2 ω ′′<br />
ω ′ ≃ K1⊔K 2<br />
ω ′′ ⎫<br />
⎬<br />
⎭ ⇒ ω < K 1⊔K 2<br />
ω ′<br />
49
La condition 1 dit que <strong>de</strong>ux modèles du profil sont équival<strong>en</strong>ts pour le pré-ordre<br />
associé et la condition 2 dit qu’un modèle du profil est toujours préféré <strong>à</strong> un contremodèle.<br />
La condition 3 dit que si <strong>de</strong>ux profils sont équival<strong>en</strong>ts alors les <strong>de</strong>ux pré-ordres<br />
associés sont équival<strong>en</strong>ts. Ces trois conditions sont une généralisation <strong><strong>de</strong>s</strong> conditions<br />
<strong>de</strong> l’assignem<strong>en</strong>t fidèle pour les opérateurs <strong>de</strong> révision [KM91b]. La condition 4 dit<br />
que pour le pré-ordre associé <strong>à</strong> un profil composé <strong>de</strong> <strong>de</strong>ux bases <strong>de</strong> croyances, pour<br />
chaque modèle <strong>de</strong> l’une, il existe un modèle <strong>de</strong> l’autre qui est au moins aussi bon. La<br />
condition 5 dit que si une interprétation est au moins aussi bonne qu’une autre pour<br />
un profil E 1 , et que cette interprétation est égalem<strong>en</strong>t au moins aussi bonne pour un<br />
profil E 2 , alors elle sera au moins aussi bonne que l’autre pour la réunion <strong><strong>de</strong>s</strong> <strong>de</strong>ux<br />
profils. La condition 6 r<strong>en</strong>force un peu ce résultat <strong>en</strong> exigeant que si une interprétation<br />
est strictem<strong>en</strong>t meilleure qu’une autre pour un profil E 1 , et que cette interprétation est<br />
au moins aussi bonne pour un profil E 2 , alors cette interprétation doit être strictem<strong>en</strong>t<br />
meilleure que l’autre pour la réunion <strong><strong>de</strong>s</strong> <strong>de</strong>ux profils. La condition 7 dit que si l’on<br />
répète un groupe E 2 suffisamm<strong>en</strong>t <strong>de</strong> fois alors les préfér<strong>en</strong>ces strictes <strong>de</strong> ce groupe<br />
seront respectées. La condition 8 dit que ce sont les choix médians qui sont préférés<br />
pour le groupe. Ce comportem<strong>en</strong>t est illustré figure 5.1 (les interprétations les plus<br />
basses sont les interprétations préférées, par exemple pour K 1 : ω ′′ < K1 ω < K1 ω ′ ).<br />
L’interprétation ω, qui n’est jamais aussi mauvaise que ω ′ et ω ′′ est préférée <strong>à</strong> celles-ci<br />
pour le résultat <strong>de</strong> la fusion.<br />
ω ′<br />
ω<br />
ω ′′<br />
(a) K 1<br />
ω ′<br />
ω<br />
(b) K 2<br />
ω ′′<br />
ω ′ ω ′′<br />
ω<br />
(c) K 1 ⊔ K 2<br />
FIGURE 5.1 – Arbitrage<br />
Nous pouvons <strong>à</strong> prés<strong>en</strong>t énoncer le théorème <strong>de</strong> représ<strong>en</strong>tation pour les opérateurs<br />
<strong>de</strong> fusion contrainte :<br />
Théorème 17 ([18, 77]) Un opérateur △ est un opérateur <strong>de</strong> fusion contrainte (respectivem<strong>en</strong>t<br />
un opérateur majoritaire ou un opérateur d’arbitrage) si et seulem<strong>en</strong>t si il<br />
existe un assignem<strong>en</strong>t syncrétique (respectivem<strong>en</strong>t un assignem<strong>en</strong>t syncrétique majoritaire<br />
ou un assignem<strong>en</strong>t syncrétique juste) qui associe <strong>à</strong> chaque profil E un pré-ordre<br />
total ≤ E tel que<br />
mod(△ µ (E)) = min(mod(µ), ≤ E )<br />
Voir [CP06] pour une généralisation <strong>de</strong> ce théorème dans le cas infini.<br />
5.2. Familles d’opérateurs <strong>de</strong> fusion<br />
Nous allons prés<strong>en</strong>ter dans cette section un panorama <strong><strong>de</strong>s</strong> principales familles d’opérateurs<br />
<strong>de</strong> fusion <strong>de</strong> la litterature.<br />
50
5.2.1. Opérateurs <strong>à</strong> base <strong>de</strong> modèles<br />
Nous avons donné une définition générale <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion <strong>à</strong> base <strong>de</strong> modèles,<br />
paramétrés par une distance et une fonction d’agrégation. Les opérateurs étudiés<br />
par [Rev97, LM99] sont <strong><strong>de</strong>s</strong> cas particuliers utilisant la distance <strong>de</strong> Hamming et les<br />
fonctions d’agrégation Σ ou max. Nous avons montré que les propriétés <strong>de</strong> ces opérateurs<br />
étai<strong>en</strong>t les mêmes quelle que soit la distance utilisée. Nous avons égalem<strong>en</strong>t proposé<br />
l’utilisation <strong>de</strong> la fonction d’agrégation GMAX (leximax) [17, 2] et les fonctions<br />
utilisant la somme <strong><strong>de</strong>s</strong> puissances, qui permett<strong>en</strong>t <strong>de</strong> choisir le <strong>de</strong>gré <strong>de</strong> cons<strong>en</strong>sualité<br />
<strong>de</strong> l’opérateur [6, 23].<br />
Nous allons rapi<strong>de</strong>m<strong>en</strong>t rappeler la définition <strong>de</strong> ces opérateurs.<br />
Définition 20 Une (pseudo-)distance 1 <strong>en</strong>tre interprétations est une fonction d : W×<br />
W ↦→ R + telle que pour tout ω, ω ′ ∈ W :<br />
• d(ω, ω ′ ) = d(ω ′ , ω), et<br />
• d(ω, ω ′ ) = 0 si et seulem<strong>en</strong>t si ω = ω ′ .<br />
Définition 21 Une fonction d’agrégation f est une fonction qui associe un réel positif<br />
<strong>à</strong> tout n-uplet ple fini <strong>de</strong> réels positifs tel que pour tout x 1 , . . . , x n , x, y ∈ IR + :<br />
• si x ≤ y, alors f(x 1 , . . . , x, . . . , x n ) ≤ f(x 1 , . . . , y, . . . , x n ) (monotonie)<br />
• f(x 1 , . . . , x n ) = 0 si et seulem<strong>en</strong>t si x 1 = . . . = x n = 0 (minimalité)<br />
• f(x) = x<br />
(i<strong>de</strong>ntité)<br />
Définition 22 Soit une distance <strong>en</strong>tre interprétations d et une fonction d’agrégation f.<br />
L’opérateur <strong>de</strong> fusion <strong>à</strong> base <strong>de</strong> modèles △ d,f est défini par :<br />
mod(△ d,f<br />
µ (E)) = min(mod(µ), ≤ E )<br />
où le pré-ordre ≤ E sur W est défini par :<br />
• ω ≤ E ω ′ si et seulem<strong>en</strong>t si d(ω, E) ≤ d(ω ′ , E), où<br />
• d(ω, E) = f(d(ω, K 1 ) . . . , d(ω, K n )), où E = {K 1 , . . . , K n }.<br />
Si la fonction d’agrégation f a <strong>de</strong> bonnes propriétés, comme les fonctions usuelles<br />
(comme le maximum, la somme, le leximax, la somme <strong><strong>de</strong>s</strong> puissances n iemes , le leximin),<br />
les opérateurs <strong>à</strong> base <strong>de</strong> modèles générés (quelle que soit la distance) sont <strong><strong>de</strong>s</strong><br />
opérateurs <strong>de</strong> fusion contrainte (ils satisfont toutes les propriétés (IC0-IC8)).<br />
Plus exactem<strong>en</strong>t on a les résultats suivants [5] :<br />
Théorème 18 Soit une distance <strong>en</strong>tre interprétations d et une fonction d’agrégation<br />
f, l’opérateur △ d,f satisfait les propriétés (IC0), (IC1), (IC2), (IC7) et (IC8).<br />
Théorème 19 Soit une distance <strong>en</strong>tre interprétations d et une fonction d’agrégation<br />
f, l’opérateur △ d,f satisfait les propriétés (IC0-IC8) si et seulem<strong>en</strong>t si la fonction<br />
d’agrégation f satisfait les propriétés suivantes :<br />
• Pour toute permutation σ, f(x 1 , . . . , x n ) = f(σ(x 1 , . . . , x n )) (symétrie)<br />
• Si f(x 1 , . . . , x n ) ≤ f(y 1 , . . . , y n ), alors f(x 1 , . . . , x n , z) ≤ f(y 1 , . . . , y n , z).<br />
(composition)<br />
• Si f(x 1 , . . . , x n , z) ≤ f(y 1 , . . . , y n , z), alors f(x 1 , . . . , x n ) ≤ f(y 1 , . . . , y n ).<br />
(décomposition)<br />
1. L’inégalité triangulaire n’est pas nécessaire.<br />
51
5.2.2. Opérateurs <strong>à</strong> base <strong>de</strong> formules<br />
Les opérateurs <strong>à</strong> base <strong>de</strong> formules sont <strong><strong>de</strong>s</strong> opérateurs syntaxiques, c’est-<strong>à</strong>-dire que<br />
le résultat dép<strong>en</strong>d <strong>de</strong> la prés<strong>en</strong>tation syntaxique <strong><strong>de</strong>s</strong> bases <strong>de</strong> croyances interv<strong>en</strong>ant<br />
dans la fusion. Lorsque les bases <strong>de</strong> croyances sont <strong><strong>de</strong>s</strong> <strong>en</strong>sembles <strong>de</strong> formules, les<br />
opérateurs <strong>de</strong> fusion <strong>à</strong> base <strong>de</strong> formules usuels sélectionn<strong>en</strong>t, dans l’union <strong><strong>de</strong>s</strong> bases,<br />
<strong><strong>de</strong>s</strong> sous-<strong>en</strong>sembles maximaux cohér<strong>en</strong>ts <strong>de</strong> formules [BKMS92, BDL + 98]. Ces métho<strong><strong>de</strong>s</strong><br />
ont l’inconvéni<strong>en</strong>t <strong>de</strong> perdre l’origine <strong><strong>de</strong>s</strong> informations et ne permett<strong>en</strong>t donc<br />
pas <strong>de</strong> t<strong>en</strong>ir compte <strong>de</strong> la distribution <strong>de</strong> l’information pour la fusion. Ceci est très<br />
gênant pour t<strong>en</strong>ir compte <strong>de</strong> la majorité par exemple.<br />
Nous avons donc proposé [20], après avoir étudié les propriétés logiques <strong>de</strong> ces<br />
opérateurs, l’utilisation <strong>de</strong> fonctions <strong>de</strong> sélection, inspirées <strong><strong>de</strong>s</strong> transitively relational<br />
partial meet contractions functions (voir section 4.1.2) dans le cadre <strong>de</strong> la révision,<br />
pour t<strong>en</strong>ir compte <strong>de</strong> cette distribution. Nous avons montré que cela permet d’obt<strong>en</strong>ir<br />
<strong><strong>de</strong>s</strong> opérateurs avec <strong>de</strong> meilleures propriétés logiques et donc un meilleur comportem<strong>en</strong>t.<br />
Ces résultats illustr<strong>en</strong>t l’utilité <strong>de</strong> la caractérisation logique <strong><strong>de</strong>s</strong> opérateurs. Cette<br />
caractérisation permet <strong>de</strong> classer les différ<strong>en</strong>ts opérateurs existants selon les propriétés<br />
satisfaites ou non, et <strong>de</strong> proposer <strong><strong>de</strong>s</strong> améliorations pour certains <strong>de</strong> ces opérateurs.<br />
La définition formelle <strong>de</strong> ces opérateurs est la suivante :<br />
Définition 23 Soit MAXCONS(K, µ) l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> maxcons <strong>de</strong> K∪{µ} qui conti<strong>en</strong>n<strong>en</strong>t<br />
µ, i.e., les sous-<strong>en</strong>sembles maximaux (pour l’inclusion <strong>en</strong>sembliste) <strong>de</strong> K ∪ {µ}<br />
qui conti<strong>en</strong>n<strong>en</strong>t µ. Formellem<strong>en</strong>t, MAXCONS(K, µ) est l’<strong>en</strong>semble <strong>de</strong> tous les M tels<br />
que :<br />
• M ⊆ K ∪ {µ},<br />
• µ ∈ M,<br />
• si M ⊂ M ′ ⊆ K ∪ {µ}, alors M ′ ⊢ ⊥.<br />
Soit MAXCONS(E, µ) = MAXCONS( ⋃ K i∈E K i, µ). Lorsque la maximalité <strong><strong>de</strong>s</strong> <strong>en</strong>sembles<br />
est défini <strong>en</strong> termes <strong>de</strong> cardinalité on utilisera l’indice « card », i.e. on notera<br />
l’<strong>en</strong>semble MAXCONS card (E, µ).<br />
On peut alors définir les opérateurs <strong>de</strong> fusion <strong>à</strong> base <strong>de</strong> formules suivants :<br />
Définition 24 Soit un profil E et une formule µ :<br />
△ C1<br />
µ (E) = ∨ MAXCONS(E, µ).<br />
△ C3<br />
△ C4<br />
△ C5<br />
µ (E) = ∨ {M : M ∈ MAXCONS(E, ⊤) et M ∪ {µ} cohér<strong>en</strong>t}.<br />
µ (E) = ∨ MAXCONS card (E, µ).<br />
µ (E) = ∨ {M ∪ {µ} : M ∈ MAXCONS(E, ⊤) et M ∪ {µ} cohér<strong>en</strong>t}<br />
si cet <strong>en</strong>semble est non vi<strong>de</strong> et µ sinon.<br />
Les opérateurs △ C1<br />
µ (E), △ C3<br />
µ (E) et △ C4<br />
µ (E) correspon<strong>de</strong>nt respectivem<strong>en</strong>t aux<br />
opérateurs Comb1(E, µ), Comb3(E, µ) et Comb4(E, µ) définis dans [BKMS92].<br />
L’opérateur △ C5 est une légère modification <strong>de</strong> △ C3 afin d’obt<strong>en</strong>ir <strong>de</strong> meilleures propriétés<br />
logiques [20].<br />
Ces opérateurs effectu<strong>en</strong>t une union <strong><strong>de</strong>s</strong> bases, puis t<strong>en</strong>t<strong>en</strong>t d’extraire une information<br />
cohér<strong>en</strong>te <strong>à</strong> partir <strong>de</strong> cette union incohér<strong>en</strong>te. C’est-<strong>à</strong>-dire qu’ils utilis<strong>en</strong>t les<br />
approches d’infér<strong>en</strong>ce <strong>à</strong> partir <strong>de</strong> base incohér<strong>en</strong>te décrites <strong>à</strong> la section 3.1.1. Du point<br />
52
<strong>de</strong> vue <strong>de</strong> la fusion c’est assez insatisfaisant car on n’utilise pas les informations sur la<br />
localisation <strong><strong>de</strong>s</strong> informations parmi les différ<strong>en</strong>tes bases, c’est-<strong>à</strong>-dire justem<strong>en</strong>t ce qui<br />
différ<strong>en</strong>cie la fusion <strong>de</strong> l’infér<strong>en</strong>ce <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce.<br />
Nous avons appelé ces opérateurs opérateurs <strong>de</strong> combinaison 2 , pour les différ<strong>en</strong>cier<br />
<strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion.<br />
Nous avons égalem<strong>en</strong>t proposé [20] <strong>de</strong> ne sélectionner que les maxcons qui satisfont<br />
au mieux un critère <strong>de</strong> fusion. Ces fonctions <strong>de</strong> sélections sont inspirées <strong>de</strong> celles<br />
utilisées dans le cadre <strong>de</strong> la révision pour la définition <strong><strong>de</strong>s</strong> partial meet contraction<br />
functions (voir section 4.1.2). Dans les <strong>de</strong>ux cas, ces fonctions <strong>de</strong> sélection ont pour<br />
but <strong>de</strong> ne sélectionner que les « meilleurs » maxcons. L’idée dans ce cadre <strong>de</strong> fusion est<br />
que ces fonctions apport<strong>en</strong>t une évaluation sociale (i.e. t<strong>en</strong>ant compte <strong>de</strong> la distribution<br />
<strong>de</strong> l’information parmi les bases).<br />
Dans [20] nous avons étudié trois critères particuliers. Le premier (△ d ) sélectionne<br />
les maxcons qui sont cohér<strong>en</strong>ts avec le plus <strong>de</strong> bases possibles. Le <strong>de</strong>uxième (△ S,Σ )<br />
sélectionne les maxcons qui ont la plus petite différ<strong>en</strong>ce (symétrique) pour la cardinalité<br />
avec les bases. Le troisième (△ ∩,Σ ) sélectionne les maxcons qui ont la plus gran<strong>de</strong><br />
intersection (pour la cardinalité) avec les bases.<br />
En terme <strong>de</strong> propriétés logiques on obti<strong>en</strong>t les résultats suivants pour les opérateurs<br />
<strong>de</strong> combinaison ( √ signifie que le postulat est vérifié) :<br />
IC0 IC1 IC2 IC3 IC4 IC5 IC6 IC7 IC8 MI Maj<br />
△ C1 √ √ √ −<br />
√ √<br />
−<br />
√<br />
−<br />
√<br />
−<br />
△ C3 − − − −<br />
√ √<br />
−<br />
√ √ √<br />
−<br />
△ C4 √ √ √ − − − −<br />
√ √ √<br />
−<br />
△ C5 √ √ √ −<br />
√ √<br />
−<br />
√ √ √<br />
−<br />
TABLE 5.1 – Propriétés <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> combinaison<br />
Si l’on utilise les fonctions <strong>de</strong> sélection, on obti<strong>en</strong>t <strong>de</strong> meilleures propriétés qu’avec<br />
les opérateurs <strong>de</strong> combinaison :<br />
IC0 IC1 IC2 IC3 IC4 IC5 IC6 IC7 IC8 MI Maj<br />
△ C1 √ √ √ −<br />
√ √<br />
−<br />
√<br />
−<br />
√<br />
−<br />
△ d √ √ √ −<br />
√ √<br />
−<br />
√<br />
− −<br />
√<br />
△ S,Σ √ √ √ −<br />
√<br />
− −<br />
√ √<br />
−<br />
√<br />
△ ∩,Σ √ √ √ − −<br />
√ √ √ √<br />
−<br />
√<br />
TABLE 5.2 – Propriétés <strong><strong>de</strong>s</strong> opérateurs <strong>à</strong> base <strong>de</strong> fonction <strong>de</strong> sélection basés sur △ C1<br />
On peut constater qu’aucun opérateur <strong>à</strong> base <strong>de</strong> formules ne parvi<strong>en</strong>t <strong>à</strong> satisfaire<br />
l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> propriétés logiques <strong>de</strong> la fusion contrainte. C’est pour cette raison que<br />
nous avons proposé une alternative décrite <strong>à</strong> la section 5.2.4.<br />
2. Terme employé dans les articles [BKM91, BKMS92].<br />
53
Nous p<strong>en</strong>sons qu’il pourrait être intéressant <strong>de</strong> continuer cette piste pour étudier<br />
plus systématiquem<strong>en</strong>t les autres opérateurs et autres fonctions <strong>de</strong> sélection. Une perspective<br />
plus générale serait d’obt<strong>en</strong>ir un théorème <strong>de</strong> représ<strong>en</strong>tation avec ces fonctions<br />
<strong>de</strong> sélection, qui serait une généralisation <strong>de</strong> celui <strong><strong>de</strong>s</strong> partial meet contraction functions<br />
pour la révision.<br />
5.2.3. Opérateurs DA 2<br />
Nous avons défini [5, 24, 63] une nouvelle famille d’opérateurs <strong>de</strong> fusion, paramétrée<br />
par une distance et <strong>de</strong>ux fonctions d’agrégation, appelés opérateurs <strong>de</strong> fusion<br />
DA 2 (Pour 1 Distance et 2 fonctions d’Agrégation). Ces opérateurs sont une généralisation<br />
<strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion <strong>à</strong> base <strong>de</strong> modèles usuels, mais permett<strong>en</strong>t égalem<strong>en</strong>t<br />
<strong>de</strong> capturer certains opérateurs <strong>de</strong> fusion <strong>à</strong> base <strong>de</strong> formules. L’inconvéni<strong>en</strong>t principal<br />
<strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion <strong>à</strong> base <strong>de</strong> modèles usuels est qu’ils ne permett<strong>en</strong>t pas <strong>de</strong><br />
pr<strong>en</strong>dre <strong>en</strong> compte les bases incohér<strong>en</strong>tes. Or, dans certains cas, il peut être nécessaire<br />
ou simplem<strong>en</strong>t utile d’utiliser ces informations. D’un autre côté, les opérateurs<br />
<strong>à</strong> base <strong>de</strong> formules permett<strong>en</strong>t <strong>de</strong> pr<strong>en</strong>dre <strong>en</strong> compte les bases incohér<strong>en</strong>tes, mais ne<br />
ti<strong>en</strong>n<strong>en</strong>t pas compte <strong>de</strong> la distribution <strong><strong>de</strong>s</strong> informations. Les opérateurs <strong>de</strong> fusion DA 2<br />
permett<strong>en</strong>t d’éviter ces <strong>de</strong>ux écueils. Ils sont définis similairem<strong>en</strong>t aux opérateurs <strong>à</strong><br />
base <strong>de</strong> modèles :<br />
Définition 25 Soit une distance <strong>en</strong>tre interprétations d et <strong>de</strong>ux fonctions d’agrégation<br />
f et g. L’opérateur <strong>de</strong> fusion DA 2 △ d,f,g est défini par :<br />
mod(△ d,f,g<br />
µ (E)) = min(mod(µ), ≤ E )<br />
où le pré-ordre ≤ E sur W est défini par :<br />
• ω ≤ E ω ′ si et seulem<strong>en</strong>t si d(ω, E) ≤ d(ω ′ , E), où<br />
• d(ω, E) = f(d(ω, K 1 ) . . . , d(ω, K n )), où E = {K 1 , . . . , K n }.<br />
• d(ω, K i ) = g(d(ω, α 1 ) . . . , d(ω, α mi )), où K i = {α 1 , . . . , α mi }.<br />
La première fonction d’agrégation g permet d’extraire une information cohér<strong>en</strong>te<br />
<strong>de</strong> la base K i même si celle-ci est incohér<strong>en</strong>te. Puis la secon<strong>de</strong> fonction f réalise<br />
l’agrégation <strong>en</strong>tre sources.<br />
Nous avons étudié ces opérateurs d’un point <strong>de</strong> vue logique et du point <strong>de</strong> vue <strong>de</strong><br />
la complexité algorithmique, <strong>à</strong> la fois pour <strong><strong>de</strong>s</strong> familles générales d’opérateurs mais<br />
égalem<strong>en</strong>t pour <strong><strong>de</strong>s</strong> opérateurs particuliers. Ces résultats <strong>de</strong> complexité permett<strong>en</strong>t<br />
d’intéressantes conclusions, ils montr<strong>en</strong>t <strong>en</strong> particulier que d’un point <strong>de</strong> vue calculatoire,<br />
ces opérateurs <strong>de</strong> fusion <strong>à</strong> partir <strong>de</strong> distances ne sont pas plus durs que les<br />
opérateurs <strong>à</strong> base <strong>de</strong> modèles usuels puisque l’on reste au même niveau <strong>de</strong> la hiérarchie<br />
polynomiale (la généralisation ne coûte donc ri<strong>en</strong> au niveau <strong>de</strong> la complexité), et<br />
ne sont pas plus durs que les opérateurs <strong>de</strong> révision.<br />
Nous ne détaillons pas plus cette approche ici, puisque l’article correspondant est<br />
inclus dans la <strong>de</strong>uxième partie du docum<strong>en</strong>t (chapitre 13).<br />
54
5.2.4. Opérateurs disjonctifs<br />
Nous avons défini une nouvelle [30, 49, 65] famille d’opérateurs <strong>de</strong> fusion, les opérateurs<br />
<strong>de</strong> fusion <strong>à</strong> quota. Ces opérateurs sont proches <strong>de</strong> métho<strong><strong>de</strong>s</strong> <strong>de</strong> vote, appelées<br />
votes par comités [BSZ91]. L’idée est qu’une interprétation est modèle du résultat si<br />
elle est modèle d’un nombre suffisant <strong>de</strong> bases. Nous avons montré que ces opérateurs<br />
propos<strong>en</strong>t un bon compromis <strong>en</strong>tre un <strong>en</strong>semble <strong>de</strong> critères importants pour <strong><strong>de</strong>s</strong> opérateurs<br />
<strong>de</strong> fusion : les propriétés logiques (section 5.1), la complexité algorithmique,<br />
la (non-)manipulabilité (voir section 5.4) et la puissance infér<strong>en</strong>tielle. Nous avons égalem<strong>en</strong>t<br />
proposé une nouvelle famille d’opérateurs, appelés opérateurs GMIN 3 , et nous<br />
avons montré qu’ils sont plus intéressants que les opérateurs <strong>de</strong> fusion <strong>à</strong> quota, car ils<br />
sont moins pru<strong>de</strong>nts et satisfont plus <strong>de</strong> propriétés logiques (d’un autre côté cela se<br />
paye par une complexité algorithmique plus élevée).<br />
Le point commun <strong>de</strong> tous ces opérateurs est qu’ils sont disjonctifs, c’est-<strong>à</strong>-dire que<br />
le résultat <strong>de</strong> la fusion est choisi parmi la disjonction <strong><strong>de</strong>s</strong> bases.<br />
Cette propriété n’est pas (et ne doit pas être) satisfaite par tous les opérateurs <strong>de</strong> fusion,<br />
car cela empêche <strong>de</strong> trouver <strong><strong>de</strong>s</strong> solutions <strong>de</strong> compromis, qui n’ont été proposées<br />
par aucune <strong><strong>de</strong>s</strong> bases.<br />
Mais dans certains cas, il est justifié d’imposer cette propriété <strong>de</strong> « disjonction »,<br />
<strong>en</strong> particulier <strong>en</strong> ce qui concerne la fusion <strong>de</strong> croyances 4 . Supposons par exemple que<br />
plusieurs mé<strong>de</strong>cins propos<strong>en</strong>t <strong><strong>de</strong>s</strong> traitem<strong>en</strong>ts possibles pour une pathologie donnée,<br />
il semble clair qu’il vaut mieux choisir parmi les traitem<strong>en</strong>ts possibles, plutôt que <strong>de</strong><br />
mélanger les traitem<strong>en</strong>ts...<br />
Une autre justification <strong>de</strong> cette propriété <strong>de</strong> disjonction est qu’elle peut-être expliquée<br />
comme l’expression d’une propriété d’unanimité. L’unanimité est une propriété<br />
classique lorsque l’on agrège <strong><strong>de</strong>s</strong> informations, et elle est considérée comme une propriété<br />
indisp<strong>en</strong>sable pour les métho<strong><strong>de</strong>s</strong> <strong>de</strong> vote par exemple. Cette propriété signifie<br />
intuitivem<strong>en</strong>t que si tous les ag<strong>en</strong>ts considèr<strong>en</strong>t qu’un candidat est le meilleur, alors il<br />
doit être considéré comme le meilleur pour le groupe. Si l’on considère l’expression<br />
<strong>de</strong> cette condition d’unanimité <strong>en</strong> fusion, il y a <strong>de</strong>ux interprétations possibles. La plus<br />
directe est l’expression <strong>de</strong> l’unanimité sur les interprétations. Ce qui s’exprime <strong>de</strong> la<br />
façon suivante :<br />
(UnaM) Si ω |= µ et ∀K ∈ E, ω |= K, alors ω |= △ µ (E)<br />
Cette condition est une conséqu<strong>en</strong>ce <strong>de</strong> la propriété (IC2), elle est donc satisfaite<br />
par tout opérateur <strong>de</strong> fusion contrainte.<br />
Mais, si l’on considère qu’une base est l’<strong>en</strong>semble <strong>de</strong> ses conséqu<strong>en</strong>ces, on peut<br />
exprimer l’unanimité sur les formules :<br />
(UnaF) Si ∃K ∈ E t.q. µ∧K est cohér<strong>en</strong>t, alors si ∀K ∈ E, K |= α, alors △ µ (E) |=<br />
α<br />
La condition <strong>de</strong> (UnaF) s’assure juste qu’il est possible <strong>de</strong> sélectionner un résultat<br />
dans la disjonction <strong><strong>de</strong>s</strong> bases qui soit compatible avec les contraintes d’intégrité.<br />
Et cette condition est équival<strong>en</strong>te au fait d’être un opérateur disjonctif :<br />
3. Ces opérateurs sont <strong><strong>de</strong>s</strong> opérateurs <strong>à</strong> base <strong>de</strong> modèle (section 5.2.1), avec une nouvelle fonction d’agrégation<br />
GMIN (leximin).<br />
4. Voir la section fusion <strong>de</strong> croyances versus fusion <strong>de</strong> but (section 5.3).<br />
55
(Disj) Si ∨ E est cohér<strong>en</strong>t avec µ, alors △ µ (E) |= ∨ E<br />
Cette propriété (Disj) est la principale raison pour justifier l’utilisation <strong><strong>de</strong>s</strong> opérateurs<br />
<strong>de</strong> fusion <strong>à</strong> base <strong>de</strong> formules (voir section 5.2.2). En effet, comparés aux opérateurs<br />
<strong>à</strong> base <strong>de</strong> modèles, ces opérateurs satisfont bi<strong>en</strong> moins <strong>de</strong> propriétés logiques et<br />
ont une complexité algorithmique souv<strong>en</strong>t plus importante.<br />
Ce travail suggère donc que les opérateurs GMIN sont un bon substitut aux opérateurs<br />
<strong>à</strong> base <strong>de</strong> formules : comme eux ils satisfont la propriété <strong>de</strong> disjonction, mais<br />
ils prés<strong>en</strong>t<strong>en</strong>t <strong>de</strong> bi<strong>en</strong> meilleures propriétés logiques (ce sont <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion<br />
contrainte), et une meilleure complexité algorithmique.<br />
5.2.5. Opérateurs <strong>à</strong> base <strong>de</strong> conflits<br />
Les opérateurs <strong>à</strong> base <strong>de</strong> modèles (section 5.2.1) sont basés sur une notion <strong>de</strong> proximité<br />
<strong>en</strong>tre modèles. Cette notion <strong>de</strong> proximité est capturée par une distance (numérique),<br />
telle que la distance <strong>de</strong> Hamming par exemple. Une autre possibilité est <strong>de</strong><br />
considérer comme « distance » l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> variables propositionnelles qui diffèr<strong>en</strong>t<br />
<strong>en</strong>tre <strong>de</strong>ux interprétations (cette distinction existe dans le cadre <strong>de</strong> la révision, si on<br />
examine le li<strong>en</strong> <strong>en</strong>tre l’opérateur <strong>de</strong> révision <strong>de</strong> Dalal [Dal88] et l’opérateur <strong>de</strong> Borgida<br />
[Bor85]). On est donc alors beaucoup plus précis qu’avec la distance <strong>de</strong> Hamming.<br />
Cela nous a conduit <strong>à</strong> définir un premier opérateur <strong>de</strong> fusion [38, 52], puis une famille<br />
<strong>en</strong>tière d’opérateurs <strong>de</strong> fusion basée sur les vecteurs <strong>de</strong> conflits [39]. Cette famille est<br />
beaucoup plus générale que les opérateurs basés sur les modèles usuels (qui sont donc<br />
un cas particulier <strong>de</strong> cette classe), et permett<strong>en</strong>t même <strong>de</strong> raffiner ces opérateurs.<br />
Voyons un petit exemple illustrant <strong>en</strong> quoi définir un vecteur <strong>de</strong> conflit permet une<br />
meilleure discrimination <strong><strong>de</strong>s</strong> interprétations que lorsque l’on résume le conflit par une<br />
distance numérique.<br />
Exemple 6 Soit un langage cont<strong>en</strong>ant les variables a, b, c, d, un profil E = {K 1 , . . . ,<br />
K 4 } et une contrainte d’intégrité µ telle que mod(µ) = {ω 1 , ω 2 }.<br />
diff(ω, ω ′ ) = {a ∈ P | ω(a) ≠ ω ′ (a)}, et diff(ω, K) = min ω′ |=K(diff(ω, ω ′ ), ⊆).<br />
diff(ω, K 1 ) diff(ω, K 2 ) diff(ω, K 3 ) diff(ω, K 4 )<br />
ω 1 {{a}} {{a}} {{a}} {{a}}<br />
ω 2 {{a}} {{b}} {{c}} {{d}}<br />
TABLE 5.3 – Différ<strong>en</strong>ce <strong>en</strong>tre ω 1 et ω 2 ?<br />
La table 5.3 indique le conflit minimal <strong>en</strong>tre chaque modèle <strong><strong>de</strong>s</strong> contraintes et<br />
chaque base du profil. Par exemple le conflit <strong>en</strong>tre ω 1 et K 1 porte sur la variable<br />
a, c’est-<strong>à</strong>-dire que le modèle <strong>de</strong> K 1 qui prés<strong>en</strong>te le moins <strong>de</strong> différ<strong>en</strong>ce par rapport <strong>à</strong><br />
ω 1 est simplem<strong>en</strong>t <strong>en</strong> désaccord avec la valeur <strong>de</strong> vérité <strong>de</strong> la variable a.<br />
Clairem<strong>en</strong>t la distance <strong>de</strong> Hamming ne permet pas <strong>de</strong> discriminer <strong>en</strong>tre ces <strong>de</strong>ux<br />
interprétations, car elles sont toutes <strong>de</strong>ux <strong>à</strong> une distance 1 <strong>de</strong> chaque base. On obti<strong>en</strong>t<br />
<strong>de</strong>ux vecteurs 〈1, 1, 1, 1〉 qui sont indiscernables quelle que soit la fonction d’agrégation<br />
utilisée.<br />
56
Si l’on utilise les vecteurs <strong>de</strong> conflits on obti<strong>en</strong>t dans un cas le vecteur 〈a, a, a, a〉<br />
et dans l’autre 〈a, b, c, d〉, ce qui montre une claire différ<strong>en</strong>ce <strong>en</strong>tre les <strong>de</strong>ux situations.<br />
Dans le cas <strong>de</strong> ω 1 tous les ag<strong>en</strong>ts s’accor<strong>de</strong>nt sur le fait que la variable « problématique<br />
» est a, alors que ce n’est pas le cas pour ω 2 . Ces <strong>de</strong>ux interprétations peuv<strong>en</strong>t donc<br />
être traitées différemm<strong>en</strong>t par les opérateurs <strong>de</strong> fusion <strong>à</strong> base <strong>de</strong> vecteurs <strong>de</strong> conflits.<br />
5.2.6. Opérateurs <strong>à</strong> base <strong>de</strong> défauts<br />
Delgran<strong>de</strong> et Schaub ont proposé <strong>de</strong>ux opérateurs <strong>de</strong> fusion <strong>à</strong> base <strong>de</strong> défauts<br />
[DS07]. L’idée est d’utiliser un langage spécifique pour chaque base, afin d’assurer<br />
que l’union <strong>de</strong> ces bases soit cohér<strong>en</strong>te, et <strong>en</strong>suite d’ajouter autant <strong>de</strong> règles <strong>de</strong> défaut<br />
que possible afin d’i<strong>de</strong>ntifier les variables correspondantes dans les différ<strong>en</strong>ts langages<br />
(ce qui rappelle l’approche <strong>de</strong> Besnard et Schaub pour l’infér<strong>en</strong>ce <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce<br />
décrite section 3.1.2).<br />
Ces opérateurs sont proches dans l’idée <strong><strong>de</strong>s</strong> opérateurs <strong>à</strong> base <strong>de</strong> conflits, puisqu’ils<br />
examin<strong>en</strong>t les conflits existants variable par variable, mais ils les trait<strong>en</strong>t <strong>de</strong> manière<br />
différ<strong>en</strong>te. Ces opérateurs ne sont donc pas définissables <strong>à</strong> partir d’une <strong><strong>de</strong>s</strong> familles<br />
précé<strong>de</strong>ntes.<br />
La définition formelle <strong>de</strong> ces opérateurs est la suivante :<br />
Définition 26 Un i-r<strong>en</strong>ommage d’un langage L est le langage L i , construit <strong>à</strong> partir<br />
<strong>de</strong> l’<strong>en</strong>semble <strong>de</strong> variables propositionnelles P i = {p i | p ∈ P}, où pour chaque<br />
α ∈ L, α i est le résultat du remplacem<strong>en</strong>t dans α <strong>de</strong> chaque variable propositionnelle<br />
p ∈ P par la variable correspondante p i ∈ P i . Soit une base K, le i-r<strong>en</strong>ommage <strong>de</strong><br />
(<strong><strong>de</strong>s</strong> formules <strong>de</strong>) K, est noté K i .<br />
Définition 27 Soit un profil E = {K 1 , . . . , K n }.<br />
• Soit EQ un sous-<strong>en</strong>semble <strong>de</strong> {p k ⇔ p l | p ∈ L et k, l ∈ {1 . . . n}} maximal<br />
(pour l’inclusion <strong>en</strong>sembliste) tel que ( ∧ K i∈E Ki i ) ∧ EQ est cohér<strong>en</strong>t.<br />
Alors {α | ∀j ∈ {1 . . . n} ( ∧ K i∈E Ki i ) ∧ EQ |= αj } est une ext<strong>en</strong>sion cohér<strong>en</strong>te<br />
symétrique <strong>de</strong> E.<br />
La fusion sceptique △ s (E) <strong>de</strong> E est l’intersection <strong>de</strong> toutes les ext<strong>en</strong>sions cohér<strong>en</strong>tes<br />
symétriques <strong>de</strong> E.<br />
• Soit EQ un sous-<strong>en</strong>semble <strong>de</strong> {p j ⇔ p | p ∈ L et j ∈ {1 . . . n}} maximal (pour<br />
l’inclusion <strong>en</strong>sembliste) tel que ( ∧ K i∈E Ki i ) ∧ EQ est cohér<strong>en</strong>t.<br />
Alors ( ∧ K i∈E Ki i ) ∧ EQ est une ext<strong>en</strong>sion cohér<strong>en</strong>te projetée <strong>de</strong> E.<br />
La fusion sceptique ∇ s (E) <strong>de</strong> E est l’intersection <strong>de</strong> toutes les ext<strong>en</strong>sions cohér<strong>en</strong>tes<br />
projetées <strong>de</strong>E.<br />
Exemple 7 Soit le profil E = {K 1 , K 2 , K 3 }, avec K 1 = (p ∧ q ∧ ¬r) ∨ (¬p ∧ q ∧ r),<br />
K 2 = (p ∧ ¬q ∧ ¬r) ∨ (¬p ∧ q ∧ ¬r) et K 3 = ¬q ∧ ¬r.<br />
Il y a quatre <strong>en</strong>sembles maximaux d’équival<strong>en</strong>ces pour ∆ s (E) :<br />
EQ1 = {p 1 ⇔ p 2 , p 1 ⇔ p 3 , p 2 ⇔ p 3 , r 1 ⇔ r 2 , r 1 ⇔ r 3 , r 2 ⇔ r 3 , q 2 ⇔ q 3 }<br />
EQ2 = {p 1 ⇔ p 3 , r 1 ⇔ r 2 , r 1 ⇔ r 3 , r 2 ⇔ r 3 , q 1 ⇔ q 2 }<br />
EQ3 = {p 2 ⇔ p 3 , r 1 ⇔ r 2 , r 1 ⇔ r 3 , r 2 ⇔ r 3 , q 1 ⇔ q 2 }<br />
57
EQ4 = {p 1 ⇔ p 2 , p 1 ⇔ p 3 , p 2 ⇔ p 3 , r 2 ⇔ r 3 , q 1 ⇔ q 2 }<br />
Donc, ∆ s (E) ≡ ¬r ∨ (¬p ∧ q).<br />
Pour ∇ s , les <strong>en</strong>sembles maximaux d’équival<strong>en</strong>ces sont les suivants :<br />
(p ⇔ p 1 ⇔ p 2 ⇔ p 3 sera utilisé comme abbréviation pour p ⇔ p 1 , p ⇔ p 2 , p ⇔ p 3 )<br />
EQ 1 = {p ⇔ p 1 ⇔ p 2 ⇔ p 3 , r ⇔ r 1 ⇔ r 2 ⇔ r 3 , q ⇔ q 2 ⇔ q 3 }<br />
EQ ′ 1 = {p ⇔ p 1 ⇔ p 2 ⇔ p 3 , r ⇔ r 1 ⇔ r 2 ⇔ r 3 , q ⇔ q 1 },<br />
EQ 2 = {p ⇔ p 1 ⇔ p 3 , r ⇔ r 1 ⇔ r 2 ⇔ r 3 , q ⇔ q 1 ⇔ q 2 }<br />
EQ 3 = {p ⇔ p 2 ⇔ p 3 , r ⇔ r 1 ⇔ r 2 ⇔ r 3 , q ⇔ q 1 ⇔ q 2 }<br />
EQ 4 = {p ⇔ p 1 ⇔ p 2 ⇔ p 3 , r ⇔ r 2 ⇔ r 3 , q ⇔ q 1 ⇔ q 2 }<br />
EQ ′ 4 = {p ⇔ p 1 ⇔ p 2 ⇔ p 3 , r ⇔ r 1 , q ⇔ q 1 ⇔ q 2 }<br />
Donc, ∇ s (E) ≡ (p ∧ ¬r) ∨ (¬p ∧ q).<br />
Ces opérateurs <strong>de</strong> fusion ont été implém<strong>en</strong>tés dans le cadre <strong>de</strong> la plate-forme COBA<br />
[DLST07].<br />
Une critique que l’on peut adresser <strong>à</strong> cette approche est que, tout comme les opérateurs<br />
<strong>à</strong> base <strong>de</strong> formules, ces opérateurs ne ti<strong>en</strong>n<strong>en</strong>t pas compte <strong>de</strong> la distribution <strong><strong>de</strong>s</strong><br />
informations parmi les sources. En particulier ils ne sont pas majoritaires, et une information<br />
qui serait crue par toutes les bases sauf une ne sera par exemple pas ret<strong>en</strong>ue dans<br />
le résultat. Mais, comme pour les opérateurs <strong>à</strong> base <strong>de</strong> formules (voir section 5.2.2),<br />
il nous semble possible <strong>de</strong> définir <strong><strong>de</strong>s</strong> politiques additionnelles afin <strong>de</strong> pr<strong>en</strong>dre ce type<br />
d’argum<strong>en</strong>ts <strong>en</strong> compte <strong>en</strong> utilisant <strong><strong>de</strong>s</strong> fonctions <strong>de</strong> sélection sur les sous-<strong>en</strong>sembles<br />
maximaux EQ. Nous p<strong>en</strong>sons donc qu’il reste <strong><strong>de</strong>s</strong> pistes intéressantes <strong>à</strong> explorer dans<br />
ce cadre.<br />
5.2.7. Opérateurs <strong>à</strong> base <strong>de</strong> similarité<br />
Récemm<strong>en</strong>t Schockaert et Pra<strong>de</strong> [SP09] ont proposé <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion basés<br />
sur une relation <strong>de</strong> similarité qualitative : pour chaque variable propositionnelle<br />
on associe un pré-ordre partiel sur les interprétations, dont cette variable est l’unique<br />
minimum, et qui représ<strong>en</strong>te la similarité <strong><strong>de</strong>s</strong> autres variables. Deux variables peuv<strong>en</strong>t<br />
évi<strong>de</strong>mm<strong>en</strong>t ne pas être <strong>en</strong> relation, d’où le pré-or<strong>de</strong> partiel.<br />
Cette relation peut-être extraite d’un graphe <strong>en</strong>tre variables propositionnelles, comme<br />
sur la figure 5.2 extraite <strong>de</strong> [SP09], où la similarité <strong>en</strong>tre <strong>de</strong>ux variables se compte<br />
<strong>en</strong> nombre d’arcs parcourus.<br />
Shockaert et Pra<strong>de</strong> se serv<strong>en</strong>t alors <strong>de</strong> cette relation <strong>de</strong> similarité pour t<strong>en</strong>ter <strong>de</strong><br />
trouver les meilleurs compromis lors <strong>de</strong> la fusion. La justification est alors <strong>de</strong> supposer<br />
que le conflit n’est pas issu d’avis diverg<strong>en</strong>ts (donc d’un conflit réel), mais <strong>en</strong> quelque<br />
sorte <strong>de</strong> problèmes d’ontologie, ou d’approximations (par exemple un ag<strong>en</strong>t qui ne fait<br />
pas <strong>de</strong> distinction <strong>en</strong>tre Single(x) et Divorced(x).<br />
L’utilisation d’une telle relation <strong>de</strong> similarité permet d’utiliser <strong><strong>de</strong>s</strong> techniques plus<br />
fines pour la résolution <strong>de</strong> conflit, qui ont <strong><strong>de</strong>s</strong> points communs avec les techniques<br />
utilisées pour la fusion <strong>de</strong> systèmes <strong>de</strong> contrainte (voir section 5.5.5).<br />
58
, thus solving conflicts by<br />
y” interpretations.<br />
atoms oc (“tomorrow the<br />
c (“tomorrow the sky will<br />
nd os (“tomorrow the sky<br />
ume that we have access<br />
h weather forecast inforon<br />
for tomorrow is cony<br />
constraint that the three<br />
tive and pairwise disjoint<br />
d K 2 = {os}. Classical<br />
ally lead to ∆(K 1 ,K 2 )=<br />
less one of the sources is<br />
le, the most intuitive con-<br />
,K 2 )={pc}.<br />
s a bor<strong>de</strong>rline case of both<br />
the result ∆(K 1 ,K 2 )=<br />
. Giv<strong>en</strong> the importance of<br />
in conflicts, it may be useat<br />
do not occur in any of<br />
es; e.g. the result of merge<br />
<strong>de</strong>fined as an os-pc borhich<br />
is op<strong>en</strong>, apart from a<br />
at there are five differ<strong>en</strong>t<br />
and oc, say pc −2 , pc −1 ,<br />
-to-guess meanings. Now<br />
omes that may be advometry<br />
principle, and trypossible,<br />
one may <strong><strong>de</strong>s</strong>ire<br />
However, also the more<br />
{pc −1 ∨ pc 0 ∨ pc 1 } and<br />
pc −1 ∨ pc 0 ∨ pc 1 ∨ pc 2 }<br />
d.<br />
knowledge bases are<br />
ategies may implem<strong>en</strong>t<br />
r, or (soft) majority prine<br />
next example.<br />
rd knowledge base K 3 =<br />
, pc −2 and pc −1 may be<br />
sible than pc 0 . Thus, the<br />
e merging operator might<br />
Figure 1. Similarity graph for atoms<br />
related to marriage.<br />
FIGURE 5.2 – Relation <strong>de</strong> similarité<br />
L’inconvéni<strong>en</strong>t est que cette approche nécessite la donnée additionnelle d’une relation<br />
<strong>de</strong> similarité, qui n’est pas toujours disponible.<br />
Cet article est <strong>à</strong> notre connaissance le premier <strong>à</strong> abor<strong>de</strong>r l’utilisation <strong>de</strong> cette notation<br />
<strong>de</strong> similarité pour la fusion <strong>de</strong> croyances. Nous p<strong>en</strong>sons qu’il reste beaucoup<br />
<strong>à</strong> faire dans cette voie. Il semble intéressant <strong>en</strong> particulier <strong>de</strong> t<strong>en</strong>ter <strong>de</strong> combiner les<br />
approches <strong>de</strong> fusion <strong>à</strong> base <strong>de</strong> modèles et cette approche <strong>à</strong> base <strong>de</strong> similarité.<br />
similarity. For each atom a, we assume that a sequ<strong>en</strong>ce<br />
of weak<strong>en</strong>ings a ∗ , a ∗∗ , a ∗∗∗ , etc. is available,<br />
corresponding to increasingly more tolerant<br />
interpretations of the assertion mo<strong>de</strong>led by atom<br />
a. In g<strong>en</strong>eral, we write a (k) for the k th elem<strong>en</strong>t in<br />
this sequ<strong>en</strong>ce, and a (0) = a. The integrity constraints<br />
C th<strong>en</strong> specify what is known about an<br />
atom such as a (k) . In the same way, we assume<br />
that a sequ<strong>en</strong>ce of tight<strong>en</strong>ings a ∗ , a ∗∗ , etc. is available,<br />
where we write a (k) for the k th elem<strong>en</strong>t.<br />
In practice, such sequ<strong>en</strong>ces of weak<strong>en</strong>ings and<br />
tight<strong>en</strong>ings can be <strong>de</strong>fined in terms of a similarity<br />
graph G =(A, S), where the set of no<strong><strong>de</strong>s</strong> A coinci<strong><strong>de</strong>s</strong><br />
with the set of atoms, and there is an edge<br />
(a, b) in S if atoms a and b are similar. Giv<strong>en</strong><br />
a similarity graph G, we can interpret a (k) (resp.<br />
a (k) ) as the disjunction (resp. conjunction) of all<br />
atoms whose distance to a in G is at most k. As<br />
an example, Figure 1 <strong>de</strong>picts a similarity graph involving<br />
predicates related to marriage.<br />
Weak<strong>en</strong>ings and tight<strong>en</strong>ings can also be applied<br />
to propositions and knowledge bases. Positive occurr<strong>en</strong>ces<br />
of an atom a are th<strong>en</strong> replaced by an<br />
atom of the form a (k) , and negative occurr<strong>en</strong>ces<br />
by an atom of the form a (k) . For instance, for<br />
K = {¬p ∨ q, r} we have K ∗ = {¬p ∗ ∨ q ∗ ,r ∗ }.<br />
5.3. Fusion <strong>de</strong> croyances versus fusion <strong>de</strong> buts<br />
Depuis le début du chapitre nous utilisons le terme <strong>de</strong> « fusion <strong>de</strong> croyances » <strong>de</strong><br />
manière générique. Mais les travaux réalisés sur la caractérisation <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong><br />
fusion sont valables autant pour la fusion <strong>de</strong> croyances proprem<strong>en</strong>t dite que pour la<br />
fusion <strong>de</strong> buts. En effet, tous les postulats prés<strong>en</strong>tés sont aussi valables dans un cas<br />
que dans l’autre. Même s’il peut sembler étrange <strong>à</strong> première vue que <strong><strong>de</strong>s</strong> concepts<br />
si différ<strong>en</strong>ts (croyances ou buts) puiss<strong>en</strong>t être traités <strong>de</strong> façon i<strong>de</strong>ntique du point <strong>de</strong><br />
vue <strong>de</strong> l’agrégation, la pertin<strong>en</strong>ce <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion contrainte pour la fusion<br />
propositionnelle (que ce soit <strong>de</strong> buts ou <strong>de</strong> croyances) n’a pas <strong>en</strong>core été remise <strong>en</strong><br />
cause par l’adjonction <strong>de</strong> nouveaux postulats qui permettrai<strong>en</strong>t <strong>de</strong> séparer les <strong>de</strong>ux<br />
types <strong>de</strong> fusion.<br />
Les seules nuances que l’on peut trouver est qu’il est plus facile <strong>de</strong> justifier les<br />
opérateurs <strong>de</strong> fusion disjonctive pour la fusion <strong>de</strong> croyances que pour la fusion <strong>de</strong> buts.<br />
Dans l’idée d’étudier plus avant les li<strong>en</strong>s <strong>en</strong>tre fusion et théorie du choix social,<br />
nous nous sommes intéressés [71] au problème <strong>de</strong> l’i<strong>de</strong>ntification du mon<strong>de</strong> réel (truth<br />
tracking). Cela permet d’i<strong>de</strong>ntifier <strong>de</strong>ux types <strong>de</strong> fusion : le point <strong>de</strong> vue synthétique et<br />
le point <strong>de</strong> vue épistémique :<br />
Vue synthétique : Sous le point <strong>de</strong> vue synthétique la fusion a pour objectif <strong>de</strong> caractériser<br />
une base qui représ<strong>en</strong>te au mieux les croyances du profil initial. C’est la<br />
vue considérée dans les travaux précé<strong>de</strong>nts concernant la fusion <strong>de</strong> croyances.<br />
Vue épistémique : Sous le point <strong>de</strong> vue épistémique, le but d’un processus <strong>de</strong> fusion<br />
est d’estimer au mieux le mon<strong>de</strong> réel, donc <strong>de</strong> supprimer autant que possible<br />
59<br />
Manipulating interpretations Using the similarity<br />
information <strong>en</strong>co<strong>de</strong>d by the sequ<strong>en</strong>ces of<br />
weak<strong>en</strong>ings a (k) , we can <strong>de</strong>fine a sequ<strong>en</strong>ce of expansions<br />
of an interpretation I as follows. Let<br />
C sim be the fragm<strong>en</strong>t of the integrity constraints
l’incertitu<strong>de</strong> du groupe d’ag<strong>en</strong>ts <strong>à</strong> son sujet.<br />
Il est intéressant <strong>de</strong> remarquer que la vue épistémique, c’est-<strong>à</strong>-dire la recherche du<br />
mon<strong>de</strong> réel, est un moy<strong>en</strong> <strong>de</strong> différ<strong>en</strong>cier la fusion <strong>de</strong> croyances <strong>de</strong> la fusion <strong>de</strong> buts.<br />
En effet, alors que la recherche du mon<strong>de</strong> réel peut être <strong>de</strong>mandée lors <strong>de</strong> la fusion <strong>de</strong><br />
croyances, le concept <strong>de</strong> recherche du mon<strong>de</strong> réel n’a pas <strong>de</strong> s<strong>en</strong>s pour <strong><strong>de</strong>s</strong> buts. Il est<br />
<strong>en</strong> effet assez clair que la notion <strong>de</strong> « vrai but », équival<strong>en</strong>t dans le cadre <strong><strong>de</strong>s</strong> buts du<br />
mon<strong>de</strong> réel, ne signifie ri<strong>en</strong>.<br />
Nous avons étudié quels opérateurs <strong>de</strong> fusion étai<strong>en</strong>t adéquats pour la vue épistémique<br />
(nous avons proposé un nouveau postulat pour cette recherche du mon<strong>de</strong> réel).<br />
En théorie du choix social, le résultat qui justifie les décisions prises <strong>à</strong> la majorité<br />
est le théorème du jury <strong>de</strong> Condorcet, qui énonce que si, pour répondre <strong>à</strong> une question<br />
binaire (<strong>de</strong> type Oui/Non), on dispose d’ag<strong>en</strong>ts fiables (qui ont moins d’une chance sur<br />
<strong>de</strong>ux <strong>de</strong> se tromper) et indép<strong>en</strong>dants, alors se fier <strong>à</strong> la majorité est la bonne chose <strong>à</strong><br />
faire (<strong>en</strong> particulier quand le nombre d’ag<strong>en</strong>ts t<strong>en</strong>d vers l’infini, la probabilité d’erreur<br />
<strong>de</strong> cette procédure t<strong>en</strong>d vers 0).<br />
C’est ce résultat théorique qui justifie l’utilisation <strong>de</strong> comités pour pr<strong>en</strong>dre <strong><strong>de</strong>s</strong><br />
décisions. Or, il est facile <strong>de</strong> constater que ses hypothèses sont très restrictives. En<br />
particulier, on ne peut choisir qu’<strong>en</strong>tre 2 alternatives, et les ag<strong>en</strong>ts ne peuv<strong>en</strong>t pas avoir<br />
d’incertitu<strong>de</strong> (ils ne peuv<strong>en</strong>t pas être indiffér<strong>en</strong>ts/hésitants <strong>en</strong>tre Oui et Non).<br />
Nous avons démontré, afin <strong>de</strong> pouvoir l’importer dans le cadre <strong>de</strong> la fusion, une<br />
généralisation du théorème du jury <strong>de</strong> Condorcet, que nous avons appelé théorème du<br />
jury sous incertitu<strong>de</strong>, lorsqu’il y a un nombre quelconque d’alternatives et lorsque<br />
les ag<strong>en</strong>ts peuv<strong>en</strong>t être incertains (ils fourniss<strong>en</strong>t un <strong>en</strong>semble d’alternatives et pas une<br />
alternative unique). Nous avons montré que le vote par approbation (approval voting)<br />
[BF83], qui permet aux individus <strong>de</strong> voter pour un nombre quelconque <strong>de</strong> candidats et<br />
qui élit le candidat ayant reçu le plus <strong>de</strong> voix, est la métho<strong>de</strong> <strong>à</strong> utiliser dans ce cas.<br />
5.4. Manipulation<br />
Nous avons étudié [9, 27, 64] la résistance <strong>à</strong> la manipulation <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong><br />
fusion. Les opérateurs existants <strong>de</strong> fusion permett<strong>en</strong>t <strong>de</strong> définir les croyances/buts du<br />
groupe d’ag<strong>en</strong>ts. Mais, si un ag<strong>en</strong>t est capable <strong>de</strong> conduire <strong><strong>de</strong>s</strong> raisonnem<strong>en</strong>ts sur le résultat<br />
<strong>de</strong> cette opération et sur l’impact qu’il peut avoir sur celui-ci, il peut être t<strong>en</strong>té <strong>de</strong><br />
m<strong>en</strong>tir sur ses véritables croyances (ou buts), afin <strong>de</strong> modifier le résultat conformém<strong>en</strong>t<br />
<strong>à</strong> ses intérêts. Nous avons défini <strong><strong>de</strong>s</strong> notions <strong>de</strong> manipulabilité pour la fusion, et nous<br />
avons étudié la manipulabilité <strong><strong>de</strong>s</strong> principales métho<strong><strong>de</strong>s</strong> <strong>de</strong> fusion <strong>de</strong> la littérature.<br />
De manière non surpr<strong>en</strong>ante, il est assez difficile d’obt<strong>en</strong>ir <strong><strong>de</strong>s</strong> garanties <strong>de</strong> non<br />
manipulabilité. En effet, la fusion <strong>de</strong> croyances est très proche techniquem<strong>en</strong>t <strong>de</strong> l’agrégation<br />
<strong>de</strong> préfér<strong>en</strong>ces. Or, un résultat important <strong>en</strong> choix social, le théorème <strong>de</strong> Gibbard-<br />
Sattherwaite [Gib73, Sat75], montre qu’il est impossible <strong>de</strong> définir une métho<strong>de</strong> d’agrégation<br />
<strong>de</strong> préfér<strong>en</strong>ces non manipulable. Il n’est donc pas étonnant d’avoir très peu <strong>de</strong><br />
cas <strong>de</strong> non manipulabilité dans le cas <strong>de</strong> la fusion.<br />
Nous avons défini la manipulabilité comme suit [9, 27, 64] :<br />
Définition 28 Soit un indice <strong>de</strong> satisfaction i, i.e., une fonction <strong>de</strong> L × L dans IR.<br />
60
• Un profil E est manipulable par une base K pour l’indice i étant donné l’opérateur<br />
△ et la contrainte d’intégrité µ si et seulem<strong>en</strong>t si il existe une base K ′ telle<br />
que<br />
i(K, ∆ µ (E ⊔ {K ′ })) > i(K, ∆ µ (E ⊔ {K}))<br />
• Un opérateur <strong>de</strong> fusion ∆ est non manipulable pour i si et seulem<strong>en</strong>t si il n’y<br />
a pas <strong>de</strong> contrainte d’intégrité µ et <strong>de</strong> profil E = {K 1 , . . . , K n } tel que E est<br />
manipulable pour i.<br />
Cette définition <strong>de</strong> la manipulabilité est assez standard. La différ<strong>en</strong>ce avec le cas <strong>de</strong><br />
l’agrégation <strong>de</strong> préfér<strong>en</strong>ce où la comparaison <strong><strong>de</strong>s</strong> situations est directe, est que dans le<br />
cas <strong>de</strong> la fusion on doit utiliser un indice <strong>de</strong> satisfaction.<br />
Nous nous sommes conc<strong>en</strong>trés sur trois indices différ<strong>en</strong>ts (qui sont les plus naturels<br />
si l’on ne dispose pas d’informations complém<strong>en</strong>taires) :<br />
Définition 29<br />
• Indice drastique faible : { 1 si K ∧ K△ est cohér<strong>en</strong>t<br />
i dw (K, K △ ) =<br />
0 sinon<br />
• Indice drastique fort :<br />
{<br />
1 si K△ |= K,<br />
i ds (K, K △ ) =<br />
0 sinon<br />
• Indice probabiliste 5 :<br />
i p (K, K △ ) = |[K]∩[K △]|<br />
|[K △ ]|<br />
On considère qu’un ag<strong>en</strong>t est satisfait par le résultat <strong>de</strong> la fusion (K △ ) avec l’indice<br />
drastique faible si ce résultat est cohér<strong>en</strong>t avec sa base. Avec l’indice drastique fort, le<br />
résultat doit impliquer sa base. L’indice probabiliste permet une mesure plus progressive<br />
<strong>de</strong> la satisfaction, qui dép<strong>en</strong>d <strong>de</strong> la proportion <strong><strong>de</strong>s</strong> modèles <strong>en</strong> commun <strong>en</strong>tre le<br />
résultat <strong>de</strong> la fusion et la base <strong>de</strong> l’ag<strong>en</strong>t.<br />
On obti<strong>en</strong>t alors les résultats <strong>de</strong> manipulabilité pour les opérateurs usuels 6 décrits<br />
<strong>à</strong> la table 5.4. La première colonne indique avec quel indice on travaille, la <strong>de</strong>uxième<br />
si l’on restreint la cardinalité du profil <strong>à</strong> 2 bases ou pas, la troisième si l’on contraint<br />
les bases <strong>à</strong> être complètes (C) ou pas, et la quatrième indique si on autorise (µ) ou<br />
pas (⊤) l’utilisation <strong>de</strong> contraintes d’intégrité. Les cases où un apparaît indiqu<strong>en</strong>t un<br />
cas <strong>de</strong> non manipulabilité. Les lignes <strong>en</strong> orange indiqu<strong>en</strong>t le cas général (sans aucune<br />
restriction). On peut remarquer que dans ce cas général très peu d’opérateurs sont non<br />
manipulables (<strong>en</strong> particulier 2 seulem<strong>en</strong>t avec l’indice probabiliste).<br />
D’autres résultats ont été obt<strong>en</strong>us avec d’autres indices et d’autres restrictions (voir<br />
[9, 27, 64]).<br />
5. Lorsque |[K △ ]| = 0, alors i p(K, K ∆ ) = 0.<br />
6. Les opérateurs △Ĉi sont une petite modification <strong><strong>de</strong>s</strong> opérateurs <strong>à</strong> base <strong>de</strong> formules △ Ci où on effectue<br />
d’abord la conjonction <strong>de</strong> toutes les formules <strong>de</strong> chaque base K j du profil avant la fusion.<br />
61
i # K µ ∆ d D ,f ∆ d H ,Σ ∆ d H ,Gmax △ C1 △ C3 △ C4 C5<br />
△ △Ĉ1 △Ĉ3 △Ĉ4 △Ĉ5<br />
2<br />
i dw<br />
n<br />
2<br />
i ds<br />
n<br />
2<br />
i p<br />
n<br />
C<br />
⊤ <br />
µ <br />
NC ⊤ <br />
µ <br />
C<br />
⊤ <br />
µ <br />
NC ⊤ <br />
µ <br />
C<br />
⊤ <br />
µ <br />
NC ⊤ <br />
µ <br />
C<br />
⊤ <br />
µ <br />
NC ⊤ <br />
µ <br />
C<br />
⊤ <br />
µ <br />
NC ⊤ <br />
µ <br />
C<br />
⊤ <br />
µ <br />
NC ⊤ <br />
µ <br />
TABLE 5.4 – Résultats <strong>de</strong> manipulabilité<br />
62
5.5. Fusion dans d’autres cadres<br />
La fusion a égalem<strong>en</strong>t été étudiée dans d’autre cadres <strong>de</strong> représ<strong>en</strong>tation. Les opérateurs<br />
<strong>de</strong> fusion prés<strong>en</strong>tés jusqu’ici étai<strong>en</strong>t tous définis dans le cadre <strong>de</strong> la logique propositionnelle<br />
et dans le cas où toutes les bases ont la même importance/priorité/fiabilité.<br />
On peut avoir besoin <strong>de</strong> fusionner <strong><strong>de</strong>s</strong> informations plus structurées que celles que l’on<br />
exprime <strong>en</strong> logique classique, ce qui génère <strong><strong>de</strong>s</strong> problèmes, et <strong><strong>de</strong>s</strong> possibilités, supplém<strong>en</strong>taires.<br />
Nous allons donner un aperçu <strong><strong>de</strong>s</strong> travaux les plus proches dans cette section.<br />
5.5.1. Fusion prioritaire, fusion et révision itérée<br />
Dans [DDL06] Delgran<strong>de</strong>, Dubois et Lang propos<strong>en</strong>t une discussion intéressante<br />
sur les opérateurs <strong>de</strong> fusion prioritaire. L’idée est <strong>de</strong> fusionner un <strong>en</strong>semble <strong>de</strong> formules<br />
7 pondérées. La pondération sert <strong>à</strong> stratifier ces formules (une formule avec un<br />
poids plus grand est plus importante, et le nombre <strong>de</strong> formules moins prioritaires la<br />
contredisant n’importe pas).<br />
Delgran<strong>de</strong>, Dubois et Lang motiv<strong>en</strong>t alors la généralité <strong>de</strong> leur approche <strong>en</strong> montrant<br />
que les opérateurs <strong>de</strong> fusion propositionnelle « classiques » (i.e. sur <strong><strong>de</strong>s</strong> bases non<br />
pondérées) et les opérateurs <strong>de</strong> révision itérée (<strong>à</strong> la Darwiche et Pearl) peuv<strong>en</strong>t être<br />
considérés comme les <strong>de</strong>ux cas extrêmes <strong>de</strong> ces opérateurs <strong>de</strong> fusion prioritaire.<br />
Leur discussion sur les opérateurs <strong>de</strong> révision itérée est particulièrem<strong>en</strong>t intéressante,<br />
et rappelle les mise <strong>en</strong> gar<strong>de</strong> <strong>de</strong> Friedman et Halpern sur les dangers <strong>de</strong> définir <strong><strong>de</strong>s</strong><br />
opérateurs <strong>de</strong> changem<strong>en</strong>t sans spécifier leur ontologie [FH96b]. L’argum<strong>en</strong>t principal<br />
est que si on fait l’hypothèse que les nouvelles informations qui arriv<strong>en</strong>t successivem<strong>en</strong>t<br />
lors d’une suite <strong>de</strong> révisions concern<strong>en</strong>t un mon<strong>de</strong> statique (hypothèse usuelle), alors il<br />
n’y a a priori aucune raison <strong>de</strong> préférer la <strong>de</strong>rnière. Si ces informations ont <strong><strong>de</strong>s</strong> fiabilités<br />
différ<strong>en</strong>tes, il est possible <strong>de</strong> représ<strong>en</strong>ter ces fiabilités explicitem<strong>en</strong>t, afin <strong>de</strong> les pr<strong>en</strong>dre<br />
correctem<strong>en</strong>t <strong>en</strong> compte dans le processus <strong>de</strong> « révision » si elles n’arriv<strong>en</strong>t pas dans<br />
l’ordre <strong>de</strong> leur fiabilité. Et la façon correcte <strong>de</strong> faire est <strong>de</strong> réaliser leur fusion prioritaire.<br />
Cette discussion est intéressante car dans un certain nombre <strong>de</strong> papiers portant sur<br />
la révision itérée, il semble que les auteurs confon<strong>de</strong>nt l’hypothèse d’informations <strong>de</strong><br />
plus <strong>en</strong> plus fiables, avec celle d’informations <strong>de</strong> plus <strong>en</strong> plus réc<strong>en</strong>tes.<br />
Le cadre <strong>de</strong> Delgran<strong>de</strong>, Dubois et Lang suppose que l’on i<strong>de</strong>ntifie un état épistémique<br />
avec la suite <strong>de</strong> formules que l’ag<strong>en</strong>t a reçues jusqu’ici, hypothèse qui avait<br />
été proposée dans la définition <strong>de</strong> révision itérée <strong>de</strong> Lehmann [Leh95] et dans notre<br />
proposition d’opérateurs <strong>à</strong> mémoire [1, 21].<br />
Delgran<strong>de</strong>, Dubois et Lang montr<strong>en</strong>t <strong>en</strong>suite que les postulats <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> révision<br />
itérée peuv<strong>en</strong>t être retrouvés <strong>à</strong> partir <strong><strong>de</strong>s</strong> postulats <strong>de</strong> base qu’ils propos<strong>en</strong>t pour<br />
la fusion prioritaire. Ils montr<strong>en</strong>t égalem<strong>en</strong>t que l’on obti<strong>en</strong>t une partie <strong><strong>de</strong>s</strong> postulats <strong>de</strong><br />
la fusion contrainte.<br />
Cette proposition est intéressante car elle offre une piste pour la caractérisation logique<br />
<strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion prioritaire. Nous p<strong>en</strong>sons qu’il y a <strong>en</strong>core beaucoup <strong>à</strong><br />
7. Chaque formule pouvant représ<strong>en</strong>ter une base si on veut faire le parallèle avec la fusion dans le cadre<br />
propositionnel.<br />
63
montrer <strong>en</strong> suivant cette voie. Les auteurs n’ont <strong>en</strong> particulier pas donné <strong>de</strong> théorème<br />
<strong>de</strong> représ<strong>en</strong>tation pour leurs opérateurs, et n’ont pas donné l’équival<strong>en</strong>t dans leur cadre<br />
<strong>de</strong> l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> postulats <strong>de</strong> la fusion contrainte. De plus, une <strong>de</strong> leur hypothèse <strong>de</strong><br />
base est que la hiérarchisation <strong><strong>de</strong>s</strong> formules est stricte, c’est-<strong>à</strong>-dire qu’une seule formule<br />
<strong>de</strong> poids supérieur l’emporte sur un nombre quelconque <strong>de</strong> formules <strong>de</strong> poids<br />
inférieur. Cela peut se justifier dans certaines applications, mais dans le cas général, il<br />
pourrait être intéressant d’avoir une possible comp<strong>en</strong>sation <strong>en</strong>tre formules <strong>de</strong> différ<strong>en</strong>ts<br />
poids. Finalem<strong>en</strong>t, ri<strong>en</strong> dans leur axiomatique n’indique comm<strong>en</strong>t doiv<strong>en</strong>t être traités<br />
les conflits <strong>en</strong>tre formules <strong>de</strong> même poids, ce qui est l’objet <strong>de</strong> la fusion propositionnelle.<br />
La fusion classique étant prés<strong>en</strong>tée comme cas particulier <strong>de</strong> fusion prioritaire,<br />
cela peut donc sembler paradoxal.<br />
Bref, cet article a ouvert une voie intéressante, qui n’a malheureusem<strong>en</strong>t pas été<br />
exploitée <strong>de</strong>puis.<br />
5.5.2. Fusion <strong>de</strong> bases pondérées<br />
Lorsque les informations cont<strong>en</strong>ues dans les bases <strong>de</strong> croyances ne sont pas toutes<br />
<strong>de</strong> même importance, on utilise <strong><strong>de</strong>s</strong> approches pondérées. L’approche la plus qualitative<br />
est <strong>de</strong> considérer, pour chaque source/ag<strong>en</strong>t, un <strong>en</strong>semble <strong>de</strong> bases (totalem<strong>en</strong>t) ordonnées<br />
<strong>en</strong> différ<strong>en</strong>tes strates, <strong>de</strong> la plus importante <strong>à</strong> la moins importante. Cette situation<br />
est habituellem<strong>en</strong>t codée <strong>en</strong> utilisant la logique possibiliste [DLP94] ou les fonctions<br />
ordinales conditionnelles [Spo87]. Dans ce cas on associe un ordinal (habituellem<strong>en</strong>t<br />
fini, i.e, un <strong>en</strong>tier naturel 8 ) <strong>à</strong> chaque formule.<br />
Dans le cadre <strong>de</strong> la logique possibiliste B<strong>en</strong>ferhat, Dubois, Kaci et Pra<strong>de</strong> ont étudié<br />
plusieurs opérateurs <strong>de</strong> fusion [KBDP00, BDKP02]. Ils ont <strong>en</strong> particulier étudié<br />
l’ext<strong>en</strong>sion <strong>de</strong> nos propriétés logiques dans ce cadre [BK03]. (voir égalem<strong>en</strong>t [QLB06]<br />
pour une généralisation <strong>de</strong> nos opérateurs dans un cadre pondéré).<br />
Dans le cadre <strong><strong>de</strong>s</strong> fonctions ordinales conditionnelles, Meyer a égalem<strong>en</strong>t défini<br />
différ<strong>en</strong>ts opérateurs <strong>de</strong> combinaison [Mey01]. Certains d’<strong>en</strong>tre eux sont la traduction<br />
<strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion <strong>à</strong> base <strong>de</strong> modèles usuels dans ce cadre pondéré,mais d’autres<br />
sembl<strong>en</strong>t très loin <strong>de</strong> ce que l’on att<strong>en</strong>d d’un opérateur <strong>de</strong> fusion.<br />
Tous ces travaux utilisant <strong><strong>de</strong>s</strong> bases pondérées revi<strong>en</strong>n<strong>en</strong>t sémantiquem<strong>en</strong>t <strong>à</strong> utiliser<br />
pour chaque ag<strong>en</strong>t une distribution <strong>de</strong> possibilités sur les mon<strong><strong>de</strong>s</strong> (ou <strong>de</strong> manière<br />
équival<strong>en</strong>te une fonction ordinale conditionnelle), c’est-<strong>à</strong>-dire que chaque interprétation<br />
est associée <strong>à</strong> un nombre par l’ag<strong>en</strong>t, ce nombre exprimant <strong>à</strong> quel point l’ag<strong>en</strong>t<br />
estime l’interprétation plausible.<br />
Les fonctions d’agrégation utilisées pour définir les opérateurs <strong>de</strong> fusion réalis<strong>en</strong>t<br />
donc un calcul <strong>à</strong> partir <strong>de</strong> ces nombres. Il se pose alors un problème <strong>de</strong> comparaison<br />
interpersonnelle <strong><strong>de</strong>s</strong> utilités, c’est-<strong>à</strong>-dire que cette opération suppose que le nombre<br />
4 chez un ag<strong>en</strong>t a la même valeur (i.e. a le même s<strong>en</strong>s) que chez n’importe quel autre<br />
ag<strong>en</strong>t. C’est ce que l’on appelle l’hypothèse <strong>de</strong> comm<strong>en</strong>surabilité.<br />
Pour être fondée, cette approche nécessite cette hypothèse <strong>de</strong> comm<strong>en</strong>surabilité. Il<br />
y a <strong><strong>de</strong>s</strong> cas où cela est parfaitem<strong>en</strong>t naturel, par exemple si les sources sont <strong><strong>de</strong>s</strong> capteurs<br />
i<strong>de</strong>ntiques. Mais dans <strong><strong>de</strong>s</strong> applications où les sources sont <strong><strong>de</strong>s</strong> ag<strong>en</strong>ts autonomes, cette<br />
8. Voir [40] pour une discussion.<br />
64
hypothèse semble clairem<strong>en</strong>t irréaliste. En particulier, lorsque l’on travaille avec <strong><strong>de</strong>s</strong><br />
bases pondérées, on est très proche <strong><strong>de</strong>s</strong> hypothèses faites <strong>en</strong> théorie du choix social<br />
pour les métho<strong><strong>de</strong>s</strong> <strong>de</strong> vote. Et il est communém<strong>en</strong>t accepté que cette hypothèse <strong>de</strong><br />
comm<strong>en</strong>surabilité n’est pas acceptable dans ce cas [Arr63]. Dans le cadre du vote seule<br />
la préfér<strong>en</strong>ce ordinale <strong>de</strong> chaque ag<strong>en</strong>t est prise <strong>en</strong> compte, c’est-<strong>à</strong>-dire l’ordre associé<br />
<strong>à</strong> ces nombres.<br />
Il nous semble que l’étu<strong>de</strong> générale, sans cette hypothèse <strong>de</strong> comm<strong>en</strong>surabilité,<br />
<strong>de</strong> la fusion <strong>de</strong> bases pondérées, surtout si on considère <strong><strong>de</strong>s</strong> opérateurs majoritaires,<br />
revi<strong>en</strong>t <strong>à</strong> étudier <strong><strong>de</strong>s</strong> métho<strong><strong>de</strong>s</strong> <strong>de</strong> vote, et qu’il faut alors se référer <strong>à</strong> la littérature <strong>de</strong><br />
choix social [Arr63, ASS02].<br />
B<strong>en</strong>ferhat, Lagrue et Rossit ont étudié <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion <strong>de</strong> bases pondérées<br />
non majoritaires sans l’hypothèse <strong>de</strong> comm<strong>en</strong>surabilité [BLR07, BLR09]. Evi<strong>de</strong>mm<strong>en</strong>t,<br />
cela conduit a <strong><strong>de</strong>s</strong> opérateurs beaucoup plus pru<strong>de</strong>nts que les opérateurs que<br />
l’on définit dans le cas comm<strong>en</strong>surable. Une question intéressante serait alors d’étudier<br />
si les opérateurs proposés dans le cadre non comm<strong>en</strong>surable correspon<strong>de</strong>nt <strong>à</strong> <strong><strong>de</strong>s</strong><br />
métho<strong><strong>de</strong>s</strong> <strong>de</strong> vote connues.<br />
5.5.3. Fusion <strong>en</strong> logique du premier ordre<br />
Lang et Bloch ont proposé <strong>de</strong> définir les opérateurs <strong>de</strong> fusion <strong>à</strong> base <strong>de</strong> modèle<br />
△ d,max , utilisant le maximum comme fonction d’agrégation, grâce <strong>à</strong> un processus <strong>de</strong><br />
dilatation [BL00]. On peut d’ailleurs noter que dans l’article <strong>de</strong> Dalal [Dal88], son<br />
opérateur <strong>de</strong> révision n’est pas défini <strong>à</strong> base <strong>de</strong> distance, mais <strong>à</strong> partir d’une telle fonction<br />
<strong>de</strong> dilatation.<br />
Gorogiannis et Hunter [GH08] ont ét<strong>en</strong>du cette approche afin <strong>de</strong> définir les opérateurs<br />
<strong>de</strong> fusion <strong>à</strong> base <strong>de</strong> modèles usuels <strong>en</strong> terme <strong>de</strong> dilatations. En plus <strong>de</strong> △ d,max ,<br />
ils ont exprimé la définition <strong>de</strong> △ d,Σ , △ d,Gmax , et △ d,Gmin .<br />
L’intérêt <strong>de</strong> cette caractérisation <strong>de</strong> ces opérateurs est que celle-ci peut-être exportée<br />
<strong>à</strong> la logique du premier ordre. En effet, la définition usuelle <strong><strong>de</strong>s</strong> opérateurs <strong>à</strong> base<br />
<strong>de</strong> modèles <strong>de</strong>man<strong>de</strong> le calcul <strong>de</strong> distances <strong>en</strong>tre l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> interprétations. Or<br />
dès que l’on passe dans <strong><strong>de</strong>s</strong> logiques plus expressives que la logique propositionnelle,<br />
comme la logique du premier ordre, le nombre <strong>de</strong> modèles <strong>de</strong>vi<strong>en</strong>t très souv<strong>en</strong>t infini.<br />
L’intérêt <strong>de</strong> la définition <strong>en</strong> termes <strong>de</strong> dilatation est que celle-ci peut-être calculée<br />
même dans ces cadres. Cela nécessite simplem<strong>en</strong>t <strong>de</strong> choisir la bonne fonction <strong>de</strong> dilatation.<br />
Voir [GH08] pour une discussion et quelques exemples sur ces fonctions <strong>de</strong><br />
dilatation dans le cadre <strong>de</strong> la logique du premier ordre.<br />
5.5.4. Fusion <strong>de</strong> programmes logiques<br />
Certains travaux ont étudié <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion pour <strong><strong>de</strong>s</strong> bases exprimées <strong>en</strong><br />
programmation logique avec sémantique <strong><strong>de</strong>s</strong> modèles stables (Answer Set Programming).<br />
C’est une question assez naturelle lorsque l’on considère qu’il y a eu beaucoup<br />
<strong>de</strong> travaux sur la révision / mise <strong>à</strong> jour <strong>de</strong> programmes logiques (voir par exemple<br />
[ZF98, ALS98, ALP + 00, DSTW08]), mais jusqu’<strong>à</strong> récemm<strong>en</strong>t aucun sur la fusion.<br />
L’approche <strong>de</strong> Hué, Papini et Würbel [HPW09] repose sur la suppression d’un<br />
certain nombre <strong>de</strong> formules dans l’union <strong><strong>de</strong>s</strong> bases, sélectionnées grâce <strong>à</strong> une fonction<br />
65
<strong>de</strong> sélection (un peu comme pour sélectionner les maximaux cohér<strong>en</strong>ts dans [20]). Ces<br />
opérateurs ne satisfont que très peu <strong><strong>de</strong>s</strong> propiétés logiques <strong>de</strong> la fusion contrainte.<br />
Delgran<strong>de</strong>, Schaub, Tompits et Woltran [DSTW09] ont égalem<strong>en</strong>t étudié la fusion<br />
dans ce cadre. Leurs opérateurs sont basés sur la définition d’une distance <strong>en</strong>tre les<br />
modèles stables. Leurs opérateurs satisfont beaucoup plus <strong>de</strong> propriétés logiques.<br />
Pour comparer rapi<strong>de</strong>m<strong>en</strong>t ces <strong>de</strong>ux approches, on peut dire que dans le cadre <strong>de</strong><br />
la programmation logique, les opérateurs <strong>de</strong> Hué, Papini et Würbel correspon<strong>de</strong>nt aux<br />
approches <strong>à</strong> base <strong>de</strong> formules, alors que les opérateurs <strong>de</strong> Delgran<strong>de</strong>, Schaub, Tompits<br />
et Woltran correspon<strong>de</strong>nt aux approches <strong>à</strong> base <strong>de</strong> modèles. Il n’est donc pas étonnant<br />
que ces <strong>de</strong>rniers satisfass<strong>en</strong>t, comme dans le cas <strong>de</strong> la logique propositionnelle,<br />
plus <strong>de</strong> propriétés logiques. Ces opérateurs étant facilem<strong>en</strong>t implém<strong>en</strong>tables, il serait<br />
intéressant <strong>de</strong> comparer leur temps <strong>de</strong> réponse sur <strong><strong>de</strong>s</strong> problèmes pratiques.<br />
5.5.5. Fusion <strong>de</strong> réseaux <strong>de</strong> contraintes<br />
Condotta, Kaci, Marquis et Schwind ont proposé <strong><strong>de</strong>s</strong> métho<strong><strong>de</strong>s</strong> pour fusionner <strong><strong>de</strong>s</strong><br />
réseaux <strong>de</strong> contraintes qualitatives [CKMS09b, CKMS09a]. Ces métho<strong><strong>de</strong>s</strong> peuv<strong>en</strong>t être<br />
très utiles pour fusionner <strong><strong>de</strong>s</strong> réseaux <strong>de</strong> contraintes représ<strong>en</strong>tant <strong><strong>de</strong>s</strong> régions spatiales ;<br />
par exemple dans le cadre <strong>de</strong> systèmes d’information géographiques (SIG), il peut<br />
être nécessaire <strong>de</strong> t<strong>en</strong>ter <strong>de</strong> fusionner <strong><strong>de</strong>s</strong> bases <strong>de</strong> données spatiales issues <strong>de</strong> sources<br />
différ<strong>en</strong>tes.<br />
Les conflits qui apparaiss<strong>en</strong>t sont plus subtils que ceux issus <strong>de</strong> problèmes exprimés<br />
<strong>en</strong> logique propositionnelle. Dans ce <strong>de</strong>rnier cas, les conflits sont <strong>de</strong> type vrai/faux,<br />
alors que dans le cas <strong><strong>de</strong>s</strong> réseaux <strong>de</strong> contraintes on peut avoir différ<strong>en</strong>ts types <strong>de</strong> conflits<br />
plus ou moins graves. Par exemple, si l’on s’intéresse <strong>à</strong> l’algèbre d’All<strong>en</strong>, qui permet<br />
<strong>de</strong> représ<strong>en</strong>ter les informations spatiales <strong>à</strong> propos <strong>de</strong> segm<strong>en</strong>ts situés sur une droite, on<br />
dispose <strong><strong>de</strong>s</strong> relations décrites figure 5.3.<br />
Un conflit <strong>en</strong>tre les assertions du type « Le segm<strong>en</strong>t A est avant le segm<strong>en</strong>t B » (A<br />
BEFORE B) et « Le segm<strong>en</strong>t A touche le segm<strong>en</strong>t B » (A MEET B) semble beaucoup<br />
moins fort que le conflit <strong>en</strong>tre « Le segm<strong>en</strong>t A est avant le segm<strong>en</strong>t B » (A BEFORE B)<br />
et « Le segm<strong>en</strong>t B est avant le segm<strong>en</strong>t A » (A iBEFORE B).<br />
Cette « int<strong>en</strong>sité » que l’on s<strong>en</strong>t naître <strong>en</strong>tre les différ<strong>en</strong>ts conflits permet d’imaginer<br />
<strong><strong>de</strong>s</strong> politiques <strong>de</strong> fusion plus variées que dans le cadre propositionnel.<br />
5.5.6. Fusion <strong>de</strong> systèmes d’argum<strong>en</strong>tation<br />
Nous nous sommes intéressés au problème <strong>de</strong> la fusion <strong>de</strong> systèmes d’argum<strong>en</strong>tation<br />
[8, 29]. Beaucoup <strong>de</strong> travaux ont étudié l’argum<strong>en</strong>tation comme moy<strong>en</strong> <strong>de</strong> raisonner<br />
<strong>à</strong> partir d’informations contradictoires. Fondam<strong>en</strong>talem<strong>en</strong>t on utilise un <strong>en</strong>semble<br />
d’argum<strong>en</strong>ts et une relation <strong>de</strong> contrariété (attaque) <strong>en</strong>tre les argum<strong>en</strong>ts. Un cadre général<br />
pour l’argum<strong>en</strong>tation a été proposé par Dung [Dun95]. Mais ces travaux sur<br />
l’argum<strong>en</strong>tation ne se préoccup<strong>en</strong>t que d’un seul ag<strong>en</strong>t. Nous avons étudié comm<strong>en</strong>t<br />
généraliser ces cadres pour pr<strong>en</strong>dre <strong>en</strong> compte le fait que les argum<strong>en</strong>ts sont distribués<br />
parmi un <strong>en</strong>semble d’ag<strong>en</strong>ts. Nous avons examiné le problème posé par le fait<br />
que différ<strong>en</strong>ts ag<strong>en</strong>ts puiss<strong>en</strong>t avoir <strong><strong>de</strong>s</strong> systèmes d’argum<strong>en</strong>tation construits <strong>à</strong> partir<br />
66
B<br />
A<br />
A<br />
B<br />
A before B<br />
A meet B<br />
A overlap B<br />
A start B<br />
A during B<br />
A finish B<br />
A equal B<br />
A ifinish B<br />
A iduring B<br />
A istart B<br />
A ioverlap B<br />
A imeet B<br />
A ibefore B<br />
FIGURE 5.3 – Algèbre <strong><strong>de</strong>s</strong> intervalles d’All<strong>en</strong><br />
d’argum<strong>en</strong>ts différ<strong>en</strong>ts. Nous avons étudié comm<strong>en</strong>t représ<strong>en</strong>ter ces systèmes d’argum<strong>en</strong>tation<br />
pour pouvoir les comparer, et comm<strong>en</strong>t les fusionner, afin <strong>de</strong> déterminer les<br />
argum<strong>en</strong>ts acceptables pour le groupe.<br />
Cela nécessite <strong>en</strong> particulier la définition d’un nouveau cadre d’argum<strong>en</strong>tation, les<br />
PAF (pour Partial Argum<strong>en</strong>tation Framework ou cadre d’argum<strong>en</strong>tation partiel). En<br />
effet la définition d’un cadre d’argum<strong>en</strong>tation <strong>de</strong> Dung est simplem<strong>en</strong>t la donnée d’un<br />
<strong>en</strong>semble d’objets (les argum<strong>en</strong>ts) et d’une relation <strong>en</strong>tre ces objets (la relation d’attaque).<br />
Ce que signifie l’abs<strong>en</strong>ce <strong>de</strong> relation <strong>en</strong>tre <strong>de</strong>ux argum<strong>en</strong>ts n’est pas clair. Est-ce<br />
que cela signifie que l’ag<strong>en</strong>t sait (croit) qu’il n’y a pas d’attaque <strong>en</strong>tre ces <strong>de</strong>ux argum<strong>en</strong>ts<br />
ou qu’il ne sait pas s’il y a une attaque <strong>en</strong>tre ces <strong>de</strong>ux argum<strong>en</strong>ts ? Lorsque l’on<br />
regar<strong>de</strong> la définition <strong><strong>de</strong>s</strong> solutions (ext<strong>en</strong>sions) dans ce cadre, on se r<strong>en</strong>d compte que<br />
l’interprétation par défaut est que l’ag<strong>en</strong>t sait qu’il n’y a pas d’attaque <strong>en</strong>tre les argum<strong>en</strong>ts.<br />
Nous avons donc défini un cadre où au lieu <strong>de</strong> la dichotomie habituelle attaque<br />
/ pas d’attaque, on a une trichotomie attaque / pas d’attaque / indéterminé. Cela est<br />
nécessaire pour pouvoir représ<strong>en</strong>ter l’abs<strong>en</strong>ce d’information d’un <strong><strong>de</strong>s</strong> ag<strong>en</strong>ts <strong>à</strong> propos<br />
d’un argum<strong>en</strong>t.<br />
Nous ne détaillons pas plus cette approche ici, puisque l’article correspondant est<br />
inclus dans la <strong>de</strong>uxième partie du docum<strong>en</strong>t (chapitre 14).<br />
67
6.1 Fusion itérée<br />
6.2 Fusion <strong>de</strong> croyances comme un jeu <strong>en</strong>tre sources<br />
6.3 Opérateurs <strong>de</strong> conflu<strong>en</strong>ce<br />
6.4 Vers une caractérisation <strong>de</strong> la conciliation<br />
Chapitre VI<br />
NÉGOCIATION<br />
Il faut jouer pour <strong>de</strong>v<strong>en</strong>ir sérieux.<br />
(Aristote)<br />
La problématique <strong>de</strong> la négociation est une question complexe. Elle a été beaucoup<br />
étudiée <strong>en</strong> économie, où <strong><strong>de</strong>s</strong> modèles <strong>de</strong> théorie <strong><strong>de</strong>s</strong> jeux (<strong>en</strong> particulier<br />
dans le cadre <strong><strong>de</strong>s</strong> jeux coopératifs), comme le modèle du marchandage (bargaining)<br />
étudié par Nash [NJ50], ont apporté quelques réponses. Elle est étudiée égalem<strong>en</strong>t<br />
<strong>de</strong>puis longtemps <strong>en</strong> intellig<strong>en</strong>ce artificielle et plus particulièrem<strong>en</strong>t dans le cadre<br />
<strong><strong>de</strong>s</strong> systèmes multi-ag<strong>en</strong>ts. Mais ce qui est étudié dans la plupart <strong>de</strong> ces travaux est la<br />
mise <strong>en</strong> oeuvre <strong>de</strong> procédures <strong>de</strong> négociation, c’est-<strong>à</strong>-dire la proposition <strong>de</strong> métho<strong><strong>de</strong>s</strong><br />
ad hoc. Ces métho<strong><strong>de</strong>s</strong> utilis<strong>en</strong>t une palette d’outils très différ<strong>en</strong>ts puisque se pos<strong>en</strong>t <strong><strong>de</strong>s</strong><br />
problèmes <strong>de</strong> communication, d’argum<strong>en</strong>tation, <strong>de</strong> modélisation <strong><strong>de</strong>s</strong> préfér<strong>en</strong>ces et <strong><strong>de</strong>s</strong><br />
croyances, <strong>de</strong> recherche <strong>de</strong> compromis, etc.<br />
Nous p<strong>en</strong>sons qu’avant d’étudier un problème mélangeant <strong><strong>de</strong>s</strong> notions aussi différ<strong>en</strong>tes,<br />
il serait utile <strong>de</strong> l’abstraire, afin <strong>de</strong> t<strong>en</strong>ter dans un premier temps <strong>de</strong> résoudre le<br />
problème c<strong>en</strong>tral, avant d’y ajouter <strong><strong>de</strong>s</strong> modalités pratiques qui complexifi<strong>en</strong>t beaucoup<br />
l’étu<strong>de</strong>.<br />
Nous souhaitons étudier et caractériser <strong><strong>de</strong>s</strong> procédures <strong>de</strong> négociations abstraites,<br />
c’est-<strong>à</strong>-dire <strong><strong>de</strong>s</strong> fonctions qui pr<strong>en</strong>n<strong>en</strong>t comme donnée un profil <strong>de</strong> bases (une par<br />
ag<strong>en</strong>t) et qui produit un nouveau profil, ou les conflits ont disparu, ou tout au moins ont<br />
été réduits. Nous avons appelé ces procédures <strong>de</strong> négociations abstraites, <strong><strong>de</strong>s</strong> opérateurs<br />
<strong>de</strong> conciliation.<br />
Nous avons déj<strong>à</strong> exploré <strong><strong>de</strong>s</strong> opérateurs particuliers <strong>de</strong> conciliation que nous détaillons<br />
dans les sections ci-<strong><strong>de</strong>s</strong>sous. Ainsi, nous n’avons étudié jusqu’ici que la <strong>de</strong>uxième<br />
étape pour ce problème : la proposition <strong>de</strong> métho<strong><strong>de</strong>s</strong> ad hoc. D’autres auteurs ont<br />
égalem<strong>en</strong>t proposé <strong><strong>de</strong>s</strong> métho<strong><strong>de</strong>s</strong> pour résoudre ce type <strong>de</strong> problème <strong>de</strong> négociation<br />
[Boo02, Boo06, MFZK04, MPP05, Zha05, ZZ08, Zha07].<br />
Ce que nous comptons faire dans le futur est <strong>de</strong> passer <strong>à</strong> la troisième étape, c’est-<strong>à</strong>-<br />
69
dire la caractérisation logique <strong>de</strong> ces opérateurs <strong>de</strong> conciliation. Nous revi<strong>en</strong>drons sur<br />
cette perspective <strong>à</strong> la <strong>de</strong>rnière section.<br />
6.1. Fusion itérée<br />
Une partie <strong>de</strong> nos travaux a glissé <strong>de</strong> la problématique <strong>de</strong> la fusion <strong>de</strong> croyances<br />
<strong>à</strong> celle <strong>de</strong> la négociation. L’exemple symptomatique <strong>de</strong> ce glissem<strong>en</strong>t est celui <strong>de</strong> la<br />
fusion itérée que nous détaillons dans cette section, avant d’évoquer dans les <strong>de</strong>ux<br />
sections suivantes d’autres travaux utilisant les outils issus <strong>de</strong> la fusion <strong>de</strong> croyances<br />
pour définir <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> conciliation.<br />
Nous avons voulu définir <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion améliorés. L’idée est que si l’on<br />
considère le résultat <strong>de</strong> la fusion comme la croyance « moy<strong>en</strong>ne » du groupe, il doit<br />
alors être possible <strong>de</strong> trouver un résultat plus précis <strong>à</strong> la fusion <strong>de</strong> croyances <strong>en</strong> réalisant<br />
une fusion itérée : le principe est, <strong>à</strong> partir d’un profil <strong>de</strong> bases <strong>de</strong> croyances, <strong>de</strong> calculer<br />
le résultat <strong>de</strong> la fusion <strong>de</strong> ces bases, puis <strong>de</strong> répercuter cette fusion dans les croyances<br />
<strong>de</strong> chaque ag<strong>en</strong>t (donc chaque ag<strong>en</strong>t ti<strong>en</strong>t compte <strong><strong>de</strong>s</strong> positions <strong><strong>de</strong>s</strong> autres) <strong>en</strong> utilisant<br />
un opérateur <strong>de</strong> révision. On itère ce processus jusqu’<strong>à</strong> ce que l’on atteigne un point<br />
fixe [7, 31, 50]. Malheureusem<strong>en</strong>t les opérateurs <strong>de</strong> fusion itérée ainsi définis n’ont pas<br />
<strong>de</strong> très bonnes propriétés logiques, <strong>en</strong> tant qu’opérateurs <strong>de</strong> fusion. En revanche, il peut<br />
être intéressant <strong>de</strong> considérer ce processus comme une opération <strong>de</strong> conciliation.<br />
6.2. Fusion <strong>de</strong> croyances comme un jeu <strong>en</strong>tre sources<br />
Nous avons défini une nouvelle famille d’opérateurs <strong>de</strong> fusion <strong>de</strong> croyances (ou<br />
<strong>de</strong> buts), basée sur un jeu <strong>en</strong>tre les sources pr<strong>en</strong>ant part <strong>à</strong> la fusion [4, 47, 55]. Le<br />
principe <strong>de</strong> ce jeu est le suivant : <strong>à</strong> chaque tour, un <strong>en</strong>semble <strong>de</strong> perdants est défini<br />
(intuitivem<strong>en</strong>t, les perdants sont les sources dont les croyances (ou les buts) sont le<br />
plus loin <strong>de</strong> celles du groupe), ces perdants doiv<strong>en</strong>t affaiblir leurs croyances (buts). Le<br />
jeu s’arrête lorsque le profil <strong><strong>de</strong>s</strong> croyances/buts <strong><strong>de</strong>s</strong> ag<strong>en</strong>ts est cohér<strong>en</strong>t. Un opérateur<br />
particulier est donc défini par la métho<strong>de</strong> <strong>de</strong> désignation <strong><strong>de</strong>s</strong> perdants et par la métho<strong>de</strong><br />
d’affaiblissem<strong>en</strong>t choisie. Cette famille d’opérateurs est intéressante, puisqu’elle peut<br />
être vue comme un compromis <strong>en</strong>tre fusion <strong>de</strong> croyances et négociation. Nous avons<br />
comparé ces opérateurs aux opérateurs existants et étudié les propriétés logiques <strong>de</strong><br />
fusion contrainte satisfaites. Nous ne détaillons pas plus cette approche ici, puisque<br />
l’article correspondant est inclus dans la <strong>de</strong>uxième partie du docum<strong>en</strong>t (chapitre 15).<br />
Il reste <strong>à</strong> trouver une caractérisation logique <strong>de</strong> ces opérateurs et <strong>à</strong> étudier plus<br />
profondém<strong>en</strong>t cette piste d’utilisation <strong>de</strong> la théorie <strong><strong>de</strong>s</strong> jeux (qui pour le mom<strong>en</strong>t est<br />
réduite <strong>à</strong> sa plus simple expression), afin <strong>de</strong> fournir un cadre formel <strong>à</strong> <strong><strong>de</strong>s</strong> opérateurs<br />
<strong>de</strong> négociation, ou pour étudier plus finem<strong>en</strong>t <strong><strong>de</strong>s</strong> phénomènes <strong>de</strong> coalitions ou <strong>de</strong><br />
manipulation dans <strong><strong>de</strong>s</strong> cadres <strong>de</strong> fusion.<br />
En particulier nous avons proposé d’utiliser les mesures d’incohér<strong>en</strong>ce que nous<br />
avons développées (section 3.2.3) pour sélectionner comme perdants les sources les<br />
plus conflictuelles [51]. Nous p<strong>en</strong>sons que cette voie est prometteuse.<br />
70
6.3. Opérateurs <strong>de</strong> conflu<strong>en</strong>ce<br />
Beaucoup d’opérateurs <strong>de</strong> changem<strong>en</strong>t ont été étudiés ces <strong>de</strong>rnières années. Un certain<br />
nombre d’<strong>en</strong>tre eux sont <strong>à</strong> prés<strong>en</strong>t assez bi<strong>en</strong> cernés, tels que la révision, qui permet<br />
d’incorporer <strong>de</strong> nouvelles informations sur le mon<strong>de</strong>, la mise-<strong>à</strong>-jour [KM91a, HR99],<br />
qui permet d’actualiser les croyances <strong>de</strong> l’ag<strong>en</strong>t pour pr<strong>en</strong>dre <strong>en</strong> compte une évolution<br />
du mon<strong>de</strong>, et la fusion qui permet <strong>de</strong> produire une information cohér<strong>en</strong>te <strong>à</strong> partir<br />
d’un <strong>en</strong>semble d’informations contradictoires. Il existe <strong>de</strong> forts li<strong>en</strong>s <strong>en</strong>tre révision et<br />
mise-<strong>à</strong>-jour et <strong>en</strong>tre révision et fusion. En effet, techniquem<strong>en</strong>t, la mise-<strong>à</strong>-jour peut être<br />
vue comme une révision « ponctuelle », et la révision peut être vue comme un cas particulier<br />
<strong>de</strong> fusion. Ces li<strong>en</strong>s suggérai<strong>en</strong>t la possibilité <strong>de</strong> définir une nouvelle famille<br />
d’opérateurs qui sont <strong>à</strong> la mise-<strong>à</strong>-jour ce que la fusion est <strong>à</strong> la révision, et qui pourrai<strong>en</strong>t<br />
donc être vus comme <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> fusion « ponctuelle ». Cela est résumé <strong>à</strong><br />
la figure 6.1.<br />
Révision<br />
Mise-<strong>à</strong>-jour<br />
Fusion<br />
Conflu<strong>en</strong>ce<br />
FIGURE 6.1 – Révision - Mise-<strong>à</strong>-jour - Fusion - Conflu<strong>en</strong>ce<br />
Nous avons défini et caractérisé ces opérateurs, que nous avons nommés opérateurs<br />
<strong>de</strong> conflu<strong>en</strong>ce [37, 54, 72]. Il apparaît que ces opérateurs sont beaucoup moins<br />
sélectifs que les opérateurs <strong>de</strong> fusion, et qu’ils peuv<strong>en</strong>t être utilisés pour définir <strong><strong>de</strong>s</strong> opérateurs<br />
<strong>de</strong> négociation <strong>en</strong> fournissant l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> points d’accord possibles <strong>en</strong>tre les<br />
opinions <strong><strong>de</strong>s</strong> différ<strong>en</strong>ts ag<strong>en</strong>ts.<br />
Nous ne détaillons pas plus cette approche ici, puisque l’article correspondant est<br />
inclus dans la <strong>de</strong>uxième partie du docum<strong>en</strong>t (chapitre 16).<br />
6.4. Vers une caractérisation <strong>de</strong> la conciliation<br />
Nous souhaitons caractériser les opérateurs <strong>de</strong> conciliation, c’est-<strong>à</strong>-dire les procédures<br />
<strong>de</strong> négociation abstraite qui pr<strong>en</strong>n<strong>en</strong>t comme donnée un profil <strong>de</strong> bases <strong>de</strong><br />
croyances (une par ag<strong>en</strong>t) et qui produis<strong>en</strong>t un nouveau profil, où les conflits ont disparu,<br />
ou tout au moins ont été réduits.<br />
Il peut sembler ambitieux, voire utopique, <strong>de</strong> vouloir caractériser aussi simplem<strong>en</strong>t<br />
un processus aussi compliqué que la négociation, nécessitant <strong><strong>de</strong>s</strong> échanges <strong>en</strong>tre ag<strong>en</strong>ts,<br />
<strong>de</strong> l’argum<strong>en</strong>tation, <strong><strong>de</strong>s</strong> compromis, etc. Mais on peut remarquer qu’un problème similaire<br />
<strong>de</strong> négociation a été résolu <strong>de</strong> manière abstraite dans le domaine <strong>de</strong> la théorie <strong><strong>de</strong>s</strong><br />
jeux. Le problème du marchandage (bargaining) s’énonce comme suit : étant donné<br />
un <strong>en</strong>semble (convexe et compact) d’issues possibles <strong>à</strong> la négociation, et un point <strong>de</strong><br />
désaccord (qui sera le résultat par défaut si les joueurs ne se mett<strong>en</strong>t pas d’accord sur<br />
71
une issue), est-il possible <strong>de</strong> trouver une issue unique résultat <strong>de</strong> la négociation ? Nash<br />
a montré que cela était possible et a caractérisé axiomatiquem<strong>en</strong>t cette solution [NJ50].<br />
Dans l’étu<strong>de</strong> <strong><strong>de</strong>s</strong> jeux <strong>de</strong> coalitions, un autre problème abstrait a été résolu. Le<br />
but est, étant donné un <strong>en</strong>semble <strong>de</strong> joueurs pouvant former les coalitions qu’ils souhait<strong>en</strong>t,<br />
chacune <strong>de</strong> ces coalitions gagnant un certain montant, <strong>de</strong> trouver le juste montant<br />
<strong>de</strong>vant rev<strong>en</strong>ir <strong>à</strong> chaque joueur (une allocation). Ce problème semble égalem<strong>en</strong>t<br />
compliqué, parce que, pour se mettre d’accord sur une allocation, les joueurs doiv<strong>en</strong>t<br />
discuter, argum<strong>en</strong>ter, etc. Mais Shapley a montré qu’il était possible <strong>de</strong> déterminer une<br />
telle allocation et il l’a caractérisée axiomatiquem<strong>en</strong>t [Sha53].<br />
Ces <strong>de</strong>ux exemples illustr<strong>en</strong>t le fait que l’on peut résoudre ce type <strong>de</strong> problèmes<br />
compliqués <strong>de</strong> manière convaincante, <strong>en</strong> capturant l’ess<strong>en</strong>ce du problème, et <strong>en</strong> ne se<br />
laissant pas <strong>en</strong>combrer par <strong><strong>de</strong>s</strong> modalités pratiques non fondam<strong>en</strong>tales.<br />
Nous souhaitons t<strong>en</strong>ter <strong>de</strong> trouver une solution similaire pour le problème <strong>de</strong> la<br />
négociation abstraite <strong>en</strong> logique (conciliation).<br />
Outre le défi sci<strong>en</strong>tifique que cela représ<strong>en</strong>te, nous p<strong>en</strong>sons que trouver une telle caractérisation<br />
est importante pour toutes les personnes travaillant sur la négociation. En<br />
effet, la plupart <strong><strong>de</strong>s</strong> travaux sur la négociation <strong>en</strong> intellig<strong>en</strong>ce artificielle, notamm<strong>en</strong>t<br />
dans le domaine <strong><strong>de</strong>s</strong> systèmes multi-ag<strong>en</strong>ts, port<strong>en</strong>t sur la proposition <strong>de</strong> métho<strong><strong>de</strong>s</strong><br />
ad hoc, utilisant différ<strong>en</strong>tes techniques (argum<strong>en</strong>tation, etc.). Il est difficile <strong>de</strong> comparer<br />
ces travaux, utilisant souv<strong>en</strong>t <strong><strong>de</strong>s</strong> hypothèses ou <strong><strong>de</strong>s</strong> techniques assez différ<strong>en</strong>tes.<br />
Disposer d’une caractérisation logique, et donc d’un <strong>en</strong>semble <strong>de</strong> propriétés logiques<br />
caractérisant le processus <strong>de</strong> conciliation (négociation), permettrait <strong>de</strong> comparer ces<br />
métho<strong><strong>de</strong>s</strong>, <strong>en</strong> examinant les propriétés satisfaites par chacune d’elles.<br />
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7.1 Mesures <strong>de</strong> conflit<br />
7.2 Fusion et choix social<br />
7.3 Economie et représ<strong>en</strong>tation <strong><strong>de</strong>s</strong> connaissances<br />
Chapitre VII<br />
CONCLUSION ET PERSPECTIVES<br />
La sci<strong>en</strong>ce consiste <strong>à</strong> passer d’un étonnem<strong>en</strong>t <strong>à</strong> un autre.<br />
(Aristote)<br />
Nous avons insisté sur la nécessité d’obt<strong>en</strong>ir <strong><strong>de</strong>s</strong> caractérisations logiques pour les<br />
différ<strong>en</strong>ts problèmes étudiés <strong>en</strong> intellig<strong>en</strong>ce artificielle. En ce qui concerne les différ<strong>en</strong>ts<br />
cadres que nous avons étudié, nous avons <strong>en</strong> particulier obt<strong>en</strong>us les caractérisations<br />
suivantes :<br />
• Incohér<strong>en</strong>ce : nous avons proposé la première caractérisation d’une valeur d’incohér<strong>en</strong>ce.<br />
Nous p<strong>en</strong>sons qu’il reste beaucoup <strong>à</strong> faire dans ce domaine. Ces perspectives<br />
seront détaillées ci-<strong><strong>de</strong>s</strong>sous.<br />
• Révision : nous avons défini et caractérisé les opérateurs d’amélioration, qui sont<br />
une généralisation <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> révision itérée usuels, et qui ouvr<strong>en</strong>t la voie<br />
<strong>à</strong> la définition <strong>de</strong> nouveaux opérateurs <strong>de</strong> changem<strong>en</strong>t.<br />
• Fusion : nous avons caractérisé les opérateurs <strong>de</strong> fusion contrainte, et étudié<br />
d’autres propriétés comme la non manipulabilité ou le « truth tracking ».<br />
• Négociation : la caractérisation <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong> conciliation est notre principale<br />
perspective. Nous avons pour le mom<strong>en</strong>t étudié <strong><strong>de</strong>s</strong> métho<strong><strong>de</strong>s</strong> particulières.<br />
Nous avons caractérisé les opérateurs <strong>de</strong> conflu<strong>en</strong>ce. Cela peut peut-être servir<br />
<strong>de</strong> point <strong>de</strong> départ pour la recherche d’une caractérisation <strong><strong>de</strong>s</strong> opérateurs <strong>de</strong><br />
conciliation.<br />
Nous avons énuméré dans les chapitres correspondants les pistes principales qui<br />
nous semblai<strong>en</strong>t prometteuses dans les différ<strong>en</strong>ts travaux prés<strong>en</strong>tés. Nous ne répéterons<br />
donc pas ici les perspectives <strong>de</strong> chacun <strong>de</strong> ces chapitres.<br />
Nous allons plutôt nous focaliser dans ce chapitre sur l’ori<strong>en</strong>tation générale que<br />
nous comptons donner <strong>à</strong> nos travaux, et donc les perspectives que nous comptons étudier.<br />
On peut résumer notre projet sci<strong>en</strong>tifique sous la formule :<br />
Du conflit <strong>en</strong>tre formules logiques vers le conflit <strong>en</strong>tre ag<strong>en</strong>ts<br />
En effet, l’ori<strong>en</strong>tation générale <strong>de</strong> notre projet <strong>de</strong> recherche consiste <strong>à</strong> continuer <strong>à</strong><br />
suivre la même démarche sci<strong>en</strong>tifique, et principalem<strong>en</strong>t les aspects <strong>de</strong> caractérisation<br />
73
logique <strong>de</strong> différ<strong>en</strong>ts problèmes, mais <strong>en</strong> glissant <strong>de</strong> problèmes <strong>de</strong> raisonnem<strong>en</strong>t (et <strong>de</strong><br />
« conflits <strong>en</strong>tre formules logiques ») vers <strong><strong>de</strong>s</strong> problématiques <strong>de</strong> conflits <strong>en</strong>tre ag<strong>en</strong>ts,<br />
et particulièrem<strong>en</strong>t vers la négociation et plus généralem<strong>en</strong>t la théorie <strong><strong>de</strong>s</strong> jeux.<br />
Plus précisém<strong>en</strong>t, on peut décrire ces perspectives selon quatre axes principaux :<br />
• Le développem<strong>en</strong>t <strong>de</strong> métho<strong><strong>de</strong>s</strong> <strong>de</strong> mesure <strong>de</strong> conflit.<br />
• L’étu<strong>de</strong> <strong><strong>de</strong>s</strong> li<strong>en</strong>s qui exist<strong>en</strong>t <strong>en</strong>tre la fusion et la théorie du choix social.<br />
• La caractérisation logique <strong>de</strong> la négociation.<br />
• La théorie <strong><strong>de</strong>s</strong> jeux dans <strong><strong>de</strong>s</strong> cadres d’incertitu<strong>de</strong> qualitative.<br />
Ces points sont ordonnés selon leur proximité avec nos thèmes <strong>de</strong> recherche actuels.<br />
Les <strong>de</strong>ux premiers points, concernant les mesures <strong>de</strong> conflit et les li<strong>en</strong>s <strong>en</strong>tre fusion<br />
et théorie du choix social, représ<strong>en</strong>t<strong>en</strong>t la continuation <strong>de</strong> nos travaux actuels, mais <strong>en</strong><br />
se tournant un peu plus vers la théorie du choix social.<br />
Le troisième point, qui nous semble être le plus prometteur et qui est celui sur lequel<br />
nous allons nous conc<strong>en</strong>trer <strong>en</strong> priorité, concerne la négociation, qui est <strong>à</strong> l’intersection<br />
<strong>de</strong> problématiques d’intellig<strong>en</strong>ce artificielle et d’économie. Nous ne le détaillerons pas<br />
ci-<strong><strong>de</strong>s</strong>sous puisqu’il a déj<strong>à</strong> été l’objet du chapitre précé<strong>de</strong>nt (voir la section 6.4).<br />
Le <strong>de</strong>rnier point est réellem<strong>en</strong>t un problème d’économie (théorie <strong><strong>de</strong>s</strong> jeux) pour<br />
lequel nous souhaitons apporter un éclairage v<strong>en</strong>ant <strong>de</strong> l’intellig<strong>en</strong>ce artificielle (problèmes<br />
<strong>de</strong> représ<strong>en</strong>tation <strong>de</strong> l’incertitu<strong>de</strong>, d’incertitu<strong>de</strong> qualitative, et d’ordinalité).<br />
7.1. Mesures <strong>de</strong> conflit<br />
La mesure du <strong>de</strong>gré <strong>de</strong> conflit n’est pas très développée pour le mom<strong>en</strong>t, malgré le<br />
réel besoin <strong>de</strong> telles mesures. En effet, pouvoir estimer <strong>à</strong> quel point un <strong>en</strong>semble d’informations<br />
(ou d’ag<strong>en</strong>ts) est conflictuel est aussi important que d’estimer <strong>à</strong> quel point<br />
il est informatif. Or, alors que les mesures d’informations sont étudiées <strong>de</strong>puis longtemps<br />
(citons Shannon (1950)), les premiers travaux sur <strong><strong>de</strong>s</strong> mesures <strong>de</strong> conflits sont<br />
très réc<strong>en</strong>ts. Les perspectives principales <strong>de</strong> l’utilisation <strong>de</strong> telles mesures <strong>de</strong> <strong>de</strong>gré <strong>de</strong><br />
conflit concern<strong>en</strong>t les coalitions d’ag<strong>en</strong>ts. Si les coalitions sont déj<strong>à</strong> fixées, ces mesures<br />
peuv<strong>en</strong>t permettre d’indiquer <strong>à</strong> quel point les coalitions sont proches, ou opposées ; et<br />
<strong>de</strong> quelles coalitions une coalition donnée est la plus proche. Elle peut égalem<strong>en</strong>t r<strong>en</strong>seigner<br />
sur la robustesse <strong><strong>de</strong>s</strong> différ<strong>en</strong>tes coalitions. Si les coalitions ne sont pas <strong>en</strong>core<br />
déterminées, c’est-<strong>à</strong>-dire que l’on a comme donnée un <strong>en</strong>semble d’ag<strong>en</strong>ts qui peuv<strong>en</strong>t<br />
former <strong><strong>de</strong>s</strong> coalitions quelconques, les mesures <strong>de</strong> <strong>de</strong>gré <strong>de</strong> conflit peuv<strong>en</strong>t r<strong>en</strong>seigner<br />
sur la coalition la plus intéressante pour un ag<strong>en</strong>t, sur les coalitions les plus probables<br />
ou sur les points posant le plus problème <strong>en</strong>tre <strong>de</strong>ux coalitions dans une optique <strong>de</strong><br />
résolution <strong>de</strong> conflits par exemple. Ces mesures <strong>de</strong> conflits peuv<strong>en</strong>t égalem<strong>en</strong>t nous indiquer<br />
les ag<strong>en</strong>ts (ou les coalitions d’ag<strong>en</strong>ts) qui <strong>en</strong>g<strong>en</strong>dr<strong>en</strong>t le plus <strong>de</strong> conflits pour le<br />
groupe dans son <strong>en</strong>semble. Elles peuv<strong>en</strong>t donc être utilisées pour gui<strong>de</strong>r un processus<br />
<strong>de</strong> négociation, afin <strong>de</strong> se conc<strong>en</strong>trer sur les points/ag<strong>en</strong>ts les plus conflictuels.<br />
Nous avons déj<strong>à</strong> travaillé <strong>à</strong> la définition <strong>de</strong> telles mesures (section 3.2), mais il<br />
reste <strong>en</strong>core beaucoup <strong>à</strong> faire. Nous comptons continuer <strong>à</strong> travailler sur ce sujet. Nous<br />
sommes convaincus qu’il reste beaucoup <strong>de</strong> mesures <strong>à</strong> définir et <strong>à</strong> caractériser (pour le<br />
mom<strong>en</strong>t la seule caractérisation d’une mesure d’incohér<strong>en</strong>ce est celle <strong>de</strong> [36]).<br />
74
Une autre piste concerne le conflit pot<strong>en</strong>tiel. Les mesures définies pour le mom<strong>en</strong>t<br />
s’intéress<strong>en</strong>t au conflit logique déj<strong>à</strong> cont<strong>en</strong>u dans les bases. Il peut être intéressant<br />
dans <strong>de</strong> nombreux cas <strong>de</strong> ne pas mesurer le conflit actuel, mais le conflit pot<strong>en</strong>tiel, qui<br />
permettra d’évaluer <strong>à</strong> quel point les exig<strong>en</strong>ces <strong>de</strong> différ<strong>en</strong>ts ag<strong>en</strong>ts peuv<strong>en</strong>t être (pot<strong>en</strong>tiellem<strong>en</strong>t)<br />
problématiques. En effet, détecter et résoudre les confits pot<strong>en</strong>tiels avant<br />
qu’ils ne survi<strong>en</strong>n<strong>en</strong>t et qu’il ne soit trop tard est une problématique intéressante. Cela<br />
peut être très utile dans <strong><strong>de</strong>s</strong> cas <strong>de</strong> conception <strong>de</strong> logiciels, lorsque les spécifications<br />
provi<strong>en</strong>n<strong>en</strong>t <strong>de</strong> plusieurs utilisateurs. Il peut être inutile et coûteux <strong>de</strong> développer le<br />
logiciel pour se r<strong>en</strong>dre compte trop tard que les spécifications <strong>de</strong>mandées n’étai<strong>en</strong>t pas<br />
compatibles. Il est donc intéressant d’i<strong>de</strong>ntifier au plus tôt les points susceptibles <strong>de</strong><br />
générer un conflit pot<strong>en</strong>tiel, afin <strong>de</strong> clarifier les spécifications et d’éviter <strong><strong>de</strong>s</strong> problèmes<br />
ultérieurs. Plus généralem<strong>en</strong>t, la détection <strong>de</strong> conflits pot<strong>en</strong>tiels peut être utile pour les<br />
processus <strong>de</strong> négociation <strong>en</strong>tre ag<strong>en</strong>ts.<br />
Une <strong>de</strong>rnière piste est <strong>de</strong> travailler sur <strong><strong>de</strong>s</strong> mesures <strong>de</strong> conflits pour <strong><strong>de</strong>s</strong> informations<br />
plus « riches », plus structurées, que la logique propositionelle, comme les pré-ordres<br />
par exemple. Les préfér<strong>en</strong>ces <strong><strong>de</strong>s</strong> ag<strong>en</strong>ts sont souv<strong>en</strong>t représ<strong>en</strong>tées avec <strong><strong>de</strong>s</strong> pré-ordres.<br />
Nous souhaitons étudier les mesures <strong>de</strong> conflits <strong>en</strong>tre pré-ordres, afin <strong>de</strong> pouvoir égalem<strong>en</strong>t<br />
mesurer le conflit <strong>en</strong>tre les préfér<strong>en</strong>ces <strong>de</strong> différ<strong>en</strong>ts ag<strong>en</strong>ts. Cela nécessitera<br />
<strong>de</strong> se tourner vers la théorie du choix social, <strong>en</strong> particulier les métho<strong><strong>de</strong>s</strong> <strong>de</strong> vote qui<br />
considèr<strong>en</strong>t <strong>de</strong> tels pré-ordres.<br />
7.2. Fusion et choix social<br />
Nous désirons continuer <strong>à</strong> étudier les li<strong>en</strong>s <strong>en</strong>tre fusion et choix social. Nous avons<br />
déj<strong>à</strong> obt<strong>en</strong>u <strong><strong>de</strong>s</strong> résultats intéressants <strong>en</strong> étudiant ce que la manipulabilité, une notion<br />
bi<strong>en</strong> étudiée <strong>en</strong> choix social, a comme conséqu<strong>en</strong>ces pour la fusion <strong>de</strong> croyances [9,<br />
27]. D’autres notions issues du choix social peuv<strong>en</strong>t se montrer intéressantes pour la<br />
fusion <strong>de</strong> croyances.<br />
Il s’est développé ces <strong>de</strong>rnières années <strong>en</strong> théorie du choix social une théorie <strong>de</strong><br />
l’agrégation <strong>de</strong> jugem<strong>en</strong>t, qui est un problème proche <strong>de</strong> la fusion. Pour la fusion <strong>de</strong><br />
croyances, on dispose comme données <strong>de</strong> l’<strong>en</strong>semble <strong><strong>de</strong>s</strong> bases <strong><strong>de</strong>s</strong> ag<strong>en</strong>ts, alors que<br />
pour l’agrégation <strong>de</strong> jugem<strong>en</strong>ts, on ne dispose que <strong>de</strong> la réponse <strong><strong>de</strong>s</strong> ag<strong>en</strong>ts <strong>à</strong> un certain<br />
nombre <strong>de</strong> questions (jugem<strong>en</strong>ts). Cela change assez s<strong>en</strong>siblem<strong>en</strong>t les données<br />
du problème. En particulier la plupart <strong><strong>de</strong>s</strong> résultats <strong>en</strong> agrégation <strong>de</strong> jugem<strong>en</strong>t sont<br />
<strong><strong>de</strong>s</strong> théorèmes d’impossibilité (voir par exemple [LP04, Lis10]), alors qu’<strong>en</strong> fusion<br />
<strong>de</strong> croyances <strong><strong>de</strong>s</strong> caractérisations logiques montr<strong>en</strong>t la faisabilité <strong>de</strong> l’approche. Il a<br />
d’ailleurs déj<strong>à</strong> été proposé d’utiliser nos opérateurs <strong>de</strong> fusion contrainte pour définir<br />
<strong><strong>de</strong>s</strong> métho<strong><strong>de</strong>s</strong> d’agrégation <strong>de</strong> jugem<strong>en</strong>t [Pig06]. Nous souhaitons examiner les li<strong>en</strong>s<br />
<strong>en</strong>tre les <strong>de</strong>ux problèmes, et étudier si les métho<strong><strong>de</strong>s</strong> issues <strong>de</strong> la fusion <strong>de</strong> croyances<br />
peuv<strong>en</strong>t apporter un nouvel éclairage aux questions posées par l’agrégation <strong>de</strong> jugem<strong>en</strong>t.<br />
Finalem<strong>en</strong>t, il nous semble intéressant d’examiner si, parmi l’<strong>en</strong>semble important<br />
<strong>de</strong> métho<strong><strong>de</strong>s</strong> <strong>de</strong> vote existantes, certaines peuv<strong>en</strong>t nous permettre <strong>de</strong> définir <strong><strong>de</strong>s</strong> opérateurs<br />
<strong>de</strong> fusion intéressants. En particulier, <strong>en</strong> <strong>de</strong>hors <strong><strong>de</strong>s</strong> métho<strong><strong>de</strong>s</strong> <strong>de</strong> votes majoritaires<br />
classiques, il existe un grand nombre <strong>de</strong> métho<strong><strong>de</strong>s</strong> définissables <strong>à</strong> partir <strong><strong>de</strong>s</strong><br />
75
graphes <strong>de</strong> majorité. Ces métho<strong><strong>de</strong>s</strong> définiss<strong>en</strong>t <strong><strong>de</strong>s</strong> <strong>en</strong>sembles <strong>de</strong> « bons » candidats <strong>à</strong><br />
partir <strong>de</strong> ces graphes. Nous souhaitons étudier si l’application <strong>de</strong> ces métho<strong><strong>de</strong>s</strong> dans<br />
le cadre <strong>de</strong> la fusion <strong>de</strong> croyances conduit <strong>à</strong> <strong><strong>de</strong>s</strong> opérateurs possédant <strong><strong>de</strong>s</strong> propriétés<br />
intéressantes.<br />
7.3. Economie et représ<strong>en</strong>tation <strong><strong>de</strong>s</strong> connaissances<br />
Les points précé<strong>de</strong>nts étai<strong>en</strong>t consacrés aux apports possibles <strong>de</strong> l’économie (théorie<br />
du choix social et théorie <strong><strong>de</strong>s</strong> jeux) <strong>à</strong> l’intellig<strong>en</strong>ce artificielle. Ce <strong>de</strong>rnier point<br />
concerne le chemin inverse, c’est-<strong>à</strong>-dire d’étudier les apports possibles <strong>de</strong> l’intellig<strong>en</strong>ce<br />
artificielle <strong>en</strong> économie.<br />
Nous avons déj<strong>à</strong> quelques résultats <strong>à</strong> ce sujet, mais nous ne les avons pas discutés<br />
dans cette synthèse car ils ne concern<strong>en</strong>t pas le raisonnem<strong>en</strong>t mais la théorie <strong>de</strong> la<br />
décision ou la théorie <strong><strong>de</strong>s</strong> jeux [53, 34, 57, 70, 33, 69].<br />
Il s’agit <strong>en</strong> particulier d’étudier comm<strong>en</strong>t les concepts <strong>de</strong> théorie <strong><strong>de</strong>s</strong> jeux support<strong>en</strong>t<br />
<strong><strong>de</strong>s</strong> cadres qualitatifs.<br />
En ce qui concerne l’incertitu<strong>de</strong>, la quasi-totalité <strong><strong>de</strong>s</strong> travaux <strong>en</strong> théorie <strong><strong>de</strong>s</strong> jeux<br />
se bas<strong>en</strong>t sur un modèle d’incertitu<strong>de</strong> bayési<strong>en</strong> (probabiliste). Nous avons déj<strong>à</strong> étudié<br />
<strong>de</strong>ux cas particuliers : la décision sous ignorance totale [53] et la théorie <strong><strong>de</strong>s</strong> jeux (non<br />
coopérative) sous incertitu<strong>de</strong> stricte [34, 57, 33]. Nous souhaitons continuer dans cette<br />
voie et étudier ce que ce changem<strong>en</strong>t <strong>de</strong> cadre d’incertitu<strong>de</strong> a comme conséqu<strong>en</strong>ces<br />
dans d’autres cadres <strong>de</strong> théorie <strong><strong>de</strong>s</strong> jeux, notamm<strong>en</strong>t dans le cadre <strong>de</strong> la théorie <strong><strong>de</strong>s</strong><br />
jeux coopérative.<br />
Il est égalem<strong>en</strong>t intéressant d’étudier ce qui survi<strong>en</strong>t lorsque l’on utilise un modèle<br />
qualitatif (ordinal) pour représ<strong>en</strong>ter les préfér<strong>en</strong>ces <strong><strong>de</strong>s</strong> ag<strong>en</strong>ts. Il est d’usage <strong>en</strong> théorie<br />
<strong><strong>de</strong>s</strong> jeux <strong>de</strong> représ<strong>en</strong>ter les préfér<strong>en</strong>ces <strong><strong>de</strong>s</strong> ag<strong>en</strong>ts <strong>à</strong> l’ai<strong>de</strong> <strong>de</strong> fonctions d’utilité numériques.<br />
Ce choix permet d’utiliser facilem<strong>en</strong>t <strong><strong>de</strong>s</strong> fonctions et résultats mathématiques.<br />
En particulier un certain nombre <strong>de</strong> résultats importants dép<strong>en</strong><strong>de</strong>nt <strong><strong>de</strong>s</strong> hypothèses <strong>de</strong><br />
continuité, <strong>de</strong> convexité et <strong>de</strong> compacité obt<strong>en</strong>ues grâce <strong>à</strong> ce choix <strong>de</strong> fonctions d’utilité<br />
numériques. Nous souhaitons étudier ce que <strong>de</strong>vi<strong>en</strong>n<strong>en</strong>t ces résultats lorsque l’on<br />
abandonne ces hypothèses et que l’on se place dans un cadre discret (et qualitatif).
Deuxième partie<br />
Articles<br />
77
LISTE DES ARTICLES<br />
Raisonnem<strong>en</strong>t sous incohér<strong>en</strong>ce<br />
• Anthony Hunter, Sébasti<strong>en</strong> Konieczny. Shapley Inconsist<strong>en</strong>cy Values. T<strong>en</strong>th International<br />
Confer<strong>en</strong>ce on Principles of Knowledge Repres<strong>en</strong>tation and Reasoning<br />
(KR’ 06). 249-259. 2006.<br />
• Anthony Hunter, Sébasti<strong>en</strong> Konieczny. Measuring inconsist<strong>en</strong>cy through minimal<br />
inconsist<strong>en</strong>t sets. Elev<strong>en</strong>th International Confer<strong>en</strong>ce on Principles of Knowledge<br />
Repres<strong>en</strong>tation and Reasoning (KR’08). 358-366. 2008.<br />
Révision<br />
• Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Improvem<strong>en</strong>t operators. Elev<strong>en</strong>th International<br />
Confer<strong>en</strong>ce on Principles of Knowledge Repres<strong>en</strong>tation and Reasoning<br />
(KR’ 08). 177-186. 2008.<br />
• Andreas Herzig, Sébasti<strong>en</strong> Konieczny, Laur<strong>en</strong>t Perussel. On iterated revision in<br />
the AGM framework. Sev<strong>en</strong>th European Confer<strong>en</strong>ce on Symbolic and Quantitative<br />
Approaches to Reasoning with Uncertainty (ECSQARU’03). 477-488.<br />
2003.<br />
Fusion<br />
• Sébasti<strong>en</strong> Konieczny, Jérôme Lang, Pierre Marquis. DA 2 merging operators. Artificial<br />
Intellig<strong>en</strong>ce. 157(1-2). 49-79. 2004.<br />
• Sylvie Coste-Marquis, Caroline Devred, Sébasti<strong>en</strong> Konieczny, Marie-Christine<br />
Lagasquie-Schiex, Pierre Marquis. On the merging of Dung’s argum<strong>en</strong>tation systems.<br />
Artificial Intellig<strong>en</strong>ce. 171. 740-753. 2007.<br />
Négociation<br />
• Sébasti<strong>en</strong> Konieczny. Belief base merging as a game. Journal of Applied Non-<br />
Classical Logics. 14(3). 275-294. 2004.<br />
• Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Conflu<strong>en</strong>ce operators. Elev<strong>en</strong>th European<br />
Confer<strong>en</strong>ce on Logics in Artificial Intellig<strong>en</strong>ce (JELIA’08). 272-284. 2008.<br />
79
SHAPLEY INCONSISTENCY VALUES<br />
Anthony Hunter, Sébasti<strong>en</strong> Konieczny.<br />
T<strong>en</strong>th International Confer<strong>en</strong>ce on Principles of Knowledge Repres<strong>en</strong>tation<br />
and Reasoning (KR’06).<br />
pages 249-259.<br />
2006.<br />
81
Shapley Inconsist<strong>en</strong>cy Values<br />
Anthony Hunter<br />
Departm<strong>en</strong>t of Computer Sci<strong>en</strong>ce<br />
University College London, UK<br />
a.hunter@cs.ucl.ac.uk<br />
Sébasti<strong>en</strong> Konieczny<br />
CRIL - CNRS<br />
Université d’Artois, L<strong>en</strong>s, France<br />
konieczny@cril.univ-artois.fr<br />
Abstract<br />
There are relatively few proposals for inconsist<strong>en</strong>cy measures<br />
for propositional belief bases. However inconsist<strong>en</strong>cy measures<br />
are pot<strong>en</strong>tially as important as information measures<br />
for artificial intellig<strong>en</strong>ce, and more g<strong>en</strong>erally for computer<br />
sci<strong>en</strong>ce. In particular, they can be useful to <strong>de</strong>fine various<br />
operators for belief revision, belief merging, and negotiation.<br />
The measures that have be<strong>en</strong> proposed so far can be split into<br />
two classes. The first class of measures takes into account the<br />
number of formulae required to produce an inconsist<strong>en</strong>cy: the<br />
more formulae required to produce an inconsist<strong>en</strong>cy, the less<br />
inconsist<strong>en</strong>t the base. The second class takes into account<br />
the proportion of the language that is affected by the inconsist<strong>en</strong>cy:<br />
the more propositional variables affected, the more<br />
inconsist<strong>en</strong>t the base. Both approaches are s<strong>en</strong>sible, but there<br />
is no proposal for combining them. We address this need in<br />
this paper: our proposal takes into account both the number of<br />
variables affected by the inconsist<strong>en</strong>cy and the distribution of<br />
the inconsist<strong>en</strong>cy among the formulae of the base. Our i<strong>de</strong>a<br />
is to use existing inconsist<strong>en</strong>cy measures (ones that takes into<br />
account the proportion of the language affected by the inconsist<strong>en</strong>cy,<br />
and so allow us to look insi<strong>de</strong> the formulae) in or<strong>de</strong>r<br />
to <strong>de</strong>fine a game in coalitional form, and th<strong>en</strong> to use the Shapley<br />
value to obtain an inconsist<strong>en</strong>cy measure that indicates<br />
the responsibility/contribution of each formula to the overall<br />
inconsist<strong>en</strong>cy in the base. This allows us to provi<strong>de</strong> a more<br />
reliable image of the belief base and of the inconsist<strong>en</strong>cy in<br />
it.<br />
Introduction<br />
There are numerous works on reasoning un<strong>de</strong>r inconsist<strong>en</strong>cy.<br />
One can quote for example paraconsist<strong>en</strong>t logics,<br />
argum<strong>en</strong>tation frameworks, belief revision and fusion, etc.<br />
All these approaches illustrate the fact that the dichotomy<br />
betwe<strong>en</strong> consist<strong>en</strong>t and inconsist<strong>en</strong>t sets of formulae that<br />
comes from classical logics is not suffici<strong>en</strong>t for <strong><strong>de</strong>s</strong>cribing<br />
these sets. As shown by these works two inconsist<strong>en</strong>t sets<br />
of formulae are not trivially equival<strong>en</strong>t. They do not contain<br />
the same information and they do not contain the same<br />
contradictions.<br />
Measures of information <strong>à</strong> la Shannon have be<strong>en</strong> studied<br />
in logical frameworks (see for example (Kem<strong>en</strong>y 1953)).<br />
Roughly they involve counting the number of mo<strong>de</strong>ls of<br />
Copyright c○ 2006, American Association for Artificial Intellig<strong>en</strong>ce<br />
(www.aaai.org). All rights reserved.<br />
the set of formulae (the less mo<strong>de</strong>ls, the more informative<br />
the set). The problem is that these measures give a<br />
null information cont<strong>en</strong>t to an inconsist<strong>en</strong>t set of formulae,<br />
which is counter-intuitive (especially giv<strong>en</strong> all the proposals<br />
for paraconsist<strong>en</strong>t reasoning). So g<strong>en</strong>eralizations of measures<br />
of information have be<strong>en</strong> proposed to solve this problem<br />
(Lozinskii 1994; Wong & Besnard 2001; Knight 2003;<br />
Konieczny, Lang, & Marquis 2003; Hunter & Konieczny<br />
2005).<br />
In comparison, there are relatively few proposals for inconsist<strong>en</strong>cy<br />
measures (Grant 1978; Hunter 2002; Knight<br />
2001; Konieczny, Lang, & Marquis 2003; Hunter 2004;<br />
Grant & Hunter 2006). However, these measures are pot<strong>en</strong>tially<br />
important in diverse applications in artificial intellig<strong>en</strong>ce,<br />
such as belief revision, belief merging, and negotiation,<br />
and more g<strong>en</strong>erally in computer sci<strong>en</strong>ce. Already<br />
measuring inconsist<strong>en</strong>cy has being se<strong>en</strong> to be a useful<br />
tool in analysing a diverse range of information types<br />
including news reports (Hunter 2006), integrity constraints<br />
(Grant & Hunter 2006), software specifications (Barragáns-<br />
Martínez, Pazos-Arias, & Fernán<strong>de</strong>z-Vilas 2004; 2005; Mu<br />
et al. 2005), and ecommerce protocols (Ch<strong>en</strong>, Zhang, &<br />
Zhang 2004).<br />
The curr<strong>en</strong>t proposals for measuring inconsistecy can be<br />
classified in two approaches. The first approach involves<br />
“counting” the minimal number of formulae nee<strong>de</strong>d to produce<br />
the inconsist<strong>en</strong>cy. The more formulae nee<strong>de</strong>d to produce<br />
the inconsist<strong>en</strong>cy, the less inconsist<strong>en</strong>t the set (Knight<br />
2001). This i<strong>de</strong>a is an interesting one, but it rejects the possibility<br />
of a more fine-grained inspection of the (cont<strong>en</strong>t of<br />
the) formulae. In particular, if one looks to singleton sets<br />
only, one is back to the initial problem, with only two values:<br />
consist<strong>en</strong>t or inconsist<strong>en</strong>t.<br />
The second approach involves looking at the proportion<br />
of the language that is touched by the inconsist<strong>en</strong>cy.<br />
This allows us to look insi<strong>de</strong> the formulae (Hunter 2002;<br />
Konieczny, Lang, & Marquis 2003; Grant & Hunter 2006).<br />
This means that two formulae (singleton sets) can have differ<strong>en</strong>t<br />
inconsist<strong>en</strong>cy measures. But, in these approaches one<br />
can i<strong>de</strong>ntify the set of formulae with its conjunction (i.e. the<br />
set {ϕ, ϕ ′ } has the same inconsist<strong>en</strong>cy measure as the set<br />
{ϕ ∧ ϕ ′ }). This can be s<strong>en</strong>sible in several applications, but<br />
this means that the distribution of the contradiction among<br />
the formulae is not tak<strong>en</strong> into account.<br />
83
What we propose in this paper is a <strong>de</strong>finition for inconsist<strong>en</strong>cy<br />
measures that allow us to take the best of the two<br />
approaches. This will allow us to build inconsist<strong>en</strong>cy measures<br />
that are able to look insi<strong>de</strong> the formulae, but also to<br />
take into account the distribution of the contradiction among<br />
the differ<strong>en</strong>t formulae of the set. The advantage of such a<br />
method is twofold. First, this allows us to know the <strong>de</strong>gree<br />
of blame/responsability of each formula of the base in the<br />
inconsist<strong>en</strong>cy, and so it provi<strong><strong>de</strong>s</strong> a very <strong>de</strong>tailed view of the<br />
inconsist<strong>en</strong>cy. Second, this allows us to <strong>de</strong>fine measures of<br />
consist<strong>en</strong>cy for the whole base that are more accurate, since<br />
they take into account those two dim<strong>en</strong>sions.<br />
To this <strong>en</strong>d we will use a notion that comes from coalitional<br />
game theory: the Shapley value. This value assigns<br />
to each player the payoff that this player can expect from<br />
her utility for each possible coalition. The i<strong>de</strong>a is to use existing<br />
inconsist<strong>en</strong>cy measures (that allow us to look insi<strong>de</strong><br />
the formulae) in or<strong>de</strong>r to <strong>de</strong>fine a game in coalitional form,<br />
and th<strong>en</strong> to use the Shapley value to obtain an inconsist<strong>en</strong>cy<br />
measure with the wanted properties. We will study these<br />
measures and show that they are more interesting than the<br />
other existing measures.<br />
After stating some notations and <strong>de</strong>finitions in the next<br />
section, we introduce inconsist<strong>en</strong>cy measures that count the<br />
number of formulae nee<strong>de</strong>d to produce an inconsist<strong>en</strong>cy.<br />
Th<strong>en</strong> we pres<strong>en</strong>t the approaches where the inconsist<strong>en</strong>cy<br />
measure is related to the number of variables touched by<br />
the inconsist<strong>en</strong>cy. The next section gives the <strong>de</strong>finition of<br />
coalitional games and of the Shapley value. Th<strong>en</strong> we introduce<br />
the inconsist<strong>en</strong>cy measures based on the Shapley value.<br />
The p<strong>en</strong>ultimate section sketches the possible applications<br />
of those measures for belief change operators. In the last<br />
section we conclu<strong>de</strong> and give perspectives of this work.<br />
Preliminaries<br />
We will consi<strong>de</strong>r a propositional language L built from a finite<br />
set of propositional symbols P. We will use a, b, c, . . .<br />
to <strong>de</strong>note the propositional variables, and Greek letters<br />
α, β, ϕ, . . . to <strong>de</strong>note the formulae. An interpretation is a<br />
total function from P to {0, 1}. The set of all interpretations<br />
is <strong>de</strong>noted W. An interpretation ω is a mo<strong>de</strong>l of a formula<br />
ϕ, <strong>de</strong>noted ω |= ϕ, if and only if it makes ϕ true in the usual<br />
truth-functional way. Mod(ϕ) <strong>de</strong>notes the set of mo<strong>de</strong>ls of<br />
the formula ϕ, i.e. Mod(ϕ) = {ω ∈ W | ω |= ϕ}. We<br />
will use ⊆ to <strong>de</strong>note the set inclusion, and we will use ⊂ to<br />
<strong>de</strong>note the strict set inclusion, i.e. A ⊂ B iff A ⊆ B and<br />
B ⊈ A. We will <strong>de</strong>note the set of real numbers by IR.<br />
A belief base K is a finite set of propositional formulae.<br />
More exactly, as we will need to i<strong>de</strong>ntify the differ<strong>en</strong>t formulae<br />
of a belief base in or<strong>de</strong>r to associate them with their<br />
inconsist<strong>en</strong>cy value, we will consi<strong>de</strong>r belief bases K as vectors<br />
of formulae. For logical properties we will need to use<br />
the set corresponding to each vector, so we suppose that we<br />
have a function such that for each vector K = (α 1 , . . . , α n ),<br />
K is the set {α 1 , . . . , α n }. As it will never be ambigous, in<br />
the following we will omit the and write K as both the<br />
vector and the set.<br />
Let us note K L the set of belief bases <strong>de</strong>finable from formulae<br />
of the language L. A belief base is consist<strong>en</strong>t if there<br />
is at least one interpretation that satisfies all its formulae.<br />
If a belief base K is not consist<strong>en</strong>t, th<strong>en</strong> one can <strong>de</strong>fine<br />
the minimal inconsist<strong>en</strong>t subsets of K as:<br />
MI(K) = {K ′ ⊆ K | K ′ ⊢ ⊥ and ∀K ′′ ⊂ K ′ , K ′′ ⊥}<br />
If one wants to recover consist<strong>en</strong>cy from an inconsist<strong>en</strong>t<br />
base K by removing some formulae, th<strong>en</strong> the minimal inconsist<strong>en</strong>t<br />
subsets can be consi<strong>de</strong>red as the purest form of<br />
inconsist<strong>en</strong>cy. To recover consist<strong>en</strong>cy, one has to remove<br />
at least one formula from each minimal inconsist<strong>en</strong>t subset<br />
(Reiter 1987).<br />
A free formula of a belief base K is a formula of K that<br />
does not belong to any minimal inconsist<strong>en</strong>t subset of the<br />
belief base K, or equival<strong>en</strong>tly any formula that belongs to<br />
any maximal consist<strong>en</strong>t subset of the belief base.<br />
Inconsist<strong>en</strong>cy Measures based on Formulae<br />
Wh<strong>en</strong> a base is not consist<strong>en</strong>t the classical infer<strong>en</strong>ce relation<br />
trivializes, since one can <strong>de</strong>duce every formula of<br />
the language from the base (ex falso quodlibet). Otherwise<br />
in this case the use of paraconsist<strong>en</strong>t reasoning techniques<br />
allows us to draw non-trivial consequ<strong>en</strong>ces from the<br />
base. One possibility is to take maximal consist<strong>en</strong>t subsets<br />
of formulae of the base (cf (Manor & Rescher 1970;<br />
B<strong>en</strong>ferhat, Dubois, & Pra<strong>de</strong> 1997; Nebel 1991)). This i<strong>de</strong>a<br />
can also be used to <strong>de</strong>fine an inconsist<strong>en</strong>cy measure. This is<br />
the way followed in (Knight 2001; 2003).<br />
Definition 1 A probability function on L is a function P :<br />
P → [0, 1] s.t.:<br />
• if |= α, th<strong>en</strong> P (α) = 1<br />
• if |= ¬(α ∧ β), th<strong>en</strong> P (α ∨ β) = P (α) + P (β)<br />
See (Paris 1994) for more <strong>de</strong>tails on this <strong>de</strong>finition. In the<br />
finite case, this <strong>de</strong>finition gives a probability distribution on<br />
the interpretations, and the probability of a formula is the<br />
sum of the probability of its mo<strong>de</strong>ls.<br />
Th<strong>en</strong> the inconsist<strong>en</strong>cy measure <strong>de</strong>fined by Knight (2001)<br />
is giv<strong>en</strong> by:<br />
Definition 2 Let K be a belief base.<br />
• K is η−consist<strong>en</strong>t (0 ≤ η ≤ 1) if there is a probability<br />
function P such that P (α) ≥ η for all α ∈ K.<br />
• K is maximally η−consist<strong>en</strong>t if η is maximal (i.e. if γ > η<br />
th<strong>en</strong> K is not γ−consist<strong>en</strong>t).<br />
The notion of maximal η-consist<strong>en</strong>cy can be used as an<br />
inconsist<strong>en</strong>cy measure. This is the direct formulation of the<br />
i<strong>de</strong>a that the more formulae are nee<strong>de</strong>d to produce the inconsist<strong>en</strong>cy,<br />
the less this inconsist<strong>en</strong>cy is problematic. As it<br />
is easily se<strong>en</strong>, in the finite case, a belief base is maximally 0-<br />
consist<strong>en</strong>t if and only if it contains a contradictory formula.<br />
And a belief base is maximally 1-consist<strong>en</strong>t if and only if it<br />
is consist<strong>en</strong>t.<br />
Example 1 Let K 1 = {a, b, ¬a ∨ ¬b}.<br />
K 1 is maximally 2 3 −consist<strong>en</strong>t.<br />
Let K 2 = {a ∧ b, ¬a ∧ ¬b, a ∧ ¬b}.<br />
K 2 is maximally 1 3−consist<strong>en</strong>t, whereas each subbase of<br />
cardinality 2 is maximally 1 2 −consist<strong>en</strong>t.<br />
84
For minimal inconsist<strong>en</strong>t sets of formulae, computing this<br />
inconsist<strong>en</strong>cy measure is easy:<br />
Proposition 1 If K ′ ∈ MI(K), th<strong>en</strong> K ′ is maximally<br />
|K ′ |−1<br />
|K ′ |<br />
−consist<strong>en</strong>t.<br />
But in g<strong>en</strong>eral this measure is har<strong>de</strong>r to compute. However<br />
it is possible to compute it using the simplex method<br />
(Knight 2001).<br />
Inconsist<strong>en</strong>cy Measures based on Variables<br />
Another method to evaluate the inconsist<strong>en</strong>cy of a belief<br />
base is to look at the proportion of the language concerned<br />
with the inconsist<strong>en</strong>cy. To this <strong>en</strong>d, it is clearly not possible<br />
to use classical logics, since the inconsist<strong>en</strong>cy contaminates<br />
the whole language. But if we look at the two bases K 1 =<br />
{a∧¬a∧b∧c∧d} and K 2 = {a∧¬a∧b∧¬b∧c∧¬c∧d∧¬d},<br />
we can observe that in K 1 the inconsist<strong>en</strong>cy is mainly about<br />
the variable a, whereas in K 2 all the variables are touched<br />
by a contradiction. This is this kind of distinction that these<br />
approaches allow.<br />
One way to circumscribe the inconsist<strong>en</strong>cy only to the<br />
variables directly concerned is to use multi-valued logics,<br />
and especially three-valued logics, with the third “truth<br />
value” <strong>de</strong>noting the fact that there is a conflict on the truth<br />
value (true-false) of the variable.<br />
We do not have space here to <strong>de</strong>tail the range of differ<strong>en</strong>t<br />
measures that have be<strong>en</strong> proposed. See (Grant 1978;<br />
Hunter 2002; Konieczny, Lang, & Marquis 2003; Hunter<br />
& Konieczny 2005; Grant & Hunter 2006) for more <strong>de</strong>tails<br />
on these approaches. We only give one such measure, that<br />
is a special case of the <strong>de</strong>grees of contradiction <strong>de</strong>fined in<br />
(Konieczny, Lang, & Marquis 2003). The i<strong>de</strong>a of the <strong>de</strong>finition<br />
of these <strong>de</strong>grees in (Konieczny, Lang, & Marquis 2003)<br />
is, giv<strong>en</strong> a set of tests on the truth value of some formulae<br />
of the language (typically on the variables), the <strong>de</strong>gree of<br />
contradiction is the cost of a minimum test plan that <strong>en</strong>sures<br />
recovery of consist<strong>en</strong>cy.<br />
The inconsist<strong>en</strong>cy measure we <strong>de</strong>fine here is the (normalized)<br />
minimum number of inconsist<strong>en</strong>t truth values in the<br />
LP m mo<strong>de</strong>ls (Priest 1991) of the belief base. Let us first<br />
introduce the LP m consequ<strong>en</strong>ce relation.<br />
• An interpretation ω for LP m maps each propositional<br />
atom to one of the three “truth values” F, B, T, the third<br />
truth value B meaning intuitively “both true and false”.<br />
3 P is the set of all interpretations for LP m . “Truth values”<br />
are or<strong>de</strong>red as follows: F < t B < t T.<br />
– ω(⊤) = T, ω(⊥) = F<br />
– ω(¬α) = B iff ω(α) = B<br />
ω(¬α) = T iff ω(α) = F<br />
– ω(α ∧ β) = min ≤t (ω(α), ω(β))<br />
– ω(α ∨ β) = max ≤t (ω(α), ω(β))<br />
• The set of mo<strong>de</strong>ls of a formula ϕ is:<br />
Mod LP (ϕ) = {ω ∈ 3 P | ω(ϕ) ∈ {T, B}}<br />
Define ω! as the set of “inconsist<strong>en</strong>t” variables in an interpretation<br />
w, i.e.<br />
ω! = {x ∈ P | ω(x) = B}<br />
Th<strong>en</strong> the minimum mo<strong>de</strong>ls of a formula are the “most<br />
classical” ones:<br />
min(Mod LP (ϕ)) = {ω ∈ Mod LP (ϕ) |<br />
∄ω ′ ∈ Mod LP (ϕ) s.t. ω ′ ! ⊂ ω!}<br />
The LP m consequ<strong>en</strong>ce relation is th<strong>en</strong> <strong>de</strong>fined by:<br />
K |= LPm ϕ iff min(Mod LP (K)) ⊆ Mod LP (ϕ)<br />
So ϕ is a consequ<strong>en</strong>ce of K if all the “most classical”<br />
mo<strong>de</strong>ls of K are mo<strong>de</strong>ls of ϕ.<br />
Th<strong>en</strong> let us <strong>de</strong>fine the LP m measure of inconsist<strong>en</strong>cy,<br />
noted I LPm , as:<br />
Definition 3 Let K be a belief base.<br />
I LPm = min ω∈ModLP (K)(| ω! |)<br />
| P |<br />
Example 2 K 4 = {a ∧ ¬a, b, ¬b, c}. I LPm (K 4 ) = 2 3<br />
In this example one can see the point in these kinds of<br />
measures compared to measures based on formulae since<br />
this base is maximally 0-consist<strong>en</strong>t because of the contradictory<br />
formula a ∧ ¬a. But there are also non-trivial formulae<br />
in the base, and this base is not very inconsist<strong>en</strong>t according<br />
to I LPm .<br />
Conversely, measures based on variables like this one are<br />
unable to take into account the distribution of the contradiction<br />
among formulae. In fact the result would be exactly the<br />
same with K 4 ′ = {a ∧ ¬a ∧ b ∧ ¬b ∧ c}. This can be s<strong>en</strong>sible<br />
in several applications, but in some cases this can also<br />
be se<strong>en</strong> as a drawback.<br />
Games in Coalitional Form - Shapley Value<br />
In this section we give the <strong>de</strong>finitions of games in coalitional<br />
form and of the Shapley value.<br />
Definition 4 Let N = {1, . . . , n} be a set of n players. A<br />
game in coalitional form is giv<strong>en</strong> by a function v : 2 N → IR,<br />
with v(∅) = 0.<br />
This framework <strong>de</strong>fines games in a very abstract way, focusing<br />
on the possible coalitions formations. A coalition is<br />
just a subset of N. This function gives what payoff can be<br />
achieved by each coalition in the game v wh<strong>en</strong> all its members<br />
act together as a unit.<br />
There are numerous questions that are worthwhile to investigate<br />
in this framework. One of these questions is to<br />
know how much each player can expect in a giv<strong>en</strong> game v.<br />
This <strong>de</strong>p<strong>en</strong>d on her position in the game, i.e. what she brings<br />
to differ<strong>en</strong>t coalitions.<br />
Oft<strong>en</strong> the games are super-additive.<br />
Definition 5 A game is super-additive if for each T, U ⊆ N<br />
with T ∩ U = ∅, v(T ∪ U) ≥ v(T ) + v(U).<br />
In super-additive games wh<strong>en</strong> two coalitions join, th<strong>en</strong><br />
the joined coalition wins at least as much as (the sum of) the<br />
initials coalitions. In particular, in super-additive games, the<br />
grand coalition N is the one that brings the higher utility for<br />
85
the society N. The problem is how this utility can be shared<br />
among the players 1 .<br />
Example 3 Let N = {1, 2, 3}, and let v be the following<br />
coalitional game:<br />
v({1}) = 1 v({2}) = 0 v({3}) = 1<br />
v({1, 2}) = 10 v({1, 3}) = 4 v({2, 3}) = 11<br />
v({1, 2, 3}) = 12<br />
This game is clearly super-additive. The grand coalition<br />
can bring 12 to the three players. This is the highest utility<br />
achievable by the group. But this is not the main aim<br />
for all the players. In particular one can note that two coalitions<br />
can bring nearly as much, namely {1, 2} and {2, 3}<br />
that gives respectively 10 and 11, that will have to be shared<br />
only betwe<strong>en</strong> 2 players. So it is far from certain that the<br />
grand coalition will form in this case. Another remark on<br />
this game is that all the players do not share the same situation.<br />
In particular player 2 is always of a great value for any<br />
coalition she joins. So she seems to be able to expect more<br />
from this game than the other players. For example she can<br />
make an offer to player 3 for making the coalition {2, 3},<br />
that brings 11, that will be split in 8 for player 2 and 3 for<br />
player 3. As it will be hard for player 3 to win more than<br />
that, 3 will certainly accept.<br />
A solution concept has to take into account these kinds of<br />
argum<strong>en</strong>ts. It means that one wants to solve this game by<br />
stating what is the payoff that is “due” to each ag<strong>en</strong>t. That<br />
requires to be able to quantify the payoff that an ag<strong>en</strong>t can<br />
claim with respect to the power that her position in the game<br />
offers (for example if she always significantly improves the<br />
payoff of the coalitions she joins, if she can threat to form<br />
another coalition, etc.).<br />
Definition 6 A value is a function that assigns to each game<br />
v a vector of payoff S(v) = (S 1 , . . . , S n ) in IR n .<br />
This function gives the payoff that can be expected by<br />
each player i for the game v, i.e. it measures i’s power in the<br />
game v.<br />
Shapley proposes a beautiful solution to this problem. Basically<br />
the i<strong>de</strong>a can be explained as follows: consi<strong>de</strong>ring that<br />
the coalitions form according to some or<strong>de</strong>r (a first player<br />
<strong>en</strong>ters the coalition, th<strong>en</strong> another one, th<strong>en</strong> a third one, etc),<br />
and that the payoff attached to a player is its marginal utility<br />
(i.e. the utility that it brings to the existing coalition), so<br />
if C is a coalition (subset of N) not containing i, player’s<br />
i marginal utility is v(C ∪ {i}) − v(C). As one can not<br />
make any hypothesis on which or<strong>de</strong>r is the correct one, suppose<br />
that each or<strong>de</strong>r is equally probable. This leads to the<br />
following formula:<br />
Let σ be a permutation on N, with σ n <strong>de</strong>noting all the<br />
possible permutations on N. Let us note<br />
p i σ = {j ∈ N | σ(j) < σ(i)}<br />
That means that p i σ repres<strong>en</strong>ts all the players that prece<strong>de</strong><br />
player i for a giv<strong>en</strong> or<strong>de</strong>r σ.<br />
1 One supposes the transferable utility (TU) assumption, i.e. the<br />
utility is a common unit betwe<strong>en</strong> the players and sharable as nee<strong>de</strong>d<br />
(roughly, one can see this utility as some kind of money).<br />
Definition 7 The Shapley value of a game v is <strong>de</strong>fined as:<br />
S i (v) = 1 ∑<br />
v(p i σ<br />
n!<br />
∪ {i}) − v(pi σ )<br />
σ∈σn<br />
The Shapley value can be directly computed from the possible<br />
coalitions (without looking at the permutations), with<br />
the following expression:<br />
S i (v) = ∑ (c − 1)!(n − c)!<br />
(v(C) − v(C \ {i}))<br />
n!<br />
C⊆N<br />
where c is the cardinality of C.<br />
Example 4 The Shapley value of the game <strong>de</strong>fined in Example<br />
3 is ( 17 6 , 35 6 , 20 6 ).<br />
These values show that it is player 2 that is the best placed<br />
in this game, accordingly to what we explained wh<strong>en</strong> we<br />
pres<strong>en</strong>ted Example 3.<br />
Besi<strong><strong>de</strong>s</strong> this value, Shapley proposes axiomatic properties<br />
a value should have.<br />
• ∑ i∈N S i(v) = v(N)<br />
(Effici<strong>en</strong>cy)<br />
• If i and j are such that for all C s.t. i, j /∈ C,<br />
v(C ∪ {i}) = v(C ∪ {j}), th<strong>en</strong> S i (v) = S j (v)<br />
(Symmetry)<br />
• If i is such that ∀C v(C ∪ {i}) = v(C), th<strong>en</strong> S i (v) = 0<br />
(Dummy)<br />
• S i (v + w) = S i (v) + S i (w)<br />
(Additivity)<br />
These four axioms seem quite s<strong>en</strong>sible. Effici<strong>en</strong>cy states<br />
that the payoff available to the grand coalition N must be effici<strong>en</strong>tly<br />
redistributed to the players (otherwise some players<br />
could expect more that what they have). Symmetry <strong>en</strong>sures<br />
that it is the role of the player in the game in coalitional<br />
form that <strong>de</strong>termines her payoff, so it is not possible to distinguish<br />
players by their name (as far as payoffs are concerned),<br />
but only by their respective merits/possibilities. So<br />
if two players always are i<strong>de</strong>ntical for the game, i.e. if they<br />
bring the same utility to every coalitions, th<strong>en</strong> they have the<br />
same value. The dummy player axiom says simply that if a<br />
player is of no use for every coalition, this player does not<br />
<strong><strong>de</strong>s</strong>erve any payoff. And additivity states that wh<strong>en</strong> we join<br />
two differ<strong>en</strong>t games v and w in a whole super-game v + w<br />
(v + w is straightforwardly <strong>de</strong>fined as the function that is<br />
the sum of the two functions v and w, that means that each<br />
coalition receive as payoff in the game v + w the payoff it<br />
has in v plus the payoff it has in w), th<strong>en</strong> the value of each<br />
player in the supergame is simply the sum of the values in<br />
the compound games.<br />
These properties look quite natural, and the nice result<br />
shown by Shapley is that they characterize exactly the value<br />
he <strong>de</strong>fined (Shapley 1953):<br />
Proposition 2 The Shapley value is the only value that satisfies<br />
all of Effici<strong>en</strong>cy, Symmetry, Dummy and Additivity.<br />
This result supports several variations : there are other<br />
equival<strong>en</strong>t axiomatizations of the Shapley value, and there<br />
are some differ<strong>en</strong>t values that can be <strong>de</strong>fined by relaxing<br />
some of the above axioms. See (Aumann & Hart 2002).<br />
86
Inconsist<strong>en</strong>cy Values using Shapley Value<br />
Giv<strong>en</strong> an inconsist<strong>en</strong>cy measure, the i<strong>de</strong>a is to take it as the<br />
payoff function <strong>de</strong>fining a game in coalitional form, and th<strong>en</strong><br />
using the Shapley value to compute the part of the inconsist<strong>en</strong>cy<br />
that can be imputed to each formula of the belief base.<br />
This allows us to combine the power of inconsist<strong>en</strong>cy<br />
measures based on variables and h<strong>en</strong>ce discriminating betwe<strong>en</strong><br />
singleton inconsist<strong>en</strong>t belief base (like Coher<strong>en</strong>ce<br />
measure in (Hunter 2002), or like the test action values of<br />
(Konieczny, Lang, & Marquis 2003)), and the use of the<br />
Shapley value for knowing what is the responsibility of a<br />
giv<strong>en</strong> formula in the inconsist<strong>en</strong>cy of the belief base.<br />
We just require some basic properties on the un<strong>de</strong>rlying<br />
inconsist<strong>en</strong>cy measure.<br />
Definition 8 An inconsist<strong>en</strong>cy measure I is called a basic<br />
inconsist<strong>en</strong>cy measure if it satisfies the following properties,<br />
∀K, K ′ ∈ K L , ∀α, β ∈ L:<br />
• I(K) = 0 iff K is consist<strong>en</strong>t<br />
(Consist<strong>en</strong>cy)<br />
• 0 ≤ I(K) ≤ 1<br />
(Normalization)<br />
• I(K ∪ K ′ ) ≥ I(K)<br />
(Monotony)<br />
• If α is a free formula of K∪{α}, th<strong>en</strong> I(K∪{α}) = I(K)<br />
(Free Formula In<strong>de</strong>p<strong>en</strong><strong>de</strong>nce)<br />
• If α ⊢ β and α ⊥, th<strong>en</strong> I(K ∪ {α}) ≥ I(K ∪ {β})<br />
(Dominance)<br />
We ask for few properties on the initial inconsist<strong>en</strong>cy<br />
measure. The consist<strong>en</strong>cy property states that a consist<strong>en</strong>t<br />
base has a null inconsist<strong>en</strong>cy measure. The monotony property<br />
says that the amount of inconsist<strong>en</strong>cy of a belief base<br />
can only grow if one adds new formulae (<strong>de</strong>fined on the<br />
same language). The free formula in<strong>de</strong>p<strong>en</strong><strong>de</strong>nce property<br />
states that adding a formula that does not cause any inconsist<strong>en</strong>cy<br />
cannot change the inconsist<strong>en</strong>cy measure of the base.<br />
The Dominance property states that logically stronger formulae<br />
bring (pot<strong>en</strong>tially) more conflicts. The normalization<br />
property of the inconsist<strong>en</strong>cy measure is not mandatory, it is<br />
asked only for simplification purposes.<br />
Now we are able to <strong>de</strong>fine the Shapley inconsist<strong>en</strong>cy values<br />
:<br />
Definition 9 Let I be a basic inconsist<strong>en</strong>cy measure. We<br />
<strong>de</strong>fine the corresponding Shapley inconsist<strong>en</strong>cy value (SIV),<br />
noted S I , as the Shapley value of the coalitional game <strong>de</strong>fined<br />
by the function I, i.e. let α ∈ K :<br />
S K I (α) = ∑ C⊆K<br />
(c − 1)!(n − c)!<br />
(I(C) − I(C \ {α}))<br />
n!<br />
where n is the cardinality of K and c is the cardinality of C.<br />
Note that this SIV gives a value for each formula of the<br />
base K, so if one consi<strong>de</strong>rs the base K as the vector K =<br />
(α 1 , . . . , α n ), th<strong>en</strong> we will use S I (K) to <strong>de</strong>note the vector<br />
of corresponding SIVs, i.e.<br />
S I (K) = (S K I (α 1), . . . , S K I (α n))<br />
This <strong>de</strong>finition allows us to <strong>de</strong>fine to what ext<strong>en</strong>t a formula<br />
insi<strong>de</strong> a belief base is concerned with the inconsist<strong>en</strong>cies<br />
of the base. It allows us to draw a precise picture of the<br />
contradiction of the base.<br />
From this value, one can <strong>de</strong>fine an inconsist<strong>en</strong>cy value for<br />
the whole belief base:<br />
Definition 10 Let K be a belief base, ŜI(K) = max<br />
α∈K S I(α)<br />
One can figure out other aggregation functions to <strong>de</strong>fine<br />
the inconsist<strong>en</strong>cy measure of the belief base from the inconsist<strong>en</strong>cy<br />
measure of its formulae, such as the leximax for<br />
instance. Taking the maximum will be suffici<strong>en</strong>t for us to<br />
have valuable results and to compare this with the existing<br />
measures from the literature. Note that taking the sum as aggregation<br />
function is not a good choice here, since as shown<br />
by the distribution property of Theorem 3 this equals I(K),<br />
“erasing” the use of the Shapley value.<br />
We think that the most interesting measure is S I , since it<br />
<strong><strong>de</strong>s</strong>cribes more accurately the inconsist<strong>en</strong>cy of the base. But<br />
we <strong>de</strong>fine ŜI since it is a more concise measure, that is of the<br />
same type as existing ones (it associates a real to each base),<br />
that is conv<strong>en</strong>i<strong>en</strong>t to compare our framework with existing<br />
measures.<br />
Let us see see now two instantiations of SIVs.<br />
Drastic Shapley Inconsist<strong>en</strong>cy Value<br />
We will start this section with the simplest inconsist<strong>en</strong>cy<br />
measure one can <strong>de</strong>fine:<br />
Definition 11 The drastic inconsist<strong>en</strong>cy value is <strong>de</strong>fined as:<br />
{<br />
0 if K is consist<strong>en</strong>t<br />
I d (K) =<br />
1 otherwise<br />
This measure is not of great interest by itself, since it<br />
corresponds to the usual dichotomy of classical logic. But<br />
it will be useful to illustrate the use of the Shapley inconsist<strong>en</strong>cy<br />
values, since, ev<strong>en</strong> with this over-simple measure,<br />
one will produce interesting results. Let us illustrate this on<br />
some examples.<br />
Example 5 K 1 = {a, ¬a, b}.<br />
Th<strong>en</strong> I d ({a, ¬a}) = I d ({a, ¬a, b}) = 1, and the value is<br />
S Id (K 1 ) = ( 1 2 , 1 2 , 0). So ŜId (K 1) = 1 2 .<br />
As b is a free formula, it has a value of 0, the two other<br />
formulae are equally responsible for the inconsist<strong>en</strong>cy.<br />
Example 6 K 2 = {a, b, b ∧ c, ¬b ∧ d}.<br />
Th<strong>en</strong> the value is S Id (K 2 ) = (0, 1 6 , 1 6 , 4 6 ).<br />
And ŜId (K 2) = 2 3 .<br />
The last three formulae are the ones that belong to some<br />
inconsist<strong>en</strong>cy, and the last one is the one that causes the most<br />
problems (removing only this formula restores the consist<strong>en</strong>cy<br />
of the base).<br />
Example 7 K 4 = {a ∧ ¬a, b, ¬b, c}.<br />
The value is S Id (K 4 ) = ( 4 6 , 1 6 , 1 6 , 0). So ŜId (K 4) = 2 3 .<br />
87
LP m Shapley Inconsist<strong>en</strong>cy Value<br />
Let us turn now to a more elaborate value. For this we use<br />
the LP m inconsist<strong>en</strong>cy measure (<strong>de</strong>fined earlier) to <strong>de</strong>fine a<br />
SIV.<br />
Example 8 Let K 4 = {a ∧ ¬a, b, ¬b, c}<br />
and K 4 ′ = {a ∧ ¬a ∧ b ∧ ¬b ∧ c}.<br />
Th<strong>en</strong> S ILPm (K 4 ) = ( 1 3 , 1 6 , 1 6 , 0), and ŜILPm (K 4) = 1 3 .<br />
Whereas S ILPm (K 4 ′ ) = ( 2 3 ) and ŜILPm (K′ 4 ) = 2 3 .<br />
As we can see on this example, the SIV value allows us to<br />
make a distinction betwe<strong>en</strong> K 4 and K 4, ′ since ŜILPm (K′ 4) =<br />
2<br />
3 whereas (K 4) = 1 ŜILPm 3<br />
. This illustrates the fact that<br />
the inconsist<strong>en</strong>cy is more distributed in K 4 than in K 4. ′ This<br />
distinction is not possible with the original I LPm value. Note<br />
that with Knight’s coher<strong>en</strong>ce value, the two bases have the<br />
worst inconsist<strong>en</strong>cy value (maximally 0-consist<strong>en</strong>t).<br />
So this example illustrates the improvem<strong>en</strong>t brought by<br />
this work, compared to inconsist<strong>en</strong>cy measures on formulae<br />
and to inconsist<strong>en</strong>cy measures on variables, since none of<br />
them was able to make a distinction betwe<strong>en</strong> K 4 and K 4 ′ ,<br />
whereas for ŜILPm K 4 is more consist<strong>en</strong>t than K 4.<br />
′<br />
Let us see a more striking example.<br />
Example 9 Let K 5 = {a, b, b ∧ c, ¬b ∧ ¬c}.<br />
Th<strong>en</strong> S ILPm (K 5 ) = (0, 1<br />
18 , 4<br />
18 , 7<br />
18 ),<br />
and ŜILPm (K 5) = 7<br />
18 .<br />
In this example one can easily see that it is the last formula<br />
that is the more problematic, and that b∧c brings more<br />
conflict than b alone, which is perfectly expressed in the obtained<br />
values.<br />
Logical properties<br />
Let us see now some properties of the <strong>de</strong>fined values.<br />
Proposition 3 Every Shapley Inconsist<strong>en</strong>cy Value satisfies:<br />
• ∑ α∈K S I(α) = I(K)<br />
(Distribution)<br />
• If ∃α, β ∈ K s.t. for all K ′ ⊆ K s.t. α, β /∈ K ′ ,<br />
I(K ′ ∪ {α}) = I(K ′ ∪ {β}), th<strong>en</strong> S I (α) = S I (β)<br />
(Symmetry)<br />
• If α is a free formula of K, th<strong>en</strong> S I (α) = 0<br />
(Free Formula)<br />
• If α ⊢ β and α ⊥, th<strong>en</strong> S I (α) ≥ S I (β) (Dominance)<br />
The distribution property states that the inconsist<strong>en</strong>cy values<br />
of the formulae sum to the total amount of inconsist<strong>en</strong>cy<br />
in the base (I(K)). The Symmetry property <strong>en</strong>sures<br />
that only the amount of inconsist<strong>en</strong>cy brought by a formula<br />
matters for computing the SIV. As one could expect, a formula<br />
that is not embed<strong>de</strong>d in any contradiction (i.e. does<br />
not belong to any minimal inconsist<strong>en</strong>t subset) will not be<br />
blamed by the Shapley inconsist<strong>en</strong>cy values. This is what<br />
is expressed in the Free formula property. The Dominance<br />
property states that logically stronger formulae bring (pot<strong>en</strong>tially)<br />
more conflicts.<br />
The first three properties are a restatem<strong>en</strong>t in this logical<br />
framework of the properties of the Shapley value. One can<br />
note that the Additivity axiom of the Shapley value is not<br />
translated here, since it makes little s<strong>en</strong>se to add differ<strong>en</strong>t<br />
inconsist<strong>en</strong>cy values.<br />
Let us turn now to the properties of the measure on belief<br />
bases.<br />
Proposition 4<br />
• ŜI(K) = 0 if and only if K is consist<strong>en</strong>t<br />
(Consist<strong>en</strong>cy)<br />
• 0 ≤ ŜI(K) ≤ 1<br />
(Normalization)<br />
• If α is a free formula of K ∪ {α}, th<strong>en</strong><br />
Ŝ I (K ∪ {α}) = ŜI(K) (Free Formula In<strong>de</strong>p<strong>en</strong><strong>de</strong>nce)<br />
• ŜI(K) ≤ I(K)<br />
(Upper Bound)<br />
• ŜI(K) = I(K) > 0 if only if ∃α ∈ K s.t. α is inconsist<strong>en</strong>t<br />
and ∀β ∈ K, β ≠ α, β is a free formula of K<br />
(Isolation)<br />
The first three properties are the ones giv<strong>en</strong> in Definition<br />
8 for the basic inconsist<strong>en</strong>cy measures. As one can easily<br />
note an important differ<strong>en</strong>ce is that the monotony property<br />
and the dominance property do not hold for the SIVs on belief<br />
bases. It is s<strong>en</strong>sible since distribution of the inconsist<strong>en</strong>cies<br />
matters for SIVs. The upper bound property shows that<br />
the use of the SIV aims at looking at the distribution of the<br />
inconsist<strong>en</strong>cies of the base, so the SIV on belief bases is always<br />
less or equal to the inconsist<strong>en</strong>cy measure giv<strong>en</strong> by the<br />
un<strong>de</strong>rlying basic inconsist<strong>en</strong>cy measure. The isolation property<br />
<strong>de</strong>tails the case where the two measures are equals. In<br />
this case, there is only one inconsist<strong>en</strong>t formula in the whole<br />
base.<br />
Let us see, on Example 10, counter-examples to<br />
monotony and dominance for SIV on belief bases:<br />
Example 10 Let K 6 = {a, ¬a, ¬a ∧ b},<br />
K 7 = {a, ¬a, ¬a ∧ b, a ∧ b},<br />
and K 8 = {a, ¬a, ¬a ∧ b, b}.<br />
Ŝ Id (K 6 ) = 2 3 , (K 7) = 1 ŜId 4 , (K 8) = 2 ŜId 3 .<br />
On this example one can see why monotony can not be<br />
satisfied by SIV on belief bases. Clearly K 6 ⊂ K 7 , but<br />
Ŝ Id (K 6 ) > (K 7). This is explained by the fact that the<br />
ŜId<br />
inconsist<strong>en</strong>cy is more diluted in K 7 , than in K 8 . In K 7 the<br />
formula a is the one that is the most blamed for the inconsist<strong>en</strong>cy<br />
(S K6<br />
(a) = (K 6) = 2 Id<br />
ŜId 3<br />
), since it appears in all<br />
inconsist<strong>en</strong>t sets. Whereas in K 7 inconsist<strong>en</strong>cies are equally<br />
caused by a and by a ∧ b, that <strong>de</strong>creases the responsability<br />
of a, and the whole inconsist<strong>en</strong>cy value of the base.<br />
For a similar reason dominance is not satisfied, we clearly<br />
have a ∧ b ⊢ b (and a ∧ b ⊥), but (K 7) < (K 8).<br />
ŜId ŜId<br />
Applications for Belief Change Operators<br />
As the measures we <strong>de</strong>fine allow us to associate with each<br />
formula its <strong>de</strong>gree of responsibility for the inconsist<strong>en</strong>cy of<br />
the base, they can be used to gui<strong>de</strong> any paraconsist<strong>en</strong>t reasoning,<br />
or any repair of the base. Let us quote two such<br />
possible uses for belief change operators, first for belief revision<br />
and th<strong>en</strong> for negotiation.<br />
88
Iterated Revision and Transmutation Policies<br />
The problem of belief revision is to incorporate a new piece<br />
of information which is more reliable than (and conflicting<br />
with) the old beliefs of the ag<strong>en</strong>t. This problem has received<br />
a nice answer in the work of Alchourron, Gar<strong>de</strong>nfors,<br />
Makinson (Alchourrón, Gär<strong>de</strong>nfors, & Makinson 1985) in<br />
the one-step case. But wh<strong>en</strong> one wants to iterate revision<br />
(i.e. to g<strong>en</strong>eralize it to the n-steps case), there are numerous<br />
problems and no <strong>de</strong>finitive answer has be<strong>en</strong> reached<br />
in the purely qualitative case (Darwiche & Pearl 1997;<br />
Friedman & Halpern 1996). Using a partially quantitative<br />
framework, some proposals have giv<strong>en</strong> interesting results<br />
(see e.g. (Williams 1995; Spohn 1987)). Here “partially<br />
quantitative” means that the incoming piece of information<br />
needs to be labeled by a <strong>de</strong>gree of confi<strong>de</strong>nce <strong>de</strong>noting how<br />
strongly we believe it. The problem in this framework is to<br />
justify the use of such a <strong>de</strong>gree, what does it mean exactly<br />
and where does it come from. One possibility is to use an<br />
inconsist<strong>en</strong>cy measure (or a composite measure computed<br />
from an information measure (Lozinskii 1994; Knight 2003;<br />
Konieczny, Lang, & Marquis 2003) and an inconsist<strong>en</strong>cy<br />
measure) to <strong>de</strong>termine this <strong>de</strong>gree of confi<strong>de</strong>nce. Th<strong>en</strong> one<br />
can <strong>de</strong>fine several policies for the ag<strong>en</strong>t (we can suppose that<br />
an ag<strong>en</strong>t accepts a new piece of information only if it brings<br />
more information than contradiction, etc). We can th<strong>en</strong> use<br />
the partially quantitative framework to <strong>de</strong>rive revision operators<br />
with a nice behaviour. In this setting, since the <strong>de</strong>gree<br />
attached to the incoming information is not a giv<strong>en</strong> data, but<br />
computed directly from the information itself and the ag<strong>en</strong>t<br />
policy (behaviour with respect to information and contradiction,<br />
<strong>en</strong>co<strong>de</strong>d by a composite measure) th<strong>en</strong> the problem of<br />
the justification of the meaning of the <strong>de</strong>grees is avoi<strong>de</strong>d.<br />
Negotiation<br />
The problem of negotiation has be<strong>en</strong> investigated rec<strong>en</strong>tly<br />
un<strong>de</strong>r the scope of belief change tools (Booth 2001; 2002;<br />
2006; Zhang et al. 2004; Meyer et al. 2004; Konieczny<br />
2004; Gauwin, Konieczny, & Marquis 2005). The problem<br />
is to <strong>de</strong>fine operators that take as input belief profiles (multiset<br />
of formulae 2 ) and that produce a new belief profile that<br />
aims to be less conflicting. We call these kind of operators<br />
conciliation operators. The i<strong>de</strong>a followed in (Booth 2002;<br />
2006; Konieczny 2004) to <strong>de</strong>fine conciliation operators is to<br />
use an iterative process where at each step a set of formulae<br />
is selected. These selected formulae are logically weak<strong>en</strong>ed.<br />
The process stops wh<strong>en</strong> one reaches a cons<strong>en</strong>sus, i.e. a consist<strong>en</strong>t<br />
belief profile 3 . Many interesting operators can be <strong>de</strong>fined<br />
wh<strong>en</strong> one fixes the selection function (the function that<br />
selects the formulae that must be weak<strong>en</strong> at each round) and<br />
the weak<strong>en</strong>ing method. In (Konieczny 2004) the selection<br />
function is based on a notion of distance. It can be s<strong>en</strong>sible<br />
if such a distance is meaningful in a particular application. If<br />
not, it is only an arbitrary choice. It would th<strong>en</strong> be s<strong>en</strong>sible to<br />
choose instead one of the inconsist<strong>en</strong>cy measures we <strong>de</strong>fined<br />
2 More exactly belief profiles are sets of belief bases. We use<br />
this simplifying assumption just for avoiding technical <strong>de</strong>tails here.<br />
3 A belief profile is consist<strong>en</strong>t if the conjunction of its formulae<br />
is consist<strong>en</strong>t.<br />
in this paper. So the selection function would choose the formulae<br />
with the highest inconsist<strong>en</strong>cy value. These formulae<br />
are clearly the more problematic ones. More g<strong>en</strong>erally SIVs<br />
can be used to <strong>de</strong>fine new belief merging methods.<br />
Conclusion<br />
We have proposed in this paper a new framework for<br />
<strong>de</strong>fining inconsist<strong>en</strong>cy values. The SIV values we introduce<br />
allow us to take into account the distribution of the inconsist<strong>en</strong>cy<br />
among the formulae of the belief base and the variables<br />
of the language. This is, as far as we know, the only<br />
<strong>de</strong>finition that allows us to take both types of information<br />
into account, thus allowing to have a more precise picture of<br />
the inconsist<strong>en</strong>cy of a belief base. The perspectives of this<br />
work are numerous. First, as sketched in the previous section,<br />
the use of inconsist<strong>en</strong>cy measures, and especially the<br />
use of Shapley inconsist<strong>en</strong>cy values, can be valuable for several<br />
belief change operators, for instance for mo<strong>de</strong>lizations<br />
of negotiation. The Shapley value is not the only solution<br />
concept for coalitional games, so an interesting question is<br />
to know if other solutions concept can be s<strong>en</strong>sible as a basis<br />
for <strong>de</strong>fining other inconsist<strong>en</strong>cy measures. But the main way<br />
of research op<strong>en</strong>ed by this work is to study more closely the<br />
connections betwe<strong>en</strong> other notions of (cooperative) game<br />
theory and the logical mo<strong>de</strong>lization of belief change operators.<br />
Acknowledgm<strong>en</strong>ts<br />
The authors would like to thank CNRS and the Royal Society<br />
for travel funding while collaborating on this research.<br />
The second author is supported by the Région Nord/Pas-<strong>de</strong>-<br />
Calais and the European Community FEDER program.<br />
Proofs<br />
Proof of Proposition 3 : To show distribution, let us<br />
recall that<br />
SI K(α) = (c−1)!(n−c)!<br />
∑C⊆K n!<br />
(I(C) − I(C \ {α}))<br />
∑<br />
1<br />
=<br />
n!<br />
I(pα σ∈σn σ ∪ {α}) − I(pα σ )<br />
where σ n is the set of possible permutations on K, and<br />
p α σ = {β ∈ K | σ(β) < σ(α)}. Now<br />
∑<br />
α∈K S I(α) = ∑ α∈K 1 ∑<br />
n!<br />
I(pα σ∈σn σ ∪ {α}) − I(p α σ)<br />
= 1 ∑ ∑<br />
n! σ∈σn α∈K I(pα σ ∪ {α}) − I(pα σ )<br />
Now note that we can or<strong>de</strong>r the elem<strong>en</strong>ts of K accordingly<br />
to σ wh<strong>en</strong> computing the insi<strong>de</strong> sum, that gives:<br />
= 1 ∑<br />
n!<br />
[I({α σ∈σn σ(1), . . . , α σ(n) })<br />
−I({α σ(1) , . . . , α σ(n−1) })]<br />
+[I({α σ(1) , . . . , α σ(n−1) })<br />
−I({α σ(1) , . . . , α σ(n−2) })]<br />
+ . . . + [I({α σ(1) }) − I(∅)]<br />
= 1 ∑<br />
n!<br />
I({α σ∈σn σ(1), . . . , α σ(n) }) − I(∅)<br />
= 1 n!<br />
n! I(K)<br />
= I(K)<br />
To show symmetry, assume that there are α, β ∈ K s.t. for<br />
all K ′ ⊆ K s.t. α, β /∈ K ′ , I(K ′ ∪ {α}) = I(K ′ ∪ {β}).<br />
89
Now by <strong>de</strong>finition<br />
S K I (α) = ∑ C⊆K<br />
(c − 1)!(n − c)!<br />
(I(C) − I(C \ {α}))<br />
n!<br />
Let us show that SI K(α) = SK I (β) by showing (by cases)<br />
that the elem<strong>en</strong>ts of the sum are the same:<br />
If α ∉ C and β ∉ C, th<strong>en</strong> I(C) = I(C\{α}) = I(C\{β}),<br />
so I(C) − I(C \ {α}) = I(C) − I(C \ {β}).<br />
If α ∈ C and β ∈ C, th<strong>en</strong> note that by hypothesis, as α, β /∈<br />
C \ {α, β}, we <strong>de</strong>duce that I(C \ {α}) = I(C \ {β}). So<br />
I(C) − I(C \ {α}) = I(C) − I(C \ {β}).<br />
If α ∈ C and β ∉ C. Th<strong>en</strong> I(C)−I(C\{β}) = 0, and let us<br />
note I(C)−I(C \{α}) = a. Let us note C = C ′ ∪{α}, and<br />
C ′′ = C ′ ∪{β}. Now notice that I(C ′′ )−I(C ′′ \{α}) = 0,<br />
and as we can <strong>de</strong>duce I(C \ {α}) = I(C ′′ \ {β}) by the<br />
hypothesis, we also have I(C ′′ ) − I(C ′′ \ {β}) = a.<br />
To show the free formula property, just note that if α is a<br />
free formula of K, th<strong>en</strong> for every subset C of K, by the free<br />
formula in<strong>de</strong>p<strong>en</strong><strong>de</strong>nce property of the basic inconsist<strong>en</strong>cy<br />
measure we have that for every C, such that α ∈ C, I(C) =<br />
I(C \ α), so I(C) − I(C \ α) = 0. Straightforwardly if α ∉<br />
C, I(C) = I(C \ α). So the whole expression SI K(α) =<br />
∑<br />
C⊆K<br />
(c−1)!(n−c)!<br />
n!<br />
(I(C) − I(C \ {α})) sums to 0.<br />
Finally, to show dominance we will proceed in a similar<br />
way than to show symmetry. Assume that α, β ∈ K<br />
are such that α ⊢ β and α ⊥. Th<strong>en</strong>, by the dominance<br />
property of the un<strong>de</strong>rlying basic inconsist<strong>en</strong>cy measure,<br />
we know that for all C ⊆ K, I(C ∪ {α}) ≥<br />
I(C ∪ {β}). Now by <strong>de</strong>finition of the SIV SI K(α) =<br />
∑<br />
C⊆K<br />
(c−1)!(n−c)!<br />
n!<br />
(I(C) − I(C \ {α})). Let us show that<br />
SI K(α) ≥ SK I (β) by showing (by cases) that the elem<strong>en</strong>ts<br />
of the first sum are greater or equal to the corresponding elem<strong>en</strong>ts<br />
of the second one:<br />
If α ∉ C and β ∉ C, th<strong>en</strong> I(C) = I(C\{α}) = I(C\{β}),<br />
so I(C) − I(C \ {α}) ≥ I(C) − I(C \ {β}).<br />
If α ∈ C and β ∈ C, th<strong>en</strong> let us note C \ {α} = C ′ ∪ {β}.<br />
So we also have C \ {β} = C ′ ∪ {α}. Now note that by<br />
hypothesis I(C ′ ∪ {β}) ≤ I(C ′ ∪ {α}), so I(C \ {α}) ≤<br />
I(C \{β}). H<strong>en</strong>ce I(C)−I(C \{α}) ≥ I(C)−I(C \{β}).<br />
If α ∈ C and β ∉ C. Th<strong>en</strong> I(C)−I(C\{β}) = 0, and let us<br />
note I(C)−I(C \{α}) = a. Let us note C = C ′ ∪{α}, and<br />
C ′′ = C ′ ∪{β}. Now notice that I(C ′′ )−I(C ′′ \{α}) = 0.<br />
So I(C) − I(C \ {β}) ≥ I(C ′′ ) − I(C ′′ \ {α}) Note that<br />
I(C)\{β} = I(C ′′ )\{α} = C ′ . As we can <strong>de</strong>duce I(C) ≥<br />
I(C ′′ ) by the hypothesis, we also have I(C)−I(C \{α}) ≥<br />
I(C ′′ ) − I(C ′′ \ {β}).<br />
Proof of Proposition 4 : To prove consist<strong>en</strong>cy note that if<br />
K is consist<strong>en</strong>t, th<strong>en</strong> for every C ⊆ K, I(C) = 0 (this is a<br />
direct consequ<strong>en</strong>ce of the consist<strong>en</strong>cy property of the un<strong>de</strong>rlying<br />
basic inconsist<strong>en</strong>cy measure). Th<strong>en</strong> for every α ∈ K,<br />
SI K(α) = 0. H<strong>en</strong>ce ŜI(K) = max α∈K S I (α) = 0. For the<br />
only if direction, by contradiction, suppose that ŜI(K) = 0<br />
□<br />
and that K is not consist<strong>en</strong>t. As K is not consist<strong>en</strong>t, th<strong>en</strong> by<br />
the consist<strong>en</strong>cy property of the un<strong>de</strong>rlying basic inconsist<strong>en</strong>cy<br />
measure I(K) = a ≠ 0. By the distribution property<br />
of the SIV we know that ∑ α∈K S I(α) = a ≠ 0, th<strong>en</strong> ∃α ∈<br />
K such that S I (α) > 0, so ŜI(K) = max α∈K S I (α) > 0.<br />
Contradiction.<br />
The normalization property is a consequ<strong>en</strong>ce of the <strong>de</strong>finition<br />
of ŜI(K) as a maximum of values that are all greater<br />
than zero, that <strong>en</strong>sures 0 ≤ ŜI(K), and that are all smaller<br />
than 1. An easy way to show ŜI(K) ≤ 1 is as a consequ<strong>en</strong>ce<br />
of the upper bound property (shown below) ŜI(K) ≤ I(K)<br />
and of I(K) ≤ 1 obtained by the normalization property of<br />
the un<strong>de</strong>rlying basic inconsist<strong>en</strong>cy measure I.<br />
To show the free formula in<strong>de</strong>p<strong>en</strong><strong>de</strong>nce property, just notice<br />
that for any formula β that is a free formula of K ∪ {β},<br />
it is also a free formula of every of its subsets. It is easy<br />
to see from the <strong>de</strong>finition that for any α ∈ K, S K I (α) =<br />
(α). This is easier if we consi<strong>de</strong>r the second form of<br />
the <strong>de</strong>finition: SI K(α) = 1 ∑<br />
n!<br />
I(pα σ∈σn σ ∪ {α}) − I(pα σ )<br />
where σ n is the set of possible permutations on K. Now<br />
note that for S K∪{β}<br />
I<br />
(α), the free formula does not bring<br />
any contradiction, so it does not change the marginal contribution<br />
of every other formulae. Let us call the ext<strong>en</strong>sions of<br />
a permutation σ on K by β, all the permutations of K ∪ {β}<br />
whose restriction on elem<strong>en</strong>ts of K is i<strong>de</strong>ntical to σ, i.e.<br />
an ext<strong>en</strong>sion of σ = (α 1 , . . . , α n ) by β is a permutation<br />
S K∪{β}<br />
I<br />
= (α 1 , . . . , α i , β, α i+1 , . . . , α n ). Now note that there<br />
are n + 1 such ext<strong>en</strong>sions, and that if σ ′ is an ext<strong>en</strong>sion of<br />
sigma, I(p α σ ∪ {α}) − I(pα σ ) = I(pα σ ∪ {α}) − ′ I(pα σ ′). So<br />
S K∪{β}<br />
I<br />
(α) = 1<br />
(n+1)! (n + 1) ∑ I(pα σ∈σn σ ∪ {α}) − I(pα σ )<br />
σ ′<br />
= 1 ∑<br />
n!<br />
I(pα σ∈σn σ ∪ {α}) − I(p α σ) = SI K (α). Now as we<br />
have for any α ∈ K, SI K (α) = SK∪{β}<br />
I<br />
(α), we have<br />
Ŝ I (K ∪ {α}) = ŜI(K).<br />
The ∑ upper bound property is stated by rewriting I(K) as<br />
α∈K S I(α) with the distribution property of the SIV, and<br />
by recalling the <strong>de</strong>finition of ŜI(K) as max α∈K S I (α).<br />
Now by noticing that for every vector a = (a 1 , . . . , a n ),<br />
max ai∈a a i ≤ ∑ ai∈a a i, we conclu<strong>de</strong> max α∈K S I (α) ≤<br />
∑<br />
α∈K S I(α), i.e. ŜI(K) ≤ I(K).<br />
Let us show isolation. The if direction is straighforward:<br />
As α is inconsist<strong>en</strong>t, K is inconsist<strong>en</strong>t, and by the consist<strong>en</strong>cy<br />
and normalization properties of the un<strong>de</strong>rlying basic<br />
inconsist<strong>en</strong>cy measure we know that I(K) > 0. By the free<br />
formula property of SIV, for every free formula β of K we<br />
∑have S I (β) = 0. As by the distribution property we have<br />
α∈K S I(α) = I(K), this means that S I (α) = I(K),<br />
and that ŜI(K) = max α∈K S I (α) = S I (α). So ŜI(K) =<br />
I(K) > 0. For the only if direction suppose that ŜI(K) =<br />
I(K), that means that max α∈K S I (α) = I(K). But, by the<br />
distribution property we know that I(K) = ∑ α∈K S I(α).<br />
So it means that max α∈K S I (α) = ∑ α∈K S I(α) = I(K).<br />
There exists α such that S I (α) = I(K) (consequ<strong>en</strong>ce of the<br />
<strong>de</strong>finition of the max), and if there exists a β ≠ α such that<br />
90
S I (β) > 0, th<strong>en</strong> ∑ α∈K S I(α) > I(K). Contradiction. So<br />
it means that there is α such that S I (α) = I(K) and for<br />
every β ≠ α, S I (β) = 0. That means that every β is a free<br />
formula, and that α is inconsist<strong>en</strong>t.<br />
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92
MEASURING INCONSISTENCY THROUGH MINIMAL<br />
INCONSISTENT SETS<br />
Anthony Hunter, Sébasti<strong>en</strong> Konieczny.<br />
Elev<strong>en</strong>th International Confer<strong>en</strong>ce on Principles of Knowledge Repres<strong>en</strong>tation<br />
and Reasoning (KR’08).<br />
pages 358-366.<br />
2008.<br />
93
Measuring Inconsist<strong>en</strong>cy through Minimal Inconsist<strong>en</strong>t Sets<br />
Anthony Hunter<br />
Departm<strong>en</strong>t of Computer Sci<strong>en</strong>ce<br />
University College London, UK<br />
a.hunter@cs.ucl.ac.uk<br />
Sébasti<strong>en</strong> Konieczny<br />
CRIL - CNRS<br />
Université d’Artois, L<strong>en</strong>s, France<br />
konieczny@cril.fr<br />
Abstract<br />
In this paper, we explore the links betwe<strong>en</strong> measures of inconsist<strong>en</strong>cy<br />
for a belief base and the minimal inconsist<strong>en</strong>t<br />
subsets of that belief base. The minimal inconsist<strong>en</strong>t subsets<br />
can be consi<strong>de</strong>red as the relevant part of the base to take into<br />
account to evaluate the amount of inconsist<strong>en</strong>cy. We <strong>de</strong>fine<br />
a very natural inconsist<strong>en</strong>cy value from these minimal inconsist<strong>en</strong>t<br />
sets. Th<strong>en</strong> we show that the inconsist<strong>en</strong>cy value we<br />
obtain is a particular Shapley Inconsist<strong>en</strong>cy Value, and we<br />
provi<strong>de</strong> a complete axiomatization of this value in terms of<br />
five simple and intuitive axioms. Defining this Shapley Inconsist<strong>en</strong>cy<br />
Value using the notion of minimal inconsist<strong>en</strong>t<br />
subsets allows us to look forward to a viable implem<strong>en</strong>tation<br />
of this value using SAT solvers.<br />
Introduction<br />
The need to <strong>de</strong>velop robust, but principled, logic-based techniques<br />
for analysing inconsist<strong>en</strong>t information is increasingly<br />
recognized as an important research area for artificial intellig<strong>en</strong>ce<br />
in particular, and for computer sci<strong>en</strong>ce in g<strong>en</strong>eral<br />
(Bertossi, Hunter, & Schaub 2004). This interest stems from<br />
the recognition that the dichotomy betwe<strong>en</strong> consist<strong>en</strong>t and<br />
inconsist<strong>en</strong>t sets of formulae that comes from classical logics<br />
is not suffici<strong>en</strong>t for <strong><strong>de</strong>s</strong>cribing inconsist<strong>en</strong>t information.<br />
A number of proposals have be<strong>en</strong> ma<strong>de</strong> for measuring the<br />
<strong>de</strong>gree of information of a belief base in the pres<strong>en</strong>ce of inconsist<strong>en</strong>cy<br />
(Lozinskii 1994; Wong & Besnard 2001; Knight<br />
2003; Konieczny, Lang, & Marquis 2003), and for measuring<br />
the <strong>de</strong>gree of inconsist<strong>en</strong>cy of a belief base (Grant 1978;<br />
Knight 2001; Hunter 2002; Knight 2003; Konieczny, Lang,<br />
& Marquis 2003; Hunter 2004; 2003; Grant & Hunter 2006;<br />
Hunter & Konieczny 2006; Grant & Hunter 2008). For a<br />
review see (Hunter & Konieczny 2004).<br />
These measures are pot<strong>en</strong>tially important in diverse applications<br />
in artificial intellig<strong>en</strong>ce, such as belief revision,<br />
belief merging, negotiation, multi-ag<strong>en</strong>t systems, <strong>de</strong>cisionsupport,<br />
and software <strong>en</strong>gineering tools. Already, measuring<br />
inconsist<strong>en</strong>cy has be<strong>en</strong> se<strong>en</strong> to be a useful tool in<br />
analysing a diverse range of information types including<br />
news reports (Hunter 2006), integrity constraints (Grant &<br />
Hunter 2006), information merging (Qi, Liu, & Bell 2005),<br />
databases (Martinez et al. 2007), ontologies (Ma et al.<br />
Copyright c○ 2008, Association for the Advancem<strong>en</strong>t of Artificial<br />
Intellig<strong>en</strong>ce (www.aaai.org). All rights reserved.<br />
2007), software specifications (Barragáns-Martńez, Pazos-<br />
Arias, & Fernán<strong>de</strong>z-Vilas 2004; Mu et al. 2005), and ecommerce<br />
protocols (Ch<strong>en</strong>, Zhang, & Zhang 2004).<br />
Each of the curr<strong>en</strong>t proposals for measuring inconsist<strong>en</strong>cy<br />
can be <strong><strong>de</strong>s</strong>cribed as being one of the following two approaches.<br />
The first approach involves “counting” the minimal number<br />
of formulae nee<strong>de</strong>d to produce the inconsist<strong>en</strong>cy in a set<br />
of formulae. The more formulae nee<strong>de</strong>d to produce the inconsist<strong>en</strong>cy,<br />
the less inconsist<strong>en</strong>t the set (Knight 2001). This<br />
i<strong>de</strong>a is an interesting one, but it rejects the possibility of a<br />
more fine-grained inspection of the (cont<strong>en</strong>t of the) formulae.<br />
In particular, if one looks to singleton sets only, one is<br />
back to the initial problem, with only two values: consist<strong>en</strong>t<br />
or inconsist<strong>en</strong>t.<br />
The second approach involves looking at the proportion<br />
of the language that is touched by the inconsist<strong>en</strong>cy in a<br />
set of formulae. This allows us to look insi<strong>de</strong> the formulae<br />
(Hunter 2002; Konieczny, Lang, & Marquis 2003;<br />
Grant & Hunter 2006; 2008). This means that two formulae<br />
(singleton sets) can have differ<strong>en</strong>t inconsist<strong>en</strong>cy measures.<br />
In these proposals one can i<strong>de</strong>ntify the set of formulae with<br />
its conjunction (i.e. the set {ϕ, ϕ ′ } has the same inconsist<strong>en</strong>cy<br />
measure as the set {ϕ∧ϕ ′ }). Whilst the lack of syntax<br />
s<strong>en</strong>sitivity may be appropriate for some applications, it does<br />
mean that the distribution of the contradiction among the<br />
formulae is not tak<strong>en</strong> into account.<br />
It seems difficult to build measures that take these two dim<strong>en</strong>sions<br />
into account. But, in (Hunter & Konieczny 2006)<br />
a unified framework was proposed with this aim. The main<br />
point was to <strong>de</strong>fine inconsist<strong>en</strong>cy values that give the inconsist<strong>en</strong>cy<br />
of each formula of the base, in constrast to the<br />
above inconsist<strong>en</strong>cy measures that give the inconsist<strong>en</strong>cy of<br />
the whole base. This allows us to draw a more precise picture<br />
of the inconsist<strong>en</strong>cies of the base. The i<strong>de</strong>a is to start<br />
from one of the measures consi<strong>de</strong>red in the two approaches<br />
above and use it to assign a measure of inconsist<strong>en</strong>cy to a set<br />
of formulae, and th<strong>en</strong> to use a technique based on cooperative<br />
game theory: the Shapley value (Shapley 1953). This<br />
th<strong>en</strong> allows us to i<strong>de</strong>ntify the blame/responsibility of each<br />
formula in the inconsist<strong>en</strong>cy of the belief base (Hunter &<br />
Konieczny 2006). This means for example that we can use a<br />
measure from the second approach which consi<strong>de</strong>rs the proportion<br />
of the language touched by inconsist<strong>en</strong>cy, and th<strong>en</strong><br />
95
using the Shapley value, apportion the blame for the inconsist<strong>en</strong>cy<br />
in the set to the individual formulae in a principled<br />
way.<br />
Against this background, it is interesting to note that the<br />
use of minimal inconsist<strong>en</strong>t subsets of a belief base has received<br />
much less att<strong>en</strong>tion as the basis for <strong>de</strong>fining inconsist<strong>en</strong>cy<br />
measures. So in this paper, we explore the nature of<br />
some interesting measures of inconsist<strong>en</strong>cy based on minimal<br />
inconsist<strong>en</strong>t subsets of the belief base, and we consi<strong>de</strong>r<br />
how these new measures relate to the family of Shapley Inconsist<strong>en</strong>cy<br />
Values.<br />
It has be<strong>en</strong> known for a long time that minimal inconsist<strong>en</strong>t<br />
subsets of the base are a cornerstone of analysing inconsist<strong>en</strong>cies.<br />
For instance, to recover consist<strong>en</strong>cy, one has just<br />
to remove one formula from each minimal inconsist<strong>en</strong>t subset<br />
(Reiter 1987). For conflict resolution, where the syntactic<br />
repres<strong>en</strong>tation of the information is important, measuring<br />
inconsist<strong>en</strong>cy in terms of the minimal inconsist<strong>en</strong>t subsets is<br />
intuitive and it is informative for <strong>de</strong>ciding how to change the<br />
set of formulae through a process such as negotiation, compromise,<br />
or resolution. So it should be natural to study how<br />
these minimal inconsist<strong>en</strong>t subsets could be used to <strong>de</strong>fine<br />
measures of inconsist<strong>en</strong>cy.<br />
The i<strong>de</strong>a to analyse minimal inconsist<strong>en</strong>t subsets of the<br />
belief base was followed in or<strong>de</strong>r to <strong>de</strong>fine scoring functions<br />
that, for each subset K ′ of a set of formulae K, gives a score<br />
that is the number of minimal inconsist<strong>en</strong>t subsets of K that<br />
would be eliminated if K ′ were removed from K. Th<strong>en</strong> this<br />
score is used to compare differ<strong>en</strong>t subsets K ′ (Hunter 2004).<br />
Obviously, these approaches are syntax s<strong>en</strong>sitive, which<br />
for some applications, is necessary. Consi<strong>de</strong>r capturing<br />
requirem<strong>en</strong>ts for a new corporate computer system in a<br />
process where a user may pres<strong>en</strong>t his or her requirem<strong>en</strong>ts<br />
in the form of a set of propositional formulae (Hunter &<br />
Nuseibeh 1998). Here pres<strong>en</strong>ting the set of requirem<strong>en</strong>ts<br />
{α, β} should be treated differ<strong>en</strong>tly to the set of requirem<strong>en</strong>ts<br />
{α ∧ β} since the first set says that there are two requirem<strong>en</strong>ts,<br />
the first being α and the second β, whereas in<br />
the second set says that is one requirem<strong>en</strong>t, namely α ∧ β.<br />
In the rest of this paper, we study how to use minimal<br />
inconsist<strong>en</strong>t subsets in or<strong>de</strong>r to <strong>de</strong>fine inconsist<strong>en</strong>cy values,<br />
that will allow us to <strong>de</strong>fine the inconsist<strong>en</strong>cy of each formula<br />
of the base (like with Shapley Inconsist<strong>en</strong>cy Values (Hunter<br />
& Konieczny 2006)). To this <strong>en</strong>d we introduce the family of<br />
MIV (for MinInc Inconsist<strong>en</strong>cy Value), and focus on a very<br />
intuitive one: MIV C . As these values have the same aim as<br />
the family of Shapley Inconsist<strong>en</strong>cy Values, it is interesting<br />
to study the relationship betwe<strong>en</strong> these two families. Moreover,<br />
we show a surprising result: the value MIV C is in<br />
fact a Shapley Inconsist<strong>en</strong>cy Value. This result is very interesting<br />
as it allows us to state interesting logical properties,<br />
and to find a way to implem<strong>en</strong>t the Shapley Inconsist<strong>en</strong>cy<br />
Values using existing automated reasoning technology. The<br />
additional interest of this value is that its intuitive logical<br />
properties have led us to a complete axiomatization through<br />
five intuitive axioms.<br />
Preliminaries<br />
We consi<strong>de</strong>r a propositional language L built from a finite<br />
set of propositional symbols P. We use a, b, c, . . . to <strong>de</strong>note<br />
the propositional variables, and Greek letters α, β, ϕ, . . . to<br />
<strong>de</strong>note the formulae. An interpretation is a total function<br />
from P to {0, 1}. The set of all interpretations is <strong>de</strong>noted<br />
W. An interpretation ω is a mo<strong>de</strong>l of a formula ϕ, <strong>de</strong>noted<br />
ω |= ϕ, if and only if it makes ϕ true in the usual truthfunctional<br />
way. Mod(ϕ) <strong>de</strong>notes the set of mo<strong>de</strong>ls of the<br />
formula ϕ, i.e. Mod(ϕ) = {ω ∈ W | ω |= ϕ}. We will use<br />
⊆ to <strong>de</strong>note the set inclusion, and we will use ⊂ to <strong>de</strong>note<br />
the strict set inclusion, i.e. A ⊂ B iff A ⊆ B and B ⊈ A.<br />
We will <strong>de</strong>note the set of natural numbers by N and the set<br />
of real numbers by R.<br />
Let A and B be two subsets of C, we note C = A⊕B if A<br />
and B form a partition of C, i.e. C = A ⊕ B iff C = A ∪ B<br />
and A ∩ B = ∅.<br />
A belief base K is a finite set of propositional formulae.<br />
More exactly, as we will need to i<strong>de</strong>ntify the differ<strong>en</strong>t formulae<br />
of a belief base in or<strong>de</strong>r to associate them with their<br />
inconsist<strong>en</strong>cy value, we will consi<strong>de</strong>r belief bases K as vectors<br />
of formulae. For logical properties we will need to use<br />
the set corresponding to each vector, so we suppose that we<br />
have a function such that for each vector K = (α 1 , . . . , α n ),<br />
K is the set {α 1 , . . . , α n }. As it will never be ambiguous,<br />
in the following we will omit the and write K as both the<br />
vector and the set. We use K L to <strong>de</strong>note the set of belief<br />
bases <strong>de</strong>finable from formulae of the language L.<br />
A belief base is consist<strong>en</strong>t if there is at least one interpretation<br />
that satisfies all its formulae. If a belief base K is<br />
not consist<strong>en</strong>t, th<strong>en</strong> one can <strong>de</strong>fine the minimal inconsist<strong>en</strong>t<br />
subsets of K as:<br />
MI(K) = {K ′ ⊆ K | K ′ ⊢ ⊥ and ∀K ′′ ⊂ K ′ , K ′′ ⊥}<br />
If one wants to recover consist<strong>en</strong>cy from an inconsist<strong>en</strong>t<br />
base K, th<strong>en</strong> the minimal inconsist<strong>en</strong>t subsets can be consi<strong>de</strong>red<br />
as the purest form of inconsist<strong>en</strong>cy, since to recover<br />
consist<strong>en</strong>cy, one has to remove at least one formula from<br />
each minimal inconsist<strong>en</strong>t subset (Reiter 1987).<br />
A free formula of a belief base K is a formula of K that<br />
does not belong to any minimal inconsist<strong>en</strong>t subset of the<br />
belief base K. This means that this formula has nothing to<br />
do with the conflicts of the base.<br />
Inconsist<strong>en</strong>cy values <strong>de</strong>fined from minimal<br />
inconsist<strong>en</strong>t sets<br />
Apart from (Hunter & Konieczny 2006), existing inconsist<strong>en</strong>cy<br />
measures allow us to evaluate the amount of inconsist<strong>en</strong>cy<br />
of a whole base, but not to evaluate the amount of<br />
inconsist<strong>en</strong>cy of each formula of the base. In other words,<br />
existing inconsist<strong>en</strong>cy measures do not allow us to evaluate<br />
the responsability of each formula in the inconsist<strong>en</strong>cy of the<br />
base. Yet it is possible to <strong>de</strong>fine some such measures using<br />
minimal inconsist<strong>en</strong>t sets. Furthermore, as these minimal inconsist<strong>en</strong>t<br />
sets are the parts of the base where inconsist<strong>en</strong>cies<br />
lie, it should be natural to use only the minimal inconsist<strong>en</strong>t<br />
sets for evaluating the amount of inconsist<strong>en</strong>cy of the bases.<br />
This is the motivation for the following <strong>de</strong>finition where f<br />
96
is some function that takes as input a formula α and the set<br />
of minimal inconsist<strong>en</strong>t subsets for a belief base K.<br />
Definition 1 A MinInc Inconsist<strong>en</strong>cy Value (MIV ) is a<br />
function MIV : K L × L → R such that MIV (K, α) =<br />
f(α, MI(K)) where f is a function of α and MI(K).<br />
Instances of a MIV (such as those giv<strong>en</strong> in Definitions 2<br />
and 4) <strong>de</strong>p<strong>en</strong>d on the choice of function f.<br />
So this <strong>de</strong>finition states exactly the fact that the inconsist<strong>en</strong>cy<br />
value only takes into account the minimal inconsist<strong>en</strong>t<br />
subsets of the base. In particular two differ<strong>en</strong>t bases K and<br />
K ′ with exactly the same minimal inconsist<strong>en</strong>t sets will have<br />
the same amount of inconsist<strong>en</strong>cy.<br />
The simplest types of MIV one can <strong>de</strong>fine are the following<br />
ones:<br />
Definition 2 MIV D and MIV # are <strong>de</strong>fined as follows:<br />
{<br />
1 if ∃M ∈ MI(K) α ∈ M<br />
• MIV D (K, α) =<br />
0 otherwise<br />
• MIV # (K, α) = |{M ∈ MI(K) | α ∈ M}|<br />
The first value is the drastic one, that takes value one if the<br />
formula belongs to a minimal inconsist<strong>en</strong>t subset, and zero<br />
otherwise. The second one is a cardinality value, that counts<br />
the number of minimal inconsist<strong>en</strong>t subsets the formula belongs<br />
to.<br />
The first value is of little interest, since it allows us just<br />
to make a distinction betwe<strong>en</strong> free formulas and the other<br />
ones. The second one is more useful, and allows us to find<br />
more interesting results. Let us check this on the following<br />
example.<br />
Example 1 Let K 1 = {a, ¬a, ¬a ∧ c, a ∨ d, ¬d, b ∧ ¬b, e},<br />
so the minimal inconsist<strong>en</strong>t subsets of K 1 are<br />
MI(K 1 ) = {{b∧¬b}, {a, ¬a}, {a, ¬a∧c}, {¬a, a∨d, ¬d}}<br />
and the MIV # value gives as a result:<br />
MIV # (K 1 , a) = 2 MIV # (K 1 , ¬d) = 1<br />
MIV # (K 1 , ¬a) = 2 MIV # (K 1 , b ∧ ¬b) = 1<br />
MIV # (K 1 , ¬a ∧ c) = 1 MIV # (K 1 , e) = 0<br />
MIV # (K 1 , a ∨ d) = 1<br />
It is easy to check that MIV # is just a scoring function<br />
(Hunter 2004) applied uniquely on formulae:<br />
Definition 3 Let K ∈ K L . Let S be the scoring function<br />
for K <strong>de</strong>fined as follows, where S : ℘(K) ↦→ N and K ′ ∈<br />
℘(K)<br />
S(K ′ ) = |MI(K)| − |MI(K − K ′ )|<br />
For a belief base K, a scoring function S gives the number<br />
of minimal inconsist<strong>en</strong>t subsets of K that would be eliminated<br />
if the subset K ′ was removed from K. See (Hunter<br />
2004) for more <strong>de</strong>tails on the use of these scoring functions.<br />
So if α ∈ K, it is straighforward to see that we have<br />
MIV # (α, K) = S({α}).<br />
The evaluation of the inconsist<strong>en</strong>cy value of each formula<br />
giv<strong>en</strong> by MIV # is still very rough. In particular, it does<br />
not take into account the cardinalities of the minimal inconsist<strong>en</strong>t<br />
subsets the formula belongs to. But, as explained in<br />
several works (see for example (Knight 2001)), the size of<br />
the minimal inconsist<strong>en</strong>t subset can have an impact on the<br />
evaluation of the inconsist<strong>en</strong>cy. The i<strong>de</strong>a is that the smaller<br />
the value of the minimal inconsist<strong>en</strong>t subset, the bigger is<br />
the inconsist<strong>en</strong>cy. To illustrate this we use the prototypical<br />
example of the lottery paradox giv<strong>en</strong> by Knight to motivate<br />
his approach.<br />
Example 2 There are a number of lottery tickets with one<br />
of them being the winning ticket. Suppose w i <strong>de</strong>notes ticket<br />
i will win, th<strong>en</strong> we have the assumption w 1 ∨ . . . ∨ w n . In<br />
addition, for each ticket i, we may pessimistically (or probabilistically<br />
if the number of tickets is important) assume that<br />
it will not win, and this is repres<strong>en</strong>ted by the assumption<br />
¬w i . So the base K L is:<br />
K L = {¬w 1 , . . . , ¬w n , w 1 ∨ . . . ∨ w n }<br />
Clearly if there are three or two (or one!) tickets in the<br />
lottery, th<strong>en</strong> this base is highly inconsist<strong>en</strong>t. But if there are<br />
millions of tickets there is intuitively (nearly) no conflict in<br />
the base.<br />
So it could prove better not to simply take the number<br />
of minimal inconsist<strong>en</strong>t subsets a formula belongs to, but to<br />
take into account their cardinalities. This i<strong>de</strong>a gives a third<br />
MIV value:<br />
Definition 4 MIV C is <strong>de</strong>fined as follows:<br />
∑ 1<br />
MIV C (K, α) =<br />
|M|<br />
M∈MI(K)s.t.α∈M<br />
This allows us to <strong>de</strong>fine a much more precise view of the<br />
inconsist<strong>en</strong>cy, as illustrated in the following example.<br />
Example 3 Let K 1 = {a, ¬a, ¬a ∧ c, a ∨ d, ¬d, b ∧ ¬b, e}<br />
and the MIV C value gives as a result:<br />
MIV C (K 1 , a) = 1 MIV C (K 1 , ¬d) = 1 3<br />
MIV C (K 1 , ¬a) = 5 6<br />
MIV C (K 1 , b ∧ ¬b) = 1<br />
MIV C (K 1 , ¬a ∧ c) = 1 2<br />
MIV C (K 1 , e) = 0<br />
MIV C (K 1 , a ∨ d) = 1 3<br />
We can compare these obtained results with the ones of<br />
Example 1, where less distinction was possible, with only<br />
three differ<strong>en</strong>t levels. We can notice that now we can make<br />
a distinction betwe<strong>en</strong> ¬a ∧ c, and a ∨ d, that both belong to<br />
only one minimal inconsist<strong>en</strong>t subset, but the one of a ∨ d is<br />
bigger. We also note that an inconsist<strong>en</strong>t formula has a high<br />
<strong>de</strong>gree of inconsist<strong>en</strong>cy according to the MIV C value. One<br />
can remark that with MIV C the formula b ∧ ¬b is evaluated<br />
as more conflicting than the formula ¬a which belongs to<br />
two larger minimal inconsist<strong>en</strong>t subsets, whereas the evaluation<br />
is the converse for MIV # .<br />
We can see more clearly on a <strong>de</strong>dicated example why<br />
MIV C gives a more precise view of the conflict brought by<br />
each fomula than MIV # .<br />
Example 4 Consi<strong>de</strong>r K 2 = {a ∧ ¬a, b ∧ ¬b} and K 3 =<br />
{a ∧ ¬b, ¬a ∧ b}. Here we see that the MIV # value assigns<br />
the same value to each formula, ev<strong>en</strong> though for instance<br />
a ∧ ¬a is <strong>en</strong>tirely responsible for an inconsist<strong>en</strong>cy, whereas<br />
a ∧ ¬b is only partially responsible for an inconsist<strong>en</strong>cy.<br />
MIV # (K 2 , a ∧ ¬a) = 1 MIV # (K 3 , a ∧ ¬b) = 1<br />
MIV # (K 2 , b ∧ ¬b) = 1 MIV # (K 3 , ¬a ∧ b) = 1<br />
97
In contrast, the MIV C value is more discriminating and so<br />
for instance a ∧ ¬a, which is <strong>en</strong>tirely responsible for an inconsist<strong>en</strong>cy,<br />
has the maximum value of 1, whereas a ∧ ¬b,<br />
which is only half of the cause of an inconsist<strong>en</strong>cy, has a<br />
value of 1/2.<br />
MIV C (K 2 , a ∧ ¬a) = 1 MIV C (K 3 , a ∧ ¬b) = 1 2<br />
MIV C (K 2 , b ∧ ¬b) = 1 MIV C (K 3 , ¬a ∧ b) = 1 2<br />
More g<strong>en</strong>erally, we see that the MIV C value is affected<br />
by the size of each minimal inconsist<strong>en</strong>t subset: Returning<br />
to Example 2, we see that for K L and for some α ∈ K L , the<br />
value of MIV C (K L , α) <strong>de</strong>creases as the cardinality of K L<br />
increases.<br />
We now give a few observations regarding the MIV C <strong>de</strong>finition.<br />
Other properties will be also <strong>de</strong>rivable from later<br />
results of the paper.<br />
Proposition 1<br />
• If α is a free formula in K, th<strong>en</strong> MIV C (K, α) = 0<br />
• MIV C (K ∪ K ′ , α) ≥ MIV C (K, α)<br />
• If α ≡ ⊥, th<strong>en</strong> MIV C (K, α) = 1<br />
• If φ ⊢ ψ and φ ⊬ ⊥ th<strong>en</strong><br />
MIV C (K ∪ {φ}, α) ≥ MIV C (K ∪ {ψ}, α)<br />
So from the examples and observations in this section, it<br />
seems that MIV C is an appealing and informative measure<br />
of inconsist<strong>en</strong>cy.<br />
Shapley Inconsist<strong>en</strong>cy Values<br />
Shapley Inconsist<strong>en</strong>cy Values were introduced in (Hunter &<br />
Konieczny 2006) in or<strong>de</strong>r to be able to <strong>de</strong>fine a measure of<br />
inconsist<strong>en</strong>cy for each formula, from a measure of inconsist<strong>en</strong>cy<br />
on belief bases.<br />
The i<strong>de</strong>a is to start from one basic measure of inconsist<strong>en</strong>cy<br />
from the literature, that allows us to evaluate the inconsist<strong>en</strong>cy<br />
of a belief base, to use this measure as the <strong>de</strong>finition<br />
of a coalitional game, and to use a notion from cooperative<br />
game theory, the Shapley value (Shapley 1953), that allows<br />
us to <strong>de</strong>fine the merits of one individual in a giv<strong>en</strong> game. In<br />
our setting, with the scale reversal, this amounts to <strong>de</strong>fine<br />
the blame/responsabity of one formula in a giv<strong>en</strong> base for<br />
the inconsist<strong>en</strong>cies.<br />
So the i<strong>de</strong>a is similar to the one that drove the <strong>de</strong>finition<br />
of MIV in the last section. Therefore it is natural to won<strong>de</strong>r<br />
if there are some links betwe<strong>en</strong> the two approaches.<br />
Let us first give the background on Shapley Inconsist<strong>en</strong>cy<br />
Values (SIV). We will just give here the <strong>de</strong>finitions nee<strong>de</strong>d<br />
for this paper and for the proofs, for more <strong>de</strong>tails see (Hunter<br />
& Konieczny 2006).<br />
First we recall the standard <strong>de</strong>finitions of games in coalitional<br />
form and of the Shapley value (Aumann & Hart 2002).<br />
Definition 5 Let N = {1, . . . , n} be a set of n players. A<br />
game in coalitional form is giv<strong>en</strong> by a function v : 2 N → R,<br />
with v(∅) = 0.<br />
This framework <strong>de</strong>fines games in a very abstract way, focusing<br />
on the possible coalition formations. A coalition is<br />
just a subset of N. This function gives what payoff can be<br />
achieved by each coalition in the game v wh<strong>en</strong> all its members<br />
act together as a unit.<br />
A natural notion of solution for this kind of game is to try<br />
to <strong>de</strong>fine the payoff that can be expected by each player i for<br />
the game v. This is what is called a value.<br />
Definition 6 A value is a function that assigns to each game<br />
v a vector of payoff S(v) = (S 1 , . . . , S n ) in R n , where S i is<br />
the payoff for player i.<br />
Despite these very abstract <strong>de</strong>finitions of the game and of<br />
the value, it is possible to <strong>de</strong>fine a notion of solution, i.e. a<br />
value, that gives for any game the expected payoff (merits)<br />
of each player. The first (and main) such value has be<strong>en</strong><br />
<strong>de</strong>fined by Shapley (Shapley 1953).<br />
Basically the i<strong>de</strong>a can be explained as follows: consi<strong>de</strong>ring<br />
that the coalitions form according to some or<strong>de</strong>r (a<br />
first player <strong>en</strong>ters the coalition, th<strong>en</strong> another one, th<strong>en</strong> a<br />
third one, etc), and that the payoff attached to a player is its<br />
marginal utility (i.e. the utility that it brings to the existing<br />
coalition), so if C is a coalition (subset of N) not containing<br />
i, player’s i marginal utility is v(C ∪ {i}) − v(C). As one<br />
can not make any hypothesis on which or<strong>de</strong>r is the correct<br />
one, we may suppose that each or<strong>de</strong>r is equally probable.<br />
This leads to the following formula:<br />
Let σ be a permutation on N, with σ n <strong>de</strong>noting all the<br />
possible permutations on N. We need the following notation:<br />
p i σ = {j ∈ N | σ(j) < σ(i)}<br />
That means that p i σ repres<strong>en</strong>ts all the players that prece<strong>de</strong><br />
player i for a giv<strong>en</strong> or<strong>de</strong>r σ.<br />
Definition 7 Let i ∈ N be a player, and n be the number of<br />
players. The Shapley value of a game v is <strong>de</strong>fined as.<br />
S i (v) = 1 ∑<br />
v(p i σ ∪ {i}) − v(p i<br />
n!<br />
σ)<br />
σ∈σn<br />
The Shapley value can be directly computed from the possible<br />
coalitions (without looking at the permutations) using<br />
the following expression:<br />
S i (v) = ∑ C⊆N<br />
(c − 1)!(n − c)!<br />
(v(C) − v(C \ {i}))<br />
n!<br />
where c is the cardinality of C.<br />
Besi<strong><strong>de</strong>s</strong> the fact that this <strong>de</strong>finition gives very s<strong>en</strong>sible results,<br />
its legitimacy is also giv<strong>en</strong> by a nice characterization<br />
result:<br />
Proposition 2 (Shapley 1953) The Shapley value is the<br />
only value that satisfies all of Effici<strong>en</strong>cy, Symmetry, Dummy<br />
and Additivity.<br />
• ∑ i∈N S i(v) = v(N)<br />
(Effici<strong>en</strong>cy)<br />
• If i and j are such that for all C s.t. i, j /∈ C,<br />
v(C ∪ {i}) = v(C ∪ {j}), th<strong>en</strong> S i (v) = S j (v)<br />
(Symmetry)<br />
98
• If i is such that ∀C v(C ∪ {i}) = v(C), th<strong>en</strong> S i (v) = 0<br />
(Dummy)<br />
• S i (v + w) = S i (v) + S i (w)<br />
(Additivity)<br />
This result supports several variations: there are other<br />
equival<strong>en</strong>t axiomatizations of the Shapley value, and there<br />
are some differ<strong>en</strong>t values that can be <strong>de</strong>fined by relaxing<br />
some of the above axioms. See (Aumann & Hart 2002).<br />
So the i<strong>de</strong>a is to consi<strong>de</strong>r an inconsist<strong>en</strong>cy measure (that<br />
allows us to evaluate the inconsist<strong>en</strong>cy of a belief base) as a<br />
game in coalitional form, and to compute the corresponding<br />
Shapley value, in or<strong>de</strong>r to be able to <strong>de</strong>fine the inconsist<strong>en</strong>cy<br />
value of each formula of the base.<br />
We ask some properties for the un<strong>de</strong>rlying inconsist<strong>en</strong>cy<br />
measure:<br />
Definition 8 An inconsist<strong>en</strong>cy measure I is called a basic<br />
inconsist<strong>en</strong>cy measure if it satisfies the following properties,<br />
∀K, K ′ ∈ K L , ∀α, β ∈ L:<br />
• I(K) = 0 iff K is consist<strong>en</strong>t<br />
(Consist<strong>en</strong>cy)<br />
• I(K ∪ K ′ ) ≥ I(K)<br />
(Monotony)<br />
• If α is a free formula of K∪{α}, th<strong>en</strong> I(K∪{α}) = I(K)<br />
(Free Formula In<strong>de</strong>p<strong>en</strong><strong>de</strong>nce)<br />
• If α ⊢ β and α ⊥, th<strong>en</strong> I(K ∪ {α}) ≥ I(K ∪ {β})<br />
(Dominance)<br />
In (Hunter & Konieczny 2006), a Normalization property<br />
was also pres<strong>en</strong>ted as an optional property. We do not consi<strong>de</strong>r<br />
that property in this paper.<br />
Definition 9 Let I be a basic inconsist<strong>en</strong>cy measure. We <strong>de</strong>fine<br />
the corresponding Shapley Inconsist<strong>en</strong>cy Value (SIV),<br />
noted S I , as the Shapley value of the coalitional game <strong>de</strong>fined<br />
by the function I, i.e. let α ∈ K :<br />
S I α(K) = ∑ C⊆K<br />
(c − 1)!(n − c)!<br />
(I(C) − I(C \ {α}))<br />
n!<br />
where n is the cardinality of K and c is the cardinality of C.<br />
Note that this SIV gives a value for each formula of the<br />
base K. This <strong>de</strong>finition allows us to <strong>de</strong>fine to what ext<strong>en</strong>t a<br />
formula insi<strong>de</strong> a belief base is concerned with the inconsist<strong>en</strong>cies<br />
of the base. It allows us to draw a precise picture of<br />
the contradiction of the base.<br />
So, from a SIV, one can <strong>de</strong>fine an inconsist<strong>en</strong>cy measure<br />
for the whole belief base:<br />
Definition 10 Let K be a belief base,<br />
Ŝ I (K) = max<br />
α∈K SI α(K)<br />
There are alternatives to Definition 10, see the discussion<br />
in (Hunter & Konieczny 2006).<br />
MI Shapley Inconsist<strong>en</strong>cy Value<br />
Since minimal inconsist<strong>en</strong>t subsets of a base can be consi<strong>de</strong>red<br />
as fundam<strong>en</strong>tal features in charaterizing inconsist<strong>en</strong>cy,<br />
we use the notion here as the basic inconsist<strong>en</strong>cy measure.<br />
Definition 11 The MI inconsist<strong>en</strong>cy measure is <strong>de</strong>fined as<br />
the number of minimal inconsist<strong>en</strong>t sets of K, i.e. :<br />
I MI (K) = |MI(K)|<br />
Example 5 K = {a, ¬a, ¬a ∧ c, a ∨ d, ¬d, b ∧ ¬b, e}<br />
H<strong>en</strong>ce, we get the following:<br />
I MI (K) = 4 I MI ({a, ¬a, ¬a ∧ c}) = 2<br />
I MI ({b ∧ ¬b, e}) = 1 I MI ({¬a, ¬a ∧ c}) = 0<br />
Proposition 3 The MI inconsist<strong>en</strong>cy measure I MI is a basic<br />
inconsist<strong>en</strong>cy measure, i.e. it satisfies the properties<br />
of Consist<strong>en</strong>cy, Monotonicity, Free Formula In<strong>de</strong>p<strong>en</strong><strong>de</strong>nce,<br />
and Dominance.<br />
And in fact the following result shows that this MI Shapley<br />
Inconsist<strong>en</strong>cy Value is exactly the MIV C measure of<br />
Definition 4:<br />
Proposition 4 Sα IMI (K) = MIV C (K, α)<br />
Proof: Let us first show the following lemma that will be<br />
useful in the proof.<br />
Lemma 1 If a simple game in coalitional form on a set<br />
of players N = {1, . . . , n} is <strong>de</strong>fined by a single winning<br />
coalition C ′ ⊆ N, i.e:<br />
{<br />
1 if C<br />
v(C) =<br />
′ ⊆ C<br />
0 otherwise<br />
Th<strong>en</strong> the corresponding Shapley value is:<br />
{ 0 if i ∉ C<br />
′<br />
S i (v) = 1<br />
|C ′ |<br />
if i ∈ C ′<br />
Proof of Lemma 1 : The proof is direct using the logical<br />
properties of the Shapley value giv<strong>en</strong> in Proposition 2.<br />
Since by (Dummy) we get that if i ∉ C ′ , th<strong>en</strong> S i (v) = 0.<br />
By (Effici<strong>en</strong>cy) we know that the outcome of the grand<br />
coalition N must be shared in the sum of the Shapley<br />
values of the players: ∑ i∈N S i(v) = 1. Since for players<br />
i ∉ C ′ we know that S i (v) = 0, it means that it has to<br />
be split betwe<strong>en</strong> members of C ′ . So ∑ i∈C S i(v) = 1.<br />
′<br />
Now by (Symmetry) we get that for all i, j ∈ C ′ , we<br />
have S i (v) = S j (v). So this implies that if i ∈ C ′ , th<strong>en</strong><br />
S i (v) = 1<br />
|C ′ | . □<br />
Let us now state the result. First suppose that α is a free<br />
formula of K, th<strong>en</strong> we have immediately by (Minimality)<br />
that Sα<br />
IMI (K) = 0. We also have immediately by <strong>de</strong>finition<br />
that MIV C (K, α) = 0. So the equality is satisfied in this<br />
case.<br />
Now suppose that α is not a free formula of K. First<br />
remark that I MI can be <strong>de</strong>composed in I MI (C) =<br />
∑ ˆM(C), M∈MI(K)<br />
where ˆM is the following characteristic<br />
function<br />
{<br />
1 if M ⊆ C<br />
ˆM(C) =<br />
0 otherwise<br />
Let us <strong>de</strong>note by ˆM(K) the game in coalitional form <strong>de</strong>fined<br />
from K and the characteristic function ˆM.<br />
99
So now let us start from the MI Shapley Inconsist<strong>en</strong>cy Value:<br />
S I MI<br />
α<br />
= X C⊆K<br />
= X C⊆K<br />
(K)<br />
(c − 1)!(n − c)!<br />
(I MI(C) − I MI(C \ {α}))<br />
n!<br />
(c − 1)!(n − c)! X<br />
( ˆM(C)<br />
n!<br />
= X (c − 1)!(n − c)!<br />
(<br />
n!<br />
C⊆K<br />
= X X<br />
C⊆K M∈MI(K)<br />
= X X<br />
M∈MI(K) C⊆K<br />
= X<br />
M∈MI(K)<br />
M∈MI(K)<br />
− P ˆM(C M∈MI(K)<br />
\ {α}))<br />
X<br />
( ˆM(C) − ˆM(C \ {α})))<br />
M∈MI(K)<br />
(c − 1)!(n − c)!<br />
(<br />
n!<br />
ˆM(C) − ˆM(C \ {α}))<br />
(c − 1)!(n − c)!<br />
(<br />
n!<br />
ˆM(C) − ˆM(C \ {α}))<br />
S α( ˆM(K))<br />
Now note that by Lemma 1 we have S α ( ˆM(K)) = 1<br />
|M| .<br />
That gives Sα IMI (K) = ∑ 1<br />
|M| = MIV C(K, α).<br />
M∈MI(K)<br />
This proposition is interesting for several reasons. First, it<br />
confirms the appeal of Shapley Inconsist<strong>en</strong>cy Values, since<br />
the very natural measure MIV C is a special case of these<br />
measures. Second, it gives a simpler <strong>de</strong>finition of Sα<br />
IMI than<br />
the one using the Shapley value. In g<strong>en</strong>eral, obtaining a<br />
Shapley value is computationally <strong>de</strong>manding (D<strong>en</strong>g & Papadimitriou<br />
1994). However, in the case of Sα<br />
IMI , the above<br />
proposition hints at the possibility of computationally viable<br />
implem<strong>en</strong>tations for calculating these values. Finally, this<br />
equality is useful to state the logical properties of this value,<br />
as done in the next Section.<br />
Logical Properties<br />
It is quite difficult to state logical properties about inconsist<strong>en</strong>cy<br />
handling (and measure of inconsist<strong>en</strong>cy) in a purely<br />
classical framework, i.e. without adding too many hypotheses,<br />
by using an (arbitrary) paraconsist<strong>en</strong>t logic to do so.<br />
In (Hunter & Konieczny 2006) some logical properties<br />
for inconsist<strong>en</strong>cy measures are <strong>de</strong>fined, as well as specific<br />
ones for Shapley Inconsist<strong>en</strong>cy Values. But there was no<br />
characterization theorem in that paper. We provi<strong>de</strong> such a<br />
theorem below.<br />
Let us first str<strong>en</strong>gth<strong>en</strong> the condition on the basic inconsist<strong>en</strong>cy<br />
measure:<br />
Definition 12 A MinInc Separable basic inconsist<strong>en</strong>cy<br />
measure (MSBIM) I is a basic inconsit<strong>en</strong>cy measure that<br />
satisfies this additional property:<br />
• If MI(K ∪ K ′ ) = MI(K) ⊕ MI(K ′ ), th<strong>en</strong> I(K ∪ K ′ ) =<br />
I(K) + I(K ′ )<br />
(MinInc Separability)<br />
□<br />
This property basically expresses the fact that the inconsist<strong>en</strong>cy<br />
measure <strong>de</strong>p<strong>en</strong>ds on the minimal inconsist<strong>en</strong>t subsets,<br />
so that if we can partition the belief base in two subbases<br />
without “breaking” any minimal inconsist<strong>en</strong>t subset,<br />
th<strong>en</strong> the global inconsist<strong>en</strong>cy measure is the sum of the inconsist<strong>en</strong>cy<br />
measure of the two subbases. Clearly, the MI<br />
inconsist<strong>en</strong>cy measure satisfies this property.<br />
Let us now <strong>en</strong>umerate the properties that we expect<br />
inconsist<strong>en</strong>cy values to satisfy:<br />
• ∑ α∈K SI α(K) = I(K)<br />
(Distribution)<br />
• If ∃α, β ∈ K s.t. for all K ′ ⊆ K s.t. α, β /∈ K ′ ,<br />
I(K ′ ∪ {α}) = I(K ′ ∪ {β}), th<strong>en</strong> Sα(K) I = Sβ I (K)<br />
(Symmetry)<br />
• If α is a free formula of K, th<strong>en</strong> Sα(K) I = 0 (Minimality)<br />
• If α ⊢ β and α ⊥, th<strong>en</strong> Sα(K) I ≥ Sβ I (K) (Dominance)<br />
• If MI(K ∪ K ′ ) = MI(K) ⊕ MI(K ′ ), th<strong>en</strong> Sα(K I ∪ K ′ ) =<br />
Sα(K) I + Sα(K I ′ )<br />
(Decomposability)<br />
The first four properties were already discussed in (Hunter<br />
& Konieczny 2006). The first three of these are closely related<br />
to original Shapley’s properties. The distribution property<br />
states that the inconsist<strong>en</strong>cy values of the formulae sum<br />
to the total amount of inconsist<strong>en</strong>cy in the base (I(K)). The<br />
symmetry property <strong>en</strong>sures that only the amount of inconsist<strong>en</strong>cy<br />
brought by a formula matters for computing the inconsist<strong>en</strong>cy<br />
value. As one could expect, a formula that is<br />
not embed<strong>de</strong>d in any contradiction (i.e. does not belong to<br />
any minimal inconsist<strong>en</strong>t subset) will not be blamed by the<br />
inconsist<strong>en</strong>cy value. This is what is expressed in the minimality<br />
property. The dominance property states that logically<br />
stronger formulae bring (pot<strong>en</strong>tially) more conflicts. It<br />
was shown in (Hunter & Konieczny 2006) that every Shapley<br />
Inconsist<strong>en</strong>cy Value satisfies these four properties.<br />
The Decomposability property is related to Shapley’s Additivity<br />
property. We explain in (Hunter & Konieczny 2006)<br />
that a direct translation of this Additivity property makes little<br />
s<strong>en</strong>se because it is not meaningful to add differ<strong>en</strong>t (basic)<br />
inconsist<strong>en</strong>cy measures. But one can consi<strong>de</strong>r another translation<br />
of the additivity property, by looking to the “addition”<br />
of two differ<strong>en</strong>t bases: the set union. So direct translation of<br />
that meaning leads to<br />
S I α(K ∪ K ′ ) = S I α(K) + S I α(K ′ )<br />
This formulation is not satisfactory because it forgets the<br />
fact that new conflicts can appear wh<strong>en</strong> making the union<br />
of the two bases. So we want this property to hold only<br />
wh<strong>en</strong> joining two bases does not create any new inconsist<strong>en</strong>cies.<br />
That is <strong>en</strong>sured by the condition of the Decomposability<br />
property. Note that this possibility of interaction betwe<strong>en</strong><br />
the two subgames that is not tak<strong>en</strong> into account in the<br />
usual Additivity condition, is one of the criticisms about this<br />
condition. Let us quote for instance the following paragraph<br />
from (Luce & Raiffa 1957):<br />
The last condition is not nearly so innoc<strong>en</strong>t as the other<br />
two. For although v + w is a game composed from v<br />
and w, we cannot in g<strong>en</strong>eral expect it to be played as<br />
if it were the two separate games. It will have its own<br />
structure which will <strong>de</strong>termine a set of equilibrium outcomes<br />
which may be differ<strong>en</strong>t from those for v and w.<br />
100
Therefore, one might very well argue that its a priori<br />
value should not necessarily be the sum of the values<br />
of the two compon<strong>en</strong>t games. This strikes us as a flaw<br />
in the concept value, but we have no alternative to suggest.<br />
In our framework the interaction betwe<strong>en</strong> the two bases is<br />
simply the new logical conflicts that appears wh<strong>en</strong> joining<br />
the bases, that allows us to say wh<strong>en</strong> this addition can hold,<br />
and wh<strong>en</strong> it is not s<strong>en</strong>sible.<br />
For setting the characterization result we have to ask one<br />
additional property that states that each minimal inconsist<strong>en</strong>t<br />
subset brings the same amount of conflict:<br />
• If M ∈ MI(K), th<strong>en</strong> I(M) = 1<br />
So now we reach the wanted characterization result<br />
(MinInc)<br />
Proposition 5 An inconsist<strong>en</strong>cy value satisfies Distribution,<br />
Symmetry, Minimality, Decomposability and MinInc if and<br />
only if it is the MI Shapley Inconsist<strong>en</strong>cy Value Sα IMI .<br />
Proof: To prove that the MI Shapley Inconsist<strong>en</strong>cy Value<br />
satisfy the logical properties is easy. (Distribution), (Symmetry),<br />
(Minimality) are satisfied by all Shapley Inconsist<strong>en</strong>cy<br />
Values (Proposition 3 of (Hunter & Konieczny 2006)).<br />
So it remains to show (Decomposability) and (MinInc).<br />
(MinInc) is satisfied by <strong>de</strong>finition since I MI (M) =<br />
|MI(M)| = 1 for any M ∈ MI(K).<br />
For (Decomposability), by <strong>de</strong>finition Sα(K I ∪ K ′ ) =<br />
(c−1)!(n−c)!<br />
∑C⊆K∪K ′ n!<br />
(I(C) − I(C \ {α})). Now split C<br />
on K and K ′ , i.e. <strong>de</strong>fine H = C ∩ K and H ′ = C ∩ K ′ .<br />
It is easy to check that C = H ∪ H ′ , and from the hypothesis<br />
that MI(K ∪ K ′ ) = MI(K) ⊕ MI(K ′ ) we <strong>de</strong>duce that<br />
MI(H ∪H ′ ) = MI(H)⊕MI(H ′ ), so as S IMI = MIV C satisfies<br />
(MinInc Separability) we have that I(C) = I(H) +<br />
I(H ′ ). So using this in the <strong>de</strong>finition we have<br />
Sα(K I ∪ K ′ )<br />
= X (c − 1)!(n − c)!<br />
(I(C) − I(C \ {α}))<br />
n!<br />
C⊆K∪K ′<br />
= X (c − 1)!(n − c)!<br />
(I(H) + I(H ′ )<br />
n!<br />
C⊆K∪K ′ −I(H \ {α}) − I(H ′ \ {α}))<br />
= X (c − 1)!(n − c)!<br />
(I(H) − I(H \ {α})<br />
n!<br />
C⊆K∪K ′<br />
+ X (c − 1)!(n − c)!<br />
(I(H ′ ) − I(H ′ \ {α}))<br />
n!<br />
C⊆K∪K ′<br />
= X (c − 1)!(n − c)!<br />
(I(H) − I(H \ {α})<br />
n!<br />
H⊆K<br />
+ X (c − 1)!(n − c)!<br />
(I(H ′ ) − I(H ′ \ {α}))<br />
n!<br />
H ′ ⊆K ′<br />
= Sα(K) I + Sα(K I ′ )<br />
For the converse implication suppose that we have an inconsist<strong>en</strong>cy<br />
value that satisfies (Distribution), (Symmetry),<br />
(Minimality), (Decomposability) and (MinInc). We want<br />
to show that it is the MI Shapley Inconsist<strong>en</strong>cy Value.<br />
First note that for any K such that MI(K) =<br />
{M 1 , . . . , M n }, if one chooses a sequ<strong>en</strong>ce M 1 , . . . , M n ,<br />
th<strong>en</strong> for all i where 1 ≤ i < n, the following holds:<br />
MI(M 1 ∪. . .∪M i ∪M i+1 ) = MI(M 1 ∪. . .∪M i )⊕MI(M i+1 )<br />
H<strong>en</strong>ce, there is a sequ<strong>en</strong>ce of the minimal inconsist<strong>en</strong>t subsets<br />
of K, such that by use of (Minimality) and successive<br />
use of (Decomposability) we have that<br />
Sα(K) I =<br />
∑<br />
Sα(M)<br />
I<br />
M∈MI(K)<br />
Now for each M if α ∉ M we have by (Minimality) that<br />
S∑<br />
α(M) I = 0. And if α ∈ M th<strong>en</strong> we have by (Distribution)<br />
α∈M SI α(M) = I(M). And by (Symmetry) we have that<br />
∀α, β ∈ M, Sα(M) I = Sβ I (M). So we obtain that<br />
and therefore<br />
∀α ∈ M, S I α(M) = I(M)<br />
|M|<br />
S I α(K) =<br />
∑<br />
M∈MI(K)s.t.α∈M<br />
I(M)<br />
|M|<br />
Now by (MinInc) we know that for all M ∈ MI(K),<br />
I(M) = 1. That gives<br />
∑<br />
Sα(K) I 1<br />
=<br />
|M|<br />
M∈MI(K)s.t.α∈M<br />
That is the <strong>de</strong>finition of MI Shapley Inconsist<strong>en</strong>cy Value.<br />
□<br />
This result means that the Shapley Inconsist<strong>en</strong>cy Value<br />
Sα<br />
IMI is completely characterized by five simple and intuitive<br />
axioms.<br />
Note that Dominance, although satisfied by SIV, is not<br />
required for stating this proposition.<br />
Towards Implem<strong>en</strong>tation of Inconsist<strong>en</strong>cy<br />
Values<br />
The <strong>de</strong>velopm<strong>en</strong>t of SAT solvers has ma<strong>de</strong> impressive<br />
progress in rec<strong>en</strong>t years, allowing, <strong><strong>de</strong>s</strong>pite the computational<br />
complexity of the problem, to practically solve a number of<br />
intractable problems (Kautz & Selman 2007).<br />
Based on SAT solvers, some techniques have be<strong>en</strong> aimed<br />
at the i<strong>de</strong>ntification of minimal inconsist<strong>en</strong>t subsets (called<br />
in these works Minimally Unsatisfiable Subformulas or<br />
MUS). Although the i<strong>de</strong>ntification problem is computationally<br />
hard, since checking whether a set of clauses is a<br />
MUS or not is DP-complete, and checking whether a formula<br />
belongs to the set of MUSes of a base, is in Σ P 2<br />
(Eiter & Gottlob 1992); it seems that finding each MUS can<br />
be practically feasible (Grégoire, Mazure, & Piette 2007;<br />
2008).<br />
Thanks to Proposition 4, we th<strong>en</strong> can <strong>de</strong>fine an easy<br />
algorithm to compute the MI Shapley Inconsist<strong>en</strong>cy Value<br />
101
of the formulae of a base:<br />
Input: A belief base K = {α 1 , . . . , α n }<br />
Output: A profile of values (S I α1 (K), . . . , SI αn (K))<br />
1- For i from 1 to n<br />
S i ← 0<br />
2- Compute MI(K)<br />
3- For each C ∈ MI(K)<br />
For each α i ∈ C<br />
S i ← S i + 1<br />
|C|<br />
4- Return (S 1 , . . . , S n )<br />
The hard step in the above algorithm is step 2. But if it<br />
can be viably computed, th<strong>en</strong> the rest of the algorithm is just<br />
polynomial in the size of the MI(K) (but of course the size<br />
of MI(K) can be expon<strong>en</strong>tial in the size of K).<br />
In future work, we plan to implem<strong>en</strong>t such an algorithm<br />
using an existing algorithm to i<strong>de</strong>ntify MUS (Grégoire,<br />
Mazure, & Piette 2007; 2008). This would gives us a practical<br />
tool to measure inconsist<strong>en</strong>cy.<br />
A possible approach to ameliorate the cost of <strong>en</strong>tailm<strong>en</strong>t<br />
in finding minimal inconsist<strong>en</strong>t subsets is to use approximate<br />
<strong>en</strong>tailm<strong>en</strong>t: Proposed in (Levesque 1984), and <strong>de</strong>veloped<br />
in (Schaerf & Cadoli 1995), classical <strong>en</strong>tailm<strong>en</strong>t is approximated<br />
by two sequ<strong>en</strong>ces of <strong>en</strong>tailm<strong>en</strong>t relations. The<br />
first is sound but not complete, and the second is complete<br />
but not sound. Both sequ<strong>en</strong>ces converge to classical <strong>en</strong>tailm<strong>en</strong>t.<br />
For a set of propositional formulae ∆, a formula<br />
α, and an approximate <strong>en</strong>tailm<strong>en</strong>t relation |= i , the <strong>de</strong>cision<br />
of whether ∆ |= i α holds or ∆ ̸|= i α holds can be computed<br />
in polynomial time. Approximate <strong>en</strong>tailm<strong>en</strong>t has be<strong>en</strong><br />
<strong>de</strong>veloped for anytime coher<strong>en</strong>ce reasoning (Koriche 2001;<br />
2002), and in furture work, we will investigate its pot<strong>en</strong>tial<br />
for an approximate version of the MI Shapley Inconsist<strong>en</strong>cy<br />
Value.<br />
By focussing on subsystems of classical logic, such as <strong><strong>de</strong>s</strong>cription<br />
logics, there appears to be much pot<strong>en</strong>tial in harnessing<br />
existing specialized reasoning systems for finding<br />
minimal inconsist<strong>en</strong>t subsets of a belief base (for example<br />
by using the Pellet reasoning system for <strong><strong>de</strong>s</strong>cription logics<br />
(Kalyanpur et al. 2005; Parsia, Sirin, & Kalyanpur 2005)).<br />
Furthermore, measuring inconsist<strong>en</strong>cy in <strong><strong>de</strong>s</strong>cription logic<br />
ontologies offers a pot<strong>en</strong>tially interesting and worthwhile<br />
application problem (Qi & Hunter 2007).<br />
Another application area for inconsist<strong>en</strong>cy measures is in<br />
supporting reasoning with inconsist<strong>en</strong>t databases (for example<br />
wh<strong>en</strong> using maximally consist<strong>en</strong>t subsets of the database<br />
(Bertossi & Bravo 2005)). Again, this is an application<br />
where language restrictions and specialized reasoning systems<br />
offer the pot<strong>en</strong>tial for viable means for finding minimal<br />
inconsist<strong>en</strong>t subsets of a belief base, and thereby finding the<br />
MI Shapley Inconsist<strong>en</strong>cy Value.<br />
Conclusion<br />
As discussed in the introduction, there are a number of proposals<br />
for measures of inconsist<strong>en</strong>cy. The main novel contributions<br />
provi<strong>de</strong>d by this paper are :<br />
• A first discussion on the <strong>de</strong>finition of inconsist<strong>en</strong>cy values<br />
based on minimal inconsist<strong>en</strong>t subsets of belief bases,<br />
and this leads to the <strong>de</strong>finition of the family of Minimal<br />
Inconsist<strong>en</strong>t Values;<br />
• An equival<strong>en</strong>ce betwe<strong>en</strong> a particular Shapley Inconsist<strong>en</strong>cy<br />
Value (the MI Shapley Inconsist<strong>en</strong>cy Value) and<br />
a simple Minimal Inconsist<strong>en</strong>t Value which gives an additional<br />
argum<strong>en</strong>t to support the Shapley Inconsist<strong>en</strong>cy<br />
Value <strong>de</strong>finition since it captures this very natural value<br />
as particular case;<br />
• The first (as far as we know) axiomatization of an inconsist<strong>en</strong>cy<br />
value;<br />
• And finally, the characterization of the MI Shapley Inconsist<strong>en</strong>cy<br />
Value in terms of the MIV C measure op<strong>en</strong>s<br />
the possibility for computationally viable calculation of<br />
inconsist<strong>en</strong>cy values.<br />
In future work, we would like to analyse the computational<br />
complexity of using the MI Shapley Inconsist<strong>en</strong>cy<br />
Value, <strong>de</strong>velop algorithms and implem<strong>en</strong>tations (possibly<br />
based on approximation techniques), and un<strong>de</strong>rtake case<br />
studies of applications of this value.<br />
Aknowledgem<strong>en</strong>ts<br />
This work has be<strong>en</strong> partly supported by a bilateral project<br />
CNRS - Royal Society.<br />
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103
IMPROVEMENT OPERATORS<br />
Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez.<br />
Elev<strong>en</strong>th International Confer<strong>en</strong>ce on Principles of Knowledge Repres<strong>en</strong>tation<br />
and Reasoning (KR’08).<br />
pages 177-186.<br />
2008.<br />
105
Improvem<strong>en</strong>t Operators<br />
Sébasti<strong>en</strong> Konieczny<br />
CRIL - CNRS<br />
Université d’Artois, L<strong>en</strong>s, France<br />
konieczny@cril.fr<br />
Ramón Pino Pérez<br />
Departam<strong>en</strong>to <strong>de</strong> Matemáticas<br />
Facultad <strong>de</strong> Ci<strong>en</strong>cias<br />
Universidad <strong>de</strong> Los An<strong><strong>de</strong>s</strong><br />
Mérida, V<strong>en</strong>ezuela<br />
pino@ula.ve<br />
Abstract<br />
We introduce a new class of change operators. They are a<br />
g<strong>en</strong>eralization of usual iterated belief revision operators. The<br />
i<strong>de</strong>a is to relax the success property, so the new information is<br />
not necessarily believed after the improvem<strong>en</strong>t. But its plausibility<br />
has increased in the epistemic state. So, iterating the<br />
process suffici<strong>en</strong>tly many times, the new information will be<br />
finally believed. We give syntactical and semantical characterizations<br />
of these operators.<br />
Introduction<br />
Mo<strong>de</strong>lling belief change is a c<strong>en</strong>tral topic in artificial intellig<strong>en</strong>ce,<br />
psychology and databases. One of the predominant<br />
approaches was proposed by Alchourrón, Gär<strong>de</strong>nfors<br />
and Makinson and is known as the AGM belief revision<br />
framework (Alchourrón, Gär<strong>de</strong>nfors, & Makinson 1985;<br />
Gär<strong>de</strong>nfors 1988; Katsuno & M<strong>en</strong><strong>de</strong>lzon 1991). The main<br />
requirem<strong>en</strong>ts imposed by AGM postulates are the principle<br />
of coher<strong>en</strong>ce asking to maintain consist<strong>en</strong>cy as far as possible,<br />
the so called principle of minimal change saying that we<br />
have to keep as much of the old information as possible, and<br />
the last important requirem<strong>en</strong>t is the principle of primacy of<br />
update (also called success property) that <strong>de</strong>mands the new<br />
information to be true in the new belief base. The postulates<br />
proposed to characterize belief revision operators (Alchourrón,<br />
Gär<strong>de</strong>nfors, & Makinson 1985; Gär<strong>de</strong>nfors 1988;<br />
Katsuno & M<strong>en</strong><strong>de</strong>lzon 1991; Hansson 1999) just aimed at<br />
capturing logically these principles.<br />
A drawback of AGM <strong>de</strong>finition of revision is that the conditions<br />
for the iteration of the process are very weak, and<br />
this is caused by the lack of expressive power of logical belief<br />
bases (Herzig, Konieczny, & Perussel 2003). In or<strong>de</strong>r<br />
to <strong>en</strong>sure good properties for the iteration of the revision<br />
process, one needs a more complex structure. So shifting<br />
from logical belief bases to epistemic states was proposed<br />
in (Darwiche & Pearl 1997). In this framework, one can<br />
<strong>de</strong>fine interesting iterated revision operators (Darwiche &<br />
Pearl 1997; Booth & Meyer 2006; Jin & Thielscher 2007;<br />
Konieczny & Pino Pérez 2000). Let us call these operators<br />
DP belief revision operators.<br />
Copyright c○ 2008, Association for the Advancem<strong>en</strong>t of Artificial<br />
Intellig<strong>en</strong>ce (www.aaai.org). All rights reserved.<br />
Another framework that allows to <strong>de</strong>fine interesting iterated<br />
change operator is the one of Ordinal Conditional<br />
Functions (OCF), also named kappa-rankings, that was proposed<br />
by Spohn in (Spohn 1988), and further <strong>de</strong>veloped in<br />
(Williams 1994). An OCF can be repres<strong>en</strong>ted by a function<br />
that associates an ordinal to every interpretation, with at least<br />
one interpretation taking the value 0. The ordinal associated<br />
to the interpretation repres<strong>en</strong>ts the <strong>de</strong>gree of disbelief of the<br />
interpretation. This notion can be used to <strong>de</strong>fine a <strong>de</strong>gree<br />
of acceptance of a formula.<br />
So a change operator in this framework, called a transmutation<br />
(Williams 1994), is a function that changes the <strong>de</strong>gree<br />
of acceptance of a formula. This means that it requires<br />
more information than AGM/DP belief revision operators,<br />
since, in addition to the new information, one needs to give<br />
its new <strong>de</strong>gree of acceptance. This has one important drawback,<br />
since one has to find this new <strong>de</strong>gree somewhere! It<br />
it is not a problem if it is giv<strong>en</strong> by the application, but if<br />
the only received input is the new information, justifying<br />
an “arbitrary” <strong>de</strong>gree of acceptance can be problematic. On<br />
the other hand, this more g<strong>en</strong>eral framework allows to <strong>de</strong>fine<br />
interesting operators. It allows to <strong>de</strong>fine revision and<br />
contraction operators, by choosing the right <strong>de</strong>gree of acceptance.<br />
In particular most of the works on DP iterared<br />
revision operators use OCF operators as examples (see e.g.<br />
(Darwiche & Pearl 1997; Jin & Thielscher 2007)). But it<br />
also allows to <strong>de</strong>fine restructuring operators (Williams 1994;<br />
Spohn 1988), that modify the OCF, without changing the believed<br />
formulae. Such operators do not exist in the classical<br />
DP belief revision framework, that obey the success property,<br />
that asks the new information to be believed after the<br />
change.<br />
The aim of this paper is to <strong>de</strong>fine such restructuring-like<br />
operators in the standard AGM/DP framework. We want to<br />
<strong>de</strong>fine change operators on epistemic states that do not (necessarily)<br />
satisfy the success property, although still improving<br />
the plausibility of the new information. We call these<br />
operators improvem<strong>en</strong>t operators. This i<strong>de</strong>a is quite intuitive<br />
since usual AGM/DP belief revision operators can be<br />
consi<strong>de</strong>red as too strong: after revising by a new information,<br />
this information will be believed. Most of the time<br />
this is the wanted behaviour for the revision operators. But<br />
in some cases it may be s<strong>en</strong>sible to take into account the<br />
new information more cautiously. Maybe because we have<br />
107
some confi<strong>de</strong>nce in the source of the new information, but<br />
not <strong>en</strong>ough for accepting unconditionally this new information.<br />
This can be se<strong>en</strong> as a kind of learning/reinforcem<strong>en</strong>t<br />
process: each time the ag<strong>en</strong>t receives a new information α,<br />
this formula will gain in plausibility in the epistemic state of<br />
the ag<strong>en</strong>t. And if the ag<strong>en</strong>t receives the same new information<br />
many times, th<strong>en</strong> he will finally believe it.<br />
Our operators are close in spirit to the bad day/good day<br />
approach of Booth and Meyer (also called abstract interval<br />
or<strong>de</strong>rs revision) (Booth & Meyer 2007; Booth, Meyer, &<br />
Wong 2006). Unlike their operators that need an extra information,<br />
our operators are <strong>de</strong>fined in the usual DP framework.<br />
We give more <strong>de</strong>tails on this relationship at the <strong>en</strong>d of<br />
the paper.<br />
The rest of the paper is organized as follows: we give<br />
the preliminaries in the first section. The second section is<br />
<strong>de</strong>voted to the introduction of improvem<strong>en</strong>t operators. The<br />
third section is <strong>de</strong>voted to a discussion of the irrelevance of<br />
syntax property. In the fourth and fifth sections we state the<br />
main results concerning syntactical and semantical characterizations,<br />
namely two repres<strong>en</strong>tation theorems. The fifth<br />
section shows an example and how to <strong>en</strong>co<strong>de</strong> improvem<strong>en</strong>t<br />
using OCF. The sixth section contains some interesting<br />
properties of improvem<strong>en</strong>t operators. The last section is the<br />
conclusion. There is also an app<strong>en</strong>dix containing the proofs<br />
of the main results.<br />
Preliminaries<br />
We consi<strong>de</strong>r a propositional language L <strong>de</strong>fined from a finite<br />
set of propositional variables P and the standard connectives.<br />
Let L ∗ <strong>de</strong>note the set of consist<strong>en</strong>t formulae of<br />
L.<br />
An interpretation ω is a total function from P to {0, 1}.<br />
The set of all interpretations is <strong>de</strong>noted W. An interpretation<br />
ω is a mo<strong>de</strong>l of a formula φ ∈ L if and only if it makes it<br />
true in the usual truth functional way. [[α]] <strong>de</strong>notes the set<br />
of mo<strong>de</strong>ls of the formula α, i.e., [[α]] = {ω ∈ W | ω |= α}.<br />
Wh<strong>en</strong> {w 1 , .., w n } is a set of mo<strong>de</strong>ls we <strong>de</strong>note by ϕ w1,..,wn<br />
a formula such that [[ϕ w1,..,wn ]] = {w 1, .., w n }.<br />
We will use epistemic states to repres<strong>en</strong>t the beliefs of the<br />
ag<strong>en</strong>t, as usual in iterated belief revision (Darwiche & Pearl<br />
1997). An epistemic state Ψ repres<strong>en</strong>ts the curr<strong>en</strong>t beliefs of<br />
the ag<strong>en</strong>t, but also additional conditional information guiding<br />
the revision process (usually repres<strong>en</strong>ted by a pre-or<strong>de</strong>r<br />
on interpretations, a set of conditionals, a sequ<strong>en</strong>ce of formulae,<br />
etc). Let E <strong>de</strong>note the set of all epistemic states. A<br />
projection function B : E −→ L ∗ associates to each epistemic<br />
state Ψ a consist<strong>en</strong>t formula B(Ψ), that repres<strong>en</strong>ts the<br />
curr<strong>en</strong>t beliefs of the ag<strong>en</strong>t in the epistemic state Ψ.<br />
For simplicity purpose we will only consi<strong>de</strong>r in this paper<br />
consist<strong>en</strong>t epistemic states and consist<strong>en</strong>t new information.<br />
Thus, we consi<strong>de</strong>r change operators as functions ◦ mapping<br />
an epistemic state and a consist<strong>en</strong>t formula into a new epistemic<br />
state, i.e. in symbols, ◦ : E × L ∗ −→ E. The image<br />
of a pair (Ψ, α) un<strong>de</strong>r ◦ will be <strong>de</strong>noted by Ψ ◦ α.<br />
We adopt the following notations:<br />
• Ψ ◦ n α <strong>de</strong>fined as: Ψ ◦ 1 α = Ψ ◦ α<br />
Ψ ◦ n+1 α = (Ψ ◦ n α) ◦ α<br />
• Ψ ⋆ α = Ψ ◦ n α, where n is the first integer such that<br />
B(Ψ ◦ n α) ⊢ α.<br />
Note that ⋆ is un<strong>de</strong>fined if there is no n such that B(Ψ ◦ n<br />
α) ⊢ α, but for all operators ◦ consi<strong>de</strong>red in this work, the<br />
associated operator ⋆ will be total, that is for any pair Ψ, α<br />
there will exist n such that B(Ψ ◦ n α) ⊢ α (see postulate<br />
(I1)) below.<br />
Finally, let ≤ be a a total pre-or<strong>de</strong>r, i.e a reflexive (x ≤ x),<br />
transitive ((x ≤ y ∧ y ≤ z) → x ≤ z) and total (x ≤<br />
y ∨ y ≤ x) relation over W. Th<strong>en</strong> the corresponding strict<br />
relation < is <strong>de</strong>fined as x < y iff x ≤ y and y ≰ x, and the<br />
corresponding equival<strong>en</strong>ce relation ≃ is <strong>de</strong>fined as x ≃ y iff<br />
x ≤ y and y ≤ x. We <strong>de</strong>note w ≪ w ′ wh<strong>en</strong> w < w ′ and<br />
there is no w ′′ such that w < w ′′ < w ′ . We also use the<br />
notation min(A, ≤) = {w ∈ A | ∄w ′ ∈ A w ′ < w}.<br />
Wh<strong>en</strong> a set W is equipped with a total pre-or<strong>de</strong>r ≤, th<strong>en</strong><br />
this set can be splitted in differ<strong>en</strong>t levels, that gives the or<strong>de</strong>red<br />
sequ<strong>en</strong>ce of its equival<strong>en</strong>ce classes W = 〈S 0 , . . . S n 〉.<br />
So ∀x, y ∈ S i x ≃ y. We say in that case that x and y are<br />
at the same level of the pre-or<strong>de</strong>r. And ∀x ∈ S i ∀y ∈ S j<br />
i < j implies x < y. We say in this case that x is in a lower<br />
level than y. We ext<strong>en</strong>d straightforwardly these <strong>de</strong>finitions<br />
to compare subsets of equival<strong>en</strong>ce classes, i.e if A ⊆ S i and<br />
B ⊆ S j th<strong>en</strong> we say that A is in a lower level than B if<br />
i < j.<br />
Improvem<strong>en</strong>t operators<br />
First, let us state the basic logical properties that are asked<br />
for improvem<strong>en</strong>t operators.<br />
Definition 1 An operator ◦ is said to be a weak improvem<strong>en</strong>t<br />
operator if it satisfies (I1) to (I6):<br />
(I1) There exists n such that B(Ψ ◦ n α) ⊢ α<br />
(I2) If B(Ψ) ∧ α ⊬ ⊥, th<strong>en</strong> B(Ψ ⋆ α) ≡ B(Ψ) ∧ α<br />
(I3) If α ⊥, th<strong>en</strong> B(Ψ ◦ α) ⊥<br />
(I4) For any positive integer n if α i ≡ β i for all i ≤ n th<strong>en</strong><br />
B(Ψ ◦ α 1 ◦ · · · ◦ α n ) ≡ B(Ψ ◦ β 1 ◦ · · · ◦ β n )<br />
(I5) B(Ψ ⋆ α) ∧ β ⊢ B(Ψ ⋆ (α ∧ β))<br />
(I6) If B(Ψ⋆α)∧β ⊬ ⊥, th<strong>en</strong> B(Ψ⋆(α∧β)) ⊢ B(Ψ⋆α)∧β<br />
We have put together these properties because they allow<br />
to obtain a first basic repres<strong>en</strong>tation theorem (see Theorem<br />
1). These properties are very close to the usual ones for<br />
iterated belief revision (Darwiche & Pearl 1997). Note nevertheless<br />
that there is a real differ<strong>en</strong>ce since in usual formulation<br />
⋆ is a revision operator, whereas here it <strong>de</strong>notes a<br />
sequ<strong>en</strong>ce of improvem<strong>en</strong>ts.<br />
Remark that (I3) is a straightforward consequ<strong>en</strong>ce of the<br />
<strong>de</strong>finition of the operator ◦, since we ask the new information<br />
and the epistemic states to be consist<strong>en</strong>t. Although<br />
(I3) is redundant in our framework, we have chos<strong>en</strong> to put it<br />
explicitly to remain close to the usual DP postulates.<br />
The main differ<strong>en</strong>ce with usual belief revision operators<br />
is that we do not ask the fundam<strong>en</strong>tal success property<br />
B(Ψ ◦ α) ⊢ α. We ask instead the weaker (I1), that just<br />
requires that after a sequ<strong>en</strong>ce of improvem<strong>en</strong>ts, we will finally<br />
imply the new information. So this means that the (revision)<br />
operator ⋆ <strong>de</strong>fined as a sequ<strong>en</strong>ce of improvem<strong>en</strong>ts ◦<br />
is always <strong>de</strong>fined.<br />
108
Postulate (I4) is also stonger than the usual version of<br />
(Darwiche & Pearl 1997). We discuss it in the next Section.<br />
Before establishing more specific postulates concerning<br />
the iteration by differ<strong>en</strong>t formulas, we have to <strong>de</strong>fine new<br />
notions that help us to keep light notations.<br />
Definition 2 Let ◦ be a change operator satisfying (I1). Let<br />
α, β and Ψ be two formulae and an epistemic state respectively.<br />
We say that α is below β with respect to Ψ, giv<strong>en</strong> ◦,<br />
<strong>de</strong>noted α ≺ ◦ Ψ β (or simply α ≺ Ψ β if there is no ambiguity<br />
about ◦) if and only if B(Ψ ⋆ α) ⊢ B(Ψ ⋆ (α ∨ β)) and<br />
B(Ψ ⋆ β) ⊬ B(Ψ ⋆ (α ∨ β)).<br />
The pair (α, β) is Ψ-consecutive, <strong>de</strong>noted α ≺ ◦ Ψ β (or<br />
simply α ≺ Ψ β if there is no ambiguity about ◦) if and only<br />
if α ≺ Ψ β and there is no formula γ such that α ≺ Ψ γ ≺ Ψ<br />
β.<br />
The i<strong>de</strong>a of these two <strong>de</strong>finitions is that α ≺ ◦ Ψ β <strong>de</strong>notes<br />
that α is more <strong>en</strong>tr<strong>en</strong>ched (plausible) than β in the epistemic<br />
state Ψ. And α ≺ ◦ Ψ β <strong>de</strong>notes the fact that α is a formula<br />
immediately more <strong>en</strong>tr<strong>en</strong>ched (plausible) than β.<br />
Now we are ready to state the postulates concerning more<br />
specific properties of iteration:<br />
Definition 3 A weak improvem<strong>en</strong>t operator is said to be an<br />
improvem<strong>en</strong>t operator if it satisfies I7 to I11<br />
(I7) If α ⊢ µ th<strong>en</strong> B((Ψ ◦ µ) ⋆ α) ≡ B(Ψ ⋆ α)<br />
(I8) If α ⊢ ¬µ th<strong>en</strong> B((Ψ ◦ µ) ⋆ α) ≡ B(Ψ ⋆ α)<br />
(I9) If B(Ψ ⋆ α) ⊬ ¬µ th<strong>en</strong> B((Ψ ◦ µ) ⋆ α) ⊢ µ<br />
(I10) If B(Ψ ⋆ α) ⊢ ¬µ th<strong>en</strong> B((Ψ ◦ µ) ⋆ α) ⊬ µ<br />
(I11) If B(Ψ ⋆ α) ⊢ ¬µ, α ∧ µ ⊬ ⊥ and α ≺ Ψ α ∧ µ th<strong>en</strong><br />
B((Ψ ◦ µ) ⋆ α) ⊬ ¬µ<br />
A first observation about these postulates is that they are<br />
expressed in terms of both ◦ and ⋆. And that it is thanks to<br />
these several iterations until revision 1 , mo<strong>de</strong>led by ⋆, that we<br />
can <strong>de</strong>fine powerful properties on ◦. Postulates (I7), (I8) are<br />
close to the properties (C1) and (C2) of (Darwiche & Pearl<br />
1997), but translated for weak improvem<strong>en</strong>t operators. Postulate<br />
(I9) is also close to the property of In<strong>de</strong>p<strong>en</strong><strong>de</strong>nce in<br />
(Jin & Thielscher 2007) (called also property (P) in (Booth<br />
& Meyer 2006)), but also translated for weak improvem<strong>en</strong>t<br />
operators. Postulates (I9) and (I11) <strong>de</strong>als with the improvem<strong>en</strong>t<br />
of the new information, i.e. the increase of its plausibility<br />
in the epistemic state. Postulates (I10) and (I11) <strong>de</strong>als<br />
with the the cautiousness of the approach, i.e. they express<br />
the fact that the increase of the plausibility of the new information<br />
is limited. Postulate (I10) says that if after a sequ<strong>en</strong>ce<br />
of improvem<strong>en</strong>ts by α, the obtained epistemic state<br />
imply µ, th<strong>en</strong>, if before the sequ<strong>en</strong>ce of improvem<strong>en</strong>ts by<br />
α we improve by µ, th<strong>en</strong> it will not be <strong>en</strong>ough to imply µ after<br />
the sequ<strong>en</strong>ce of improvem<strong>en</strong>ts. This means that it is not<br />
possible to go directly by an improvem<strong>en</strong>t from an epistemic<br />
state where a formula is believed to one where its negation<br />
is believed. And postulate (I11) captures some of the i<strong>de</strong>as<br />
behind improvem<strong>en</strong>t operators as “small change” operators.<br />
1 Note that the ⋆ operator satisfies the success property, so it can<br />
be called revision operator. We will use this term in the following.<br />
The fact that ⋆ is a true AGM/DP revision operator will be proved<br />
in Corollary 1.<br />
Basically it says that if ¬µ is believed wh<strong>en</strong> revising by α,<br />
but µ is quite plausible giv<strong>en</strong> α, th<strong>en</strong> improving by µ before<br />
starting the sequ<strong>en</strong>ce of improvem<strong>en</strong>ts nee<strong>de</strong>d to revise by<br />
α will be <strong>en</strong>ough to <strong>en</strong>sure the result to be consist<strong>en</strong>t with µ.<br />
Irrelevance of syntax<br />
As it has be<strong>en</strong> pointed out by many authors (see for instance<br />
(Darwiche & Pearl 1997; Booth & Meyer 2006)) the postulate<br />
of in<strong>de</strong>p<strong>en</strong><strong>de</strong>nce (or irrelevance) of syntax is a <strong>de</strong>licate<br />
matter for epistemic states. Actually a basic translation of<br />
Darwiche and Pearl (R*4) in our framework would lead to:<br />
If α ≡ β, th<strong>en</strong> B(Ψ ◦ α) ≡ B(Ψ ◦ β) (1)<br />
But ev<strong>en</strong> adding this postulate is not suffici<strong>en</strong>t. Booth<br />
and Meyer have well illustrated this i<strong>de</strong>a in (Booth & Meyer<br />
2006). Actually, it is not <strong>en</strong>ough that (1) holds in or<strong>de</strong>r to<br />
have a good iterative behavior with respect to revision by<br />
sequ<strong>en</strong>ces of equival<strong>en</strong>t formulae. Consi<strong>de</strong>r:<br />
If (α ≡ β & γ ≡ θ), th<strong>en</strong> B(Ψ◦α◦γ) ≡ B(Ψ◦β ◦θ) (2)<br />
Postulate (1) doesn’t <strong>en</strong>tail postulate (2). So the good behaviour<br />
with respect to equival<strong>en</strong>t formulae is not guaranteed<br />
on two iterations. This is why Booth and Meyer have<br />
proposed in (Booth & Meyer 2006) to replace the usual postulate<br />
(1) by (2) in the usual DP framework.<br />
We agree with Booth and Meyer that postulate (1) is not<br />
<strong>en</strong>ough. But we think that (2) does not go far <strong>en</strong>ough. In<br />
fact the example they give for showing that Postulate (1)<br />
does not avoid the problem at the second iteration can be<br />
easily ext<strong>en</strong><strong>de</strong>d to show that Postulate (2) does not avoid the<br />
problem at the third iteration. So one has to specify this for<br />
every number of iterations. That leads to the postulate (I4).<br />
The following example, which follows the same lines of<br />
the Example 1 in (Booth & Meyer 2006), shows that replacing<br />
the postulate (1) by the postulate (2) in the basic RAGM 2<br />
framework is not <strong>en</strong>ough to get the postulate (I4).<br />
Example 1 Take a language with only two propositional<br />
letters p and q (in this or<strong>de</strong>r wh<strong>en</strong> we consi<strong>de</strong>r the interpretations).<br />
Let ϕ 1 = p ∨ ¬p and ϕ 2 = q ∨ ¬q. Let Φ such that<br />
Φ ◦ ϕ 1 ≠ Φ ◦ ϕ 2 . Note that this is compatible with RAGM<br />
plus (2), because the only constraint imposed by (2) is that<br />
≤ Φ◦ϕ1 =≤ Φ◦ϕ2 but not that Φ ◦ ϕ 1 = Φ ◦ ϕ 2 . Moreover we<br />
can take B(Φ◦ϕ 1 ) = p∧q = B(Φ◦ϕ 2 ). Let Φ 1 , Φ 2 be two<br />
epistemic states such that B(Φ 1 ) = p = B(Φ 2 ), 00 < Φ1 01<br />
and 01 < Φ2 00. Now, it is compatible with RAGM plus (2)<br />
to put ≤ Φ◦ϕ1◦p=≤ Φ1 and ≤ Φ◦ϕ2◦¬¬p=≤ Φ2 . But th<strong>en</strong>, by<br />
the repres<strong>en</strong>tation, we have B(Φ ◦ ϕ 1 ◦ p ◦ ¬p) = ¬p ∧ ¬q<br />
and B(Φ ◦ ϕ 2 ◦ ¬¬p ◦ ¬p) = ¬p ∧ q, what clearly is a<br />
counter-example to (I4).<br />
Nevertheless, it worths noticing that all the well-known<br />
iterated revision operators, as natural revision (Boutilier<br />
1996), Darwiche and Pearl • operator (Darwiche &<br />
Pearl 1997), Nayak’s lexicographic revision (Nayak 1994;<br />
2 RAGM is the name that Booth and Meyer give to AGM/DP<br />
belief revision operators satisfying (R*1)-(R*6) (Darwiche & Pearl<br />
1997).<br />
109
Konieczny & Pino Pérez 2000) for instance, satisfy (I4).<br />
The reason behind this ph<strong>en</strong>om<strong>en</strong>on is that all these operators<br />
satisfy the following property<br />
If α ≡ β and ≤ Ψ =≤ Φ th<strong>en</strong> ≤ Ψ◦α =≤ Φ◦β (3)<br />
In the pres<strong>en</strong>ce of the other RAGM properties, the previous<br />
property <strong>en</strong>tails the property (2). But the converse is not<br />
true as we can see via the example 1.<br />
Remark also that usual belief revision operators satisfy all<br />
other weak improvem<strong>en</strong>t properties (with n = 1 in (I1)), so<br />
weak improvem<strong>en</strong>ts operators are a g<strong>en</strong>eralization of usual<br />
DP belief revision operators (Darwiche & Pearl 1997).<br />
Repres<strong>en</strong>tation theorem<br />
Let us first <strong>de</strong>fine strong faithful assignem<strong>en</strong>ts.<br />
Definition 4 A function Ψ ↦→≤ Ψ that maps each epistemic<br />
state Ψ to a total pre-or<strong>de</strong>r on interpretations ≤ Ψ is said to<br />
be a strong faithful assignm<strong>en</strong>t if and only if:<br />
1. If w |= B(Ψ) and w ′ |= B(Ψ), th<strong>en</strong> w ≃ Ψ w ′<br />
2. If w |= B(Ψ) and w ′ ̸|= B(Ψ), th<strong>en</strong> w < Ψ w ′<br />
3. For any positive integer n if α i ≡ β i for any i ≤ n th<strong>en</strong><br />
≤ Ψ◦α1◦···◦αn= ≤ Ψ◦β1◦···◦βn<br />
Note that conditions 1 and 2 are equival<strong>en</strong>t to [[B(Ψ)]] =<br />
min(W, ≤ Ψ ), and are the usual ones for faithful assignm<strong>en</strong>t<br />
(Darwiche & Pearl 1997). Condition 3 is a very natural<br />
condition that links pre-or<strong>de</strong>rs associated to iteration of improvem<strong>en</strong>ts:<br />
two sequ<strong>en</strong>ces of improvem<strong>en</strong>ts of the same<br />
pre-or<strong>de</strong>r by equival<strong>en</strong>t formulae lead to the same pre-or<strong>de</strong>r.<br />
Let us first show a first repres<strong>en</strong>tation theorem on weak<br />
improvem<strong>en</strong>t operators, before turning on the more interesting<br />
iteration properties.<br />
Theorem 1 A change operator ◦ is a weak improvem<strong>en</strong>t operator<br />
if and only if there exists a strong faithful assignm<strong>en</strong>t<br />
that maps each epistemic state Ψ to a total pre-or<strong>de</strong>r on interpretations<br />
≤ Ψ such that<br />
[[B(Ψ ⋆ α)]] = min([[α]], ≤ Ψ ) (4)<br />
It is easy to check that the faithful assignm<strong>en</strong>t repres<strong>en</strong>ting<br />
◦ in the previous theorem is unique.<br />
An obvious corrolary of the previous Theorem and its<br />
proof is the following one:<br />
Corollary 1 If ◦ is a weak improvem<strong>en</strong>t operator, th<strong>en</strong> ⋆ is<br />
an AGM/DP revision operator, i.e. it satisfies (R*1)-(R*6)<br />
of (Darwiche & Pearl 1997).<br />
As a consequ<strong>en</strong>ce of the previous theorem we have also<br />
the following trichotomy property:<br />
Proposition 1 Let ◦ be a weak improvem<strong>en</strong>t operator. Th<strong>en</strong><br />
{ B(Ψ ⋆ α) or<br />
B(Ψ ⋆ (α ∨ β)) = B(Ψ ⋆ β) or<br />
B(Ψ ⋆ α) ∨ B(Ψ ⋆ β)<br />
Let us now give two corollaries of these results, that are<br />
useful to un<strong>de</strong>rstand the <strong>de</strong>finitions of ≺ Ψ and ≺ Ψ , and that<br />
will be useful in the proof of the main Theorem (Theorem<br />
2).<br />
Corollary 2 Let ◦ be a weak improvem<strong>en</strong>t operator. Th<strong>en</strong><br />
α ≺ Ψ β if and only if there exist w, w ′ such that w ∈<br />
[[B(Ψ ⋆ α)]], w ′ ∈ [[B(Ψ ⋆ β)]], w < Ψ w ′ .<br />
Corollary 3 Let ◦ be a weak improvem<strong>en</strong>t operator. Th<strong>en</strong><br />
α ≺ Ψ β if and only if there exist w, w ′ such that w ∈<br />
[[B(Ψ ⋆ α)]], w ′ ∈ [[B(Ψ ⋆ β)]], w < Ψ w ′ and there is no<br />
w ′′ such that w < Ψ w ′′ < Ψ w ′ .<br />
Main result<br />
Let us turn now to the main repres<strong>en</strong>tation result about improvem<strong>en</strong>t<br />
operators.<br />
Definition 5 Let ◦ be a weak improvem<strong>en</strong>t operator and<br />
Ψ ↦→≤ Ψ its corresponding strong faithful assignm<strong>en</strong>t. The<br />
assignm<strong>en</strong>t will be called a gradual assignm<strong>en</strong>t if the properties<br />
S1, S2, S3, S4 and S5 are satisfied<br />
(S1) If w, w ′ ∈ [[α]] th<strong>en</strong> w ≤ Ψ w ′ ⇔ w ≤ Ψ◦α w ′<br />
(S2) If w, w ′ ∈ [[¬α]] th<strong>en</strong> w ≤ Ψ w ′ ⇔ w ≤ Ψ◦α w ′<br />
(S3) If w ∈ [[α]], w ′ ∈ [[¬α]] th<strong>en</strong> w ≤ Ψ w ′ ⇒ w < Ψ◦α w ′<br />
(S4) If w ∈ [[α]], w ′ ∈ [[¬α]] th<strong>en</strong> w ′ < Ψ w ⇒ w ′ ≤ Ψ◦α w<br />
(S5) If w ∈ [[α]], w ′ ∈ [[¬α]] th<strong>en</strong> w ′ ≪ Ψ w ⇒ w ≤ Ψ◦α w ′<br />
Properties (S1) and (S2) correspond to usual properties<br />
(CR1) and (CR2) for DP iterated revision operators (Darwiche<br />
& Pearl 1997). Property (S3) is the new property<br />
proposed in (Jin & Thielscher 2007; Booth & Meyer<br />
2006), and that forces to increase the plausibility of the<br />
mo<strong>de</strong>ls of the new information. Property (S4) shows how<br />
the increase of plausibility of the mo<strong>de</strong>ls of the new information<br />
is limited by improvem<strong>en</strong>t operators. This is<br />
an important differ<strong>en</strong>ce with usual DP iterated revision operators<br />
(Darwiche & Pearl 1997; Jin & Thielscher 2007;<br />
Booth & Meyer 2006). Property (S5) asks (together with<br />
(S4)) that if a mo<strong>de</strong>l of ¬α is just a little more plausible than<br />
a mo<strong>de</strong>l of α, th<strong>en</strong> after improvem<strong>en</strong>t the two mo<strong>de</strong>ls will<br />
have the same plausibility.<br />
Theorem 2 A change operator ◦ is an improvem<strong>en</strong>t operator<br />
if and only if there exists a gradual assignm<strong>en</strong>t such that<br />
[[B(Ψ ⋆ α)]] = min([[α]], ≤ Ψ )<br />
This theorem has important consequ<strong>en</strong>ces. In particular<br />
the relationship betwe<strong>en</strong> ≤ Ψ and ≤ Ψ◦α imposed by Definition<br />
5 is very tight. Actually, the total pre-or<strong>de</strong>r ≤ Ψ◦α is<br />
completely <strong>de</strong>termined by ≤ Ψ and α as it will be stated in<br />
Proposition 2. We first need the following lemma:<br />
Lemma 1 Let ◦ be an improvem<strong>en</strong>t operator and Ψ ↦→≤ Ψ<br />
its gradual assignm<strong>en</strong>t. If w < Ψ w ′ , w ∈ [[¬α]], w ′ ∈ [[α]]<br />
and w ̸≪ Ψ w ′ th<strong>en</strong> w < Ψ◦α w ′ .<br />
This Lemma is interesting since it gives the missing relation<br />
betwe<strong>en</strong> ≤ Ψ◦α and ≤ Ψ , since all other relations are<br />
giv<strong>en</strong> by the properties of Definition 5.<br />
Proposition 2 Let ◦ be an improvem<strong>en</strong>t operator and<br />
Ψ ↦→≤ Ψ its gradual assignm<strong>en</strong>t. Th<strong>en</strong> for every formula<br />
α, the pre-or<strong>de</strong>r ≤ Ψ◦α is completely <strong>de</strong>termined by ≤ Ψ and<br />
[[α]].<br />
110
w ∈ [[α]] w ′ ∈ [[α]] w ≤ Ψ w ′ ⇔ w ≤ Ψ◦α w ′ (S1)<br />
w ∈ [[¬α]] w ′ ∈ [[¬α]] w ≤ Ψ w ′ ⇔ w ≤ Ψ◦α w ′ (S2)<br />
w ∈ [[α]]<br />
w ′ ∈ [[¬α]]<br />
w < Ψ w ′ ⇔ w < Ψ◦α w ′ (S3)<br />
w ≃ Ψ w ′ ⇒ w < Ψ◦α w ′ (S3)<br />
w ′ ≪ Ψ w ⇒ w ≃ Ψ◦α w ′ (S4) & (S5)<br />
w ′ < Ψ w ∧ w ′ ̸≪ Ψ w ⇒ w < Ψ◦α w ′ (Lemma 1)<br />
Table 1: From ≤ Ψ to ≤ Ψ◦α<br />
This proposition is a very important one since it says, in<br />
a s<strong>en</strong>se, that there is a unique improvem<strong>en</strong>t operator. In fact<br />
one can <strong>de</strong>fine differ<strong>en</strong>t improvem<strong>en</strong>t operators by assigning<br />
to the initial epistemic state of the sequ<strong>en</strong>ce differ<strong>en</strong>t preor<strong>de</strong>rs.<br />
Once this pre-or<strong>de</strong>r is known, Proposition 2 tells us<br />
that there is no more freedom on the choice of subsequ<strong>en</strong>t<br />
pre-or<strong>de</strong>rs.<br />
So clearly if ones consi<strong>de</strong>rs pre-or<strong>de</strong>rs on interpretations<br />
as epistemic states (recall that the repres<strong>en</strong>tation theorem<br />
just says that we can associate a pre-or<strong>de</strong>r on interpretation<br />
to each epistemic state, it does not presume anything on the<br />
exact nature of these epistemic states), th<strong>en</strong> there is a unique<br />
improvem<strong>en</strong>t operator.<br />
The exact construction of ≤ Ψ◦α from ≤ Ψ is giv<strong>en</strong> in Table<br />
1. Roughly speaking, ≤ Ψ◦α is obtained by shifting down<br />
one level the mo<strong>de</strong>ls of α in the total pre-or<strong>de</strong>r ≤ Ψ . This<br />
will be stated more formally in the next section.<br />
Concrete example: Improvem<strong>en</strong>t via OCF<br />
Let us now show how to implem<strong>en</strong>t an improvem<strong>en</strong>t operator<br />
via OCF. Let us <strong>de</strong>note Ord the class of ordinals.<br />
Definition 6 An Ordinal Conditional Function (OCF) κ is<br />
a function from the set of interpretations W to the set of<br />
ordinals such that at least one interpretation is assigned 0.<br />
A function from the set of interpretations W to the set of<br />
ordinals will be called a free OCF.<br />
The set of OCF will be <strong>de</strong>noted K.<br />
Let us now state how to implem<strong>en</strong>t improvem<strong>en</strong>t using the<br />
framework of OCF. More precisely we will give two results:<br />
first we will show how to simply compute the resulting preor<strong>de</strong>r<br />
after an improvem<strong>en</strong>t, using a translation through free<br />
OCFs. Th<strong>en</strong>, we will see how to <strong>de</strong>fine an improvem<strong>en</strong>t<br />
operator in the OCF framework.<br />
So, what we do first is the following: giving ≤ Ψ and α we<br />
<strong><strong>de</strong>s</strong>cribe ≤ Ψ◦α using the machinery of OCF. At this point<br />
let us recall the equival<strong>en</strong>t view of a total pre-or<strong>de</strong>r ≤ introduced<br />
in the preliminaries: a total pre-or<strong>de</strong>r over W can<br />
be se<strong>en</strong> as the splitting of the set W in differ<strong>en</strong>t levels (the<br />
equival<strong>en</strong>t classes), 〈S 0 , . . . S n 〉m the or<strong>de</strong>red sequ<strong>en</strong>ce of<br />
its equival<strong>en</strong>ce classes. Thus, ∀x, y ∈ S i x ≃ y and ∀x ∈ S i<br />
∀y ∈ S j i < j implies x < y.<br />
Let κ be the canonical repres<strong>en</strong>tative of ≤ Ψ , i.e. if ≤ Ψ<br />
has n levels and w is in the level i, κ(w) = i.<br />
Consi<strong>de</strong>r now the following free OCF:<br />
{<br />
κ(w) if w |= α<br />
κ α (w) =<br />
κ(w) + 1 if w |= ¬α<br />
It is not hard to see that this free OCF repres<strong>en</strong>ts ≤ Ψ◦α ,<br />
that is, the total pre-or<strong>de</strong>r associated to this function in the<br />
natural way (w ≤ κα w ′ iff κ α (w) ≤ κ α (w ′ )) satisfies all the<br />
properties of Table 1. So this result just aims at illustrating<br />
simply the behaviour of improvem<strong>en</strong>t operators in terms of<br />
total pre-or<strong>de</strong>rs via free OCF.<br />
Note however that the free OCF used above is very particular,<br />
since it is build from the pre-or<strong>de</strong>r ≤ Ψ . In or<strong>de</strong>r<br />
to be able to <strong>de</strong>fine an improvem<strong>en</strong>t operator on any giv<strong>en</strong><br />
OCF, this requires much more difficult <strong>de</strong>finitions in or<strong>de</strong>r<br />
to mo<strong>de</strong>lize the smooth increase of plausibility of improvem<strong>en</strong>t<br />
operators.<br />
So now we turn to a plain repres<strong>en</strong>tation of an improvem<strong>en</strong>t<br />
operator ◦ in the full OCF framework. Thus, we assume<br />
that epistemic states are in<strong>de</strong>ed OCF’s and ◦ : K ×<br />
L ∗ −→ K. In this framework, we <strong>de</strong>fine the function B by<br />
putting [[B(κ)]] = {w : κ(w) = 0}.<br />
Remember that κ(α) = min{κ(w) : w ∈ [[α]]}. Giv<strong>en</strong><br />
κ and α, an OCF and a consist<strong>en</strong>t formula respectively, we<br />
are going to <strong>de</strong>fine the new OCF κ ◦ α by cases according<br />
to κ(α) > 0 or κ(α) = 0. In the first case (κ(α) > 0)<br />
we perform the sliding down the mo<strong>de</strong>ls of α via an auxiliary<br />
function called fα↓ κ <strong>de</strong>fined below. In the second case<br />
(κ(α) = 0) we simulate the sliding down the mo<strong>de</strong>ls of<br />
α via an auxiliary function called fα↑ κ <strong>de</strong>fined below that<br />
performs the sliding up the mo<strong>de</strong>ls of ¬α. The functions<br />
fα↓ κ : [[α]] −→ Ord and f α↑ κ : [[¬α]] −→ Ord are <strong>de</strong>fined<br />
by putting<br />
⎧<br />
max{ρ : ∃w ⎪⎨<br />
′ ∈ [[¬α]] κ(w ′ ) = ρ & ρ < κ(w)<br />
fα↓(w) κ & ̸ ∃w<br />
=<br />
′′ ∈ [[α]] ρ < κ(w ′′ ) < κ(w)}<br />
⎪⎩<br />
if this set is nonempty<br />
κ(w) − 1 otherwise<br />
fα↓ κ maps w, a mo<strong>de</strong>l of α, into the first rank below κ(w)<br />
where there is a mo<strong>de</strong>l of ¬α in the case that there is no<br />
mo<strong>de</strong>ls of α strictly in betwe<strong>en</strong> this two levels. Otherwise<br />
fα↓ κ maps w into κ(w) − 1.<br />
⎧<br />
min{ρ : ∃w ⎪⎨<br />
′ ∈ [[α]] κ(w ′ ) = ρ & κ(w) < ρ<br />
fα↑(w) κ & ̸ ∃w<br />
=<br />
′′ ∈ [[¬α]] κ(w) < κ(w ′′ ) < ρ}<br />
⎪⎩<br />
if this set is nonempty<br />
κ(w) + 1 otherwise<br />
111
fα↑ κ maps w, a mo<strong>de</strong>l of ¬α, into the first rank above κ(w)<br />
where there is a mo<strong>de</strong>l of α in the case that there is no mo<strong>de</strong>ls<br />
of ¬α strictly in betwe<strong>en</strong> this two ranks. Otherwise fα↑<br />
κ<br />
maps w into κ(w) + 1.<br />
Again, we <strong>de</strong>fine two functions mapping worlds into ordinals<br />
according to whether or not κ(α) = 0. Wh<strong>en</strong> κ(α) = 0<br />
we put<br />
{ κ(w) if w |= α<br />
κ ↑ ¬α(w) =<br />
fα↑ κ (w) if w |= ¬α<br />
and wh<strong>en</strong> κ(α) > 0 we put<br />
{ κ(w) if w |= ¬α<br />
κ ↓ α(w) =<br />
fα↓ κ (w) if w |= α<br />
Finally we <strong>de</strong>fine κ ◦ α by putting<br />
{<br />
κ ↑ ¬α if κ(α) = 0<br />
κ ◦ α =<br />
κ ↓ α if κ(α) > 0<br />
Let us now take an example in or<strong>de</strong>r to see how it works.<br />
Example 2 Consi<strong>de</strong>r a language with propositional variables<br />
p, q and r in this or<strong>de</strong>r. Let κ be the OCF with image<br />
{0, 1, 2, 4, 5} <strong><strong>de</strong>s</strong>cribed in the diagram below and α a<br />
formula such that α = ¬p. The following diagrams shows<br />
κ, κ ◦ α, κ ◦ α ◦ α and κ ◦ α ◦ α ◦ α (the mo<strong>de</strong>ls of α are in<br />
boldface):<br />
5<br />
4<br />
3<br />
2<br />
1<br />
0<br />
6<br />
5<br />
4<br />
3<br />
2<br />
1<br />
0<br />
110<br />
011<br />
− − − − −−<br />
010 000 001<br />
101 100<br />
111<br />
κ<br />
110<br />
− − − − −−<br />
− − − − −−<br />
− − − − −−<br />
101 100 011<br />
111 010 000 001<br />
κ ◦ α ◦ α<br />
110<br />
− − − − −−<br />
011<br />
− − − − −−<br />
101 100 010 000 001<br />
111<br />
κ ◦ α<br />
110<br />
− − − − −−<br />
− − − − −−<br />
− − − − −−<br />
101 100<br />
111 011<br />
010 000 001<br />
κ ◦ α ◦ α ◦ α<br />
Properties of improvem<strong>en</strong>t operators<br />
Let us give now some additional properties on improvem<strong>en</strong>t<br />
operators, that illustrate how it relates with existing operators.<br />
Proposition 3 Improvem<strong>en</strong>t operators can not be repres<strong>en</strong>ted<br />
as Spohn’s Conditionalisation nor Williams’ Adjustm<strong>en</strong>t.<br />
This is quite an intuitive result since Conditionalisation<br />
and Adjustem<strong>en</strong>t operate the same “global” change on the<br />
interpretation ranks, whereas, as sum up in Table 1, improvem<strong>en</strong>t<br />
requires a more adaptative behaviour (that <strong>de</strong>p<strong>en</strong>ds<br />
more on the ranks of the other interpretations).<br />
Proposition 4 • There exists n such that Ψ ◦ n α is Nayak’s<br />
lexicographic revision (Nayak 1994; Konieczny & Pino<br />
Pérez 2000). Let us note Ψ ⋆ lex α = Ψ ◦ n α.<br />
5<br />
4<br />
3<br />
2<br />
1<br />
0<br />
6<br />
5<br />
4<br />
3<br />
2<br />
1<br />
0<br />
• Actually, the first n such that Ψ ⋆ lex α = Ψ ◦ n α is<br />
a fixed point for improvem<strong>en</strong>t by α, in the s<strong>en</strong>se that<br />
≤ Ψ⋆lexα=≤ Ψ⋆lexα◦α.<br />
This can be shown easily with the help of Proposition 2<br />
or with the repres<strong>en</strong>tation of ≤ Ψ◦α via the free OCF. In fact<br />
it requires at most k iterations where k is the level in ≤ Ψ of<br />
the worst world of α (i.e. the mo<strong>de</strong>l of α at the highest level)<br />
to reach this fixed point.<br />
This last proposition is interesting since it illustrate the<br />
fact that the process of improvem<strong>en</strong>t does not stop as soon<br />
as the new information is believed. So in particular:<br />
Proposition 5 B(Ψ) ⊢ α does not imply that ≤ Ψ◦α =≤ Ψ<br />
and therefore does not imply Ψ ◦ α = Ψ.<br />
It is worth noticing that ev<strong>en</strong> if we have a fixed point in<br />
the s<strong>en</strong>se of Proposition 4, i.e. ≤ Ψ =≤ Ψ◦α we can have Ψ ≠<br />
Ψ ◦ α. The operator <strong>de</strong>fined via the OCF is an example of<br />
such a situation.<br />
Remark 1 Improvem<strong>en</strong>ts operators can be used to <strong>de</strong>fine<br />
contractions operators. Actualy, <strong>de</strong>fine Ψ ⊙ α = Ψ ◦ n ¬α<br />
where n is the smallest integer such that Ψ◦ n ¬α ⊬ α. Th<strong>en</strong>,<br />
⊙ is a contraction operator.<br />
Let us now elaborate on the links betwe<strong>en</strong> improvem<strong>en</strong>t<br />
operators and the bad day/good day approach of Booth et al.<br />
(Booth & Meyer 2007; Booth, Meyer, & Wong 2006) (also<br />
called abstract interval or<strong>de</strong>rs revision). In both cases the<br />
change is small, in the s<strong>en</strong>se that the increase of plausibility<br />
of the mo<strong>de</strong>ls of the new information is limited. A first<br />
differ<strong>en</strong>ce is that their operators are <strong>de</strong>fined as revision of total<br />
pre-or<strong>de</strong>rs, whereas improvem<strong>en</strong>ts are <strong>de</strong>fined on g<strong>en</strong>eral<br />
DP epistemic states. A second, more important differ<strong>en</strong>ce<br />
betwe<strong>en</strong> Booth et al. approach and ours is that they need<br />
an extra-logical information in or<strong>de</strong>r to gui<strong>de</strong> the process,<br />
whereas our operators are completely <strong>de</strong>fined in the usual<br />
DP framework. This is an important improvem<strong>en</strong>t, that allows<br />
for instance to easily iterate the process.<br />
Actually, giv<strong>en</strong> ≤ Ψ , it is possible to <strong>de</strong>fine ≼, a ≤ Ψ -<br />
faithful tpo (see (Booth, Meyer, & Wong 2006)), such that<br />
the revision of ≼ by α, in the s<strong>en</strong>se of Booth-Meyer-Wong,<br />
is exactly ≤ Ψ◦α . So, according to this link, improvem<strong>en</strong>t<br />
operators could be consi<strong>de</strong>red in a s<strong>en</strong>se as a special case of<br />
Booth and Meyer operators.<br />
Finally there is a very interesting behaviour of improvem<strong>en</strong>t<br />
operator with respect to long term behaviour. Wh<strong>en</strong><br />
working in a finite framework, no existing iterated belief revision<br />
operator escapes one of the following limit cases after<br />
a long course of revisions: maxichoice revision, or full meet<br />
revision.<br />
Full meet revision means that the beliefs of the new epistemic<br />
state is either the conjunction of the new information<br />
with the beliefs of the old epistemic state it if is consist<strong>en</strong>t,<br />
or just the new information otherwise. This is problematic,<br />
since it means that after a long course of revision the ag<strong>en</strong>t<br />
has lost all his beliefs. But for instance Lehmann’s operators<br />
(Lehmann 1995) lead to this limit case wh<strong>en</strong> working<br />
on finite frameworks.<br />
Maxichoice revision means that the revision leads to an<br />
epistemic states whose beliefs are a complete formula. This<br />
112
is also problematic, since it means that wh<strong>en</strong> this situation is<br />
reached, any revision by any formula will allow to be completely<br />
<strong>de</strong>termined about each issue. It can be argued that<br />
this is due to the long revision history, that allows the ag<strong>en</strong>t<br />
to have a very precise view of the world. But still it seems<br />
s<strong>en</strong>sible to be able to be uncertain about some issues, and to<br />
lose some certainties sometimes. Note that most of DP-like<br />
operators lead to this limit case (Darwiche & Pearl 1997;<br />
Nayak 1994; Boutilier 1996; Konieczny & Pino Pérez 2000;<br />
Booth & Meyer 2006; Jin & Thielscher 2007).<br />
Note that it is not the case in the framework of OCFs, after<br />
any sequ<strong>en</strong>ce of conditionalization, or adjustm<strong>en</strong>t, it is<br />
possible to reach any other OCF by some sequ<strong>en</strong>ce. This<br />
is one of the advantage of using a more quantitative framework<br />
(using a <strong>de</strong>gree of acceptance for each input formula),<br />
compared to the fully qualitative one that is the DP iterated<br />
belief revision framework.<br />
It is interesting to note that improvem<strong>en</strong>t operators have<br />
no limit case. Actually, after any sequ<strong>en</strong>ce of improvem<strong>en</strong>ts,<br />
it is possible to reach any formula (as beliefs of an epistemic<br />
state) by an a<strong>de</strong>quate sequ<strong>en</strong>ce of improvem<strong>en</strong>ts (the<br />
same result holds for associated pre-or<strong>de</strong>rs: any pre-or<strong>de</strong>r<br />
can be reached after an a<strong>de</strong>quate sequ<strong>en</strong>ce of improvem<strong>en</strong>ts<br />
starting from any other pre-or<strong>de</strong>r). Whereas for DP iterated<br />
belief revision operators it is not the case: after some sequ<strong>en</strong>ces<br />
of revisions, some formulae (or pre-or<strong>de</strong>rs) are not<br />
reachable anymore.<br />
The following proposition summarizes this property of<br />
improvem<strong>en</strong>t operators:<br />
Proposition 6 Let ≤ be any pre-or<strong>de</strong>r on interpretations<br />
and ≤ Ψ the pre-or<strong>de</strong>r associated to Ψ th<strong>en</strong> there exists a sequ<strong>en</strong>ce<br />
of formulae α 1 , . . . , α n such that ≤ Ψ◦α1◦...◦αn=≤.<br />
Improvem<strong>en</strong>t operators are, as far as we know, the first<br />
change operators <strong>de</strong>fined in the DP framework that allows<br />
to avoid these limit cases.<br />
Conclusion<br />
We have introduced a new family of change operators called<br />
improvem<strong>en</strong>t operators. These operators have a more cautious<br />
behaviour than usual DP iterated revision operators.<br />
The main iterated revision operators of the literature satisfy<br />
all the properties of weak improvem<strong>en</strong>t operators. In that<br />
respect weak improvem<strong>en</strong>t operators can be consi<strong>de</strong>red as a<br />
g<strong>en</strong>eralization of iterated revision operators.<br />
An ess<strong>en</strong>tial point for being able to state logical properties<br />
and theorems on improvem<strong>en</strong>t operators is the interesting relationship<br />
betwe<strong>en</strong> ◦ and its corresponding revision operator<br />
⋆.<br />
(I11), the last postulate required for improvem<strong>en</strong>t operators<br />
is very strong, in the s<strong>en</strong>se that it <strong>de</strong>termines in a unique<br />
way the pre-or<strong>de</strong>rs associated to the improvem<strong>en</strong>t. Thus,<br />
there are room to explore some variants of (I11) leading to<br />
other interesting weak improvem<strong>en</strong>ts operators. We keep<br />
this as future work.<br />
Acknowledgem<strong>en</strong>ts<br />
The authors would like to thank the reviewers of the paper<br />
for their helpful comm<strong>en</strong>ts.<br />
The second author was partially supported by a research<br />
grant of the Mairie <strong>de</strong> Paris and by the project CDCHT-<br />
ULA N ◦ C-1451-07-05-A. Part of this work was done wh<strong>en</strong><br />
the second author was a visiting professor at CRIL (CNRS<br />
UMR 8188) from September to December 2007 and a visiting<br />
researcher at TSI Departm<strong>en</strong>t of Telecom ParisTech<br />
(CNRS UMR 5141 LTCI) from January to April 2008. The<br />
second author thanks to CRIL and TSI Departm<strong>en</strong>t for the<br />
excell<strong>en</strong>t working conditions.<br />
App<strong>en</strong>dix<br />
Proof of Theorem 1: (only if) Let ◦ be a weak improvem<strong>en</strong>t<br />
operator. We <strong>de</strong>fine an assignm<strong>en</strong>t Ψ ↦→≤ Ψ by putting<br />
w ≤ Ψ w ′ if and only if w |= B(Ψ ⋆ ϕ w,w ′)<br />
By I4, the relation ≤ Ψ is well <strong>de</strong>fined, i.e. it does not <strong>de</strong>p<strong>en</strong>d<br />
on the choice of the formula ϕ w,w ′. We prove now that ≤ Ψ<br />
is a total pre-or<strong>de</strong>r.<br />
Totality: Let w, w ′ be any interpretations (ev<strong>en</strong>tually<br />
w = w ′ ). By I1, [[Ψ ⋆ ϕ w,w ′]] ⊆ {w, w ′ } and by <strong>de</strong>finition<br />
[[Ψ ⋆ ϕ w,w ′]] ≠ ∅, so w ∈ [[Ψ ⋆ ϕ w,w ′]] or w ′ ∈<br />
[[Ψ ⋆ ϕ w,w ′]] (or both), i.e. w ≤ Ψ w ′ or w ′ ≤ Ψ w.<br />
Transitivity: Suppose w ≤ Ψ w ′ and w ′ ≤ Ψ w ′′ . We<br />
want to show that w ≤ Ψ w ′′ , that is to say w ∈<br />
[[B(Ψ ⋆ ϕ w,w ′′)]]. Suppose towards a contradiction that it<br />
is not the case, so w ∉ [[B(Ψ ⋆ ϕ w,w ′′)]]. As by <strong>de</strong>finition<br />
[[B(Ψ ⋆ ϕ w,w ′′)]] ≠ ∅, this means that [[B(Ψ ⋆ ϕ w,w ′′)]] =<br />
{w ′′ }. Let us consi<strong>de</strong>r two cases:<br />
1) If B(Ψ ⋆ ϕ w,w ′ ,w ′′) ∧ ϕ w,w ′′ is not consist<strong>en</strong>t. Th<strong>en</strong>,<br />
as by <strong>de</strong>finition B(Ψ ⋆ ϕ w,w ′ ,w′′) ⊥, we have<br />
[[B(Ψ ⋆ ϕ w,w ′ ,w ′′)]] = {w′ }. In this case as B(Ψ ⋆<br />
ϕ w,w ′ ,w ′′) ∧ ϕ w,w ′ is consist<strong>en</strong>t, by I5 and I6 and I4 we get<br />
that [[B(Ψ ⋆ ϕ w,w ′)]] = [[B(Ψ ⋆ ϕ w,w′ ,w ′′) ∧ ϕ w,w ′]] =<br />
{w ′ }. This means by <strong>de</strong>finition that w ′ < Ψ w. Contradiction.<br />
2) If B(Ψ ⋆ ϕ w,w′ ,w ′′) ∧ ϕ w,w ′′ is consist<strong>en</strong>t. Th<strong>en</strong><br />
by I5 and I6 and I4 we get that [[B(Ψ ⋆ ϕ w,w ′′)]] =<br />
[[B(Ψ ⋆ ϕ w,w′ ,w ′′) ∧ ϕ w,w ′′]] = {w′′ }. This means by <strong>de</strong>finition<br />
that w ′′ < Ψ w. Contradiction.<br />
Let us prove now equation (4). First we will prove that<br />
[[B(Ψ ⋆ α)]] ⊆ min([[α]], ≤ Ψ ). Take w in [[B(Ψ ⋆ α)]].<br />
Thus, B(Ψ ⋆ α) ∧ ϕ w,w ′ ⊬ ⊥ for any w ′ ∈ [[α]]. Th<strong>en</strong>, by I5<br />
and I6 and I4, B(Ψ⋆α)∧ϕ w,w ′ ≡ B(Ψ⋆ϕ w,w ′). Therefore<br />
w ∈ [[B(Ψ ⋆ ϕ w,w ′)]], that is w ≤ Ψ w ′ for any w ′ ∈ [[α]]<br />
what exactly means w ∈ min([[α]], ≤ Ψ ).<br />
Now we will prove the converse inclusion that is<br />
min([[α]], ≤ Ψ ) ⊆ [[B(Ψ ⋆ α)]]. Suppose that w ∈<br />
min([[α]], ≤ Ψ ). We want to show that w ∈ [[B(Ψ ⋆ α)]].<br />
Towards a contradiction suppose that w ∉ [[B(Ψ ⋆ α)]].<br />
Let w ′ be a mo<strong>de</strong>l of B(Ψ ⋆ α).Th<strong>en</strong>, by I5 and I6 and<br />
I4, B(Ψ ⋆ α) ∧ ϕ w,w ′ ≡ B(Ψ ⋆ ϕ w,w ′). By assumption<br />
w ∉ [[B(Ψ ⋆ α)]] so [[B(Ψ ⋆ ϕ w,w ′)]] = {w ′ }. Therefore<br />
w ′ < Ψ w, contradicting the minimality of w in [[α]] with<br />
respect to ≤ Ψ .<br />
Now we prove the conditions of the strong faithful assignm<strong>en</strong>t.<br />
First to show conditions 1 and 2 it is equivall<strong>en</strong>t to<br />
show that [[B(Ψ)]] = min(W, ≤ Ψ ). Suppose that w |=<br />
113
B(Ψ). We want to see that w ≤ Ψ w ′ for any interpretation<br />
w ′ . In or<strong>de</strong>r to do that, let w ′ be a interpretation.<br />
Note that w |= B(Ψ) ∧ ϕ w,w ′, so B(Ψ) ∧ ϕ w,w ′ ⊬ ⊥.<br />
Th<strong>en</strong>, by I2, B(Ψ ⋆ ϕ w,w ′) ≡ B(Ψ) ∧ ϕ w,w ′. Therefore,<br />
w |= B(Ψ ⋆ ϕ w,w ′), i.e. w ≤ Ψ w ′ . This proves<br />
that [[B(Ψ)]] ⊆ min(W, ≤ Ψ ). For the converse inclusion<br />
take w ∈ min(W, ≤ Ψ ). Towards a contradiction, suppose<br />
that w ∉ [[B(Ψ)]]. Let w ′ be a mo<strong>de</strong>l of B(Ψ). Th<strong>en</strong><br />
[[B(Ψ) ∧ ϕ w,w ′]] = {w ′ }. Thus, by I2, B(Ψ ⋆ ϕ w,w ′) ≡<br />
B(Ψ) ∧ ϕ w,w ′ and therefore [[B(Ψ ⋆ ϕ w,w ′)]] = {w ′ }, i.e.<br />
w ′ < Ψ w, contradicting the minimality of w with respect to<br />
≤ Ψ .<br />
Now for condition 3 suppose α i ≡ β i for any i ≤ n we want<br />
to show ≤ Ψ◦α1◦···◦αn= ≤ Ψ◦β1◦···◦βn. We proceed by induction<br />
on k = 0, . . . , n. For k = 0 is trivial because ≤ Ψ =≤ Ψ .<br />
For short<strong>en</strong>ing the notation put Θ k = Ψ ◦ α 1 ◦ · · · ◦ α k and<br />
Γ k = Ψ ◦ β 1 ◦ · · · ◦ β k . Thus our induction hypothesis is<br />
≤ Θk = ≤ Γk . We want to show that ≤ = ≤ .<br />
Θk◦αk+1 Γk◦βk+1<br />
In or<strong>de</strong>r to do that, we prove that each level of ≤ Θk◦αk+1<br />
is equal to the corresponding level of ≤ . Γk◦βk+1<br />
This is<br />
done by induction on the number of levels of ≤ .<br />
Θk◦αk+1<br />
We sketch the proof. For the level 0: we want to see<br />
that min(W, ≤ ) = min(W, ≤ Θk◦αk+1<br />
). By equation<br />
(4), we have min(W, ≤ ) = [[B(Γ Θk◦αk+1 k ◦ α k+1 )]]<br />
Γk◦βk+1<br />
and min(W, ≤ ) = [[B(Γ Γk◦βk+1 k ◦ β k+1 )]]. By<br />
I4 + , [[B(Θ k ◦ α k+1 )]] = [[B(Γ k ◦ β k+1 )]]. Therefore,<br />
min(W, ≤ ) = min(W, ≤ ).<br />
Θk◦αk+1<br />
Now suppose<br />
that the first i levels of ≤ Θk◦αk+1<br />
Γk◦βk+1<br />
correspond exactly<br />
to the first i levels of ≤ . Γk◦βk+1<br />
We will prove that the<br />
level i + 1 of ≤ Θk◦αk+1<br />
is contained in the level i + 1<br />
of ≤ Γk◦βk+1<br />
(and with a symmetrical argum<strong>en</strong>t we will<br />
prove the inverse inclusion). Towards a contradiction suppose<br />
that w is in the level i + 1 of ≤ Θk◦αk+1<br />
and w is<br />
not in the level i + 1 of ≤ . Take w′ Γk◦βk+1<br />
in the level<br />
i + 1 of ≤ . As the first i levels of ≤ Γk◦βk+1 Θk◦αk+1<br />
and ≤ are equal, w′ Γk◦βk+1<br />
is in a level j, with j > i<br />
for the pre-or<strong>de</strong>r ≤ .<br />
Θk◦αk+1<br />
Consi<strong>de</strong>r now the formula<br />
ϕ w,w ′. Th<strong>en</strong> it is clear that w ∈ min([[ϕ w,w ′]], ≤ Θk◦αk+1<br />
) and w ∉ min([[ϕ w,w ′]], ≤ Γk◦βk+1<br />
). From this,<br />
by equation (4), follows [[B(Θ k ◦ α k+1 ◦ ϕ w,w ′)]] ≠<br />
[[B(Γ k ◦ β k+1 ◦ ϕ w,w ′)]], contradicting I4 + .<br />
(if) Suppose that we have a strong faithful assignm<strong>en</strong>t<br />
Ψ ↦→≤ Ψ such that equation (4) holds. We want to check<br />
that I1-I6 hold.<br />
(I1) Follows from equation 4.<br />
(I2) Let us first show that B(Ψ) ∧ α ⊢ B(Ψ ⋆ α). If w |=<br />
B(Ψ) ∧ α this means that w ∈ min(W, ≤ Ψ ). So for any<br />
w ′ ∈ W, we have w ≤ Ψ w ′ . This is in particular true for all<br />
the mo<strong>de</strong>ls of α, so w ∈ min(α, ≤ Ψ ), that is, by <strong>de</strong>finition,<br />
w |= B(Ψ⋆α). Let us now show that B(Ψ⋆α) ⊢ B(Ψ)∧α.<br />
By <strong>de</strong>finition w |= B(Ψ⋆α) means w ∈ min([[α]], ≤ Ψ ). So<br />
w |= α. Let us show that w |= B(Ψ). Suppose that it is not<br />
the case. In this case, and since by hypothesis B(Ψ)∧α ⊥<br />
we can choose a w ′ ∈ [[B(Ψ) ∧ α]]. So, as w ′ ∈ [[B(Ψ)]]<br />
and w /∈ [[B(Ψ)]], we have w ′ < Ψ w. But as w ′ , w |= α,<br />
this implies that w /∈ min([[α]], ≤ Ψ ). Contradiction.<br />
(I4) Suppose α i ≡ β i for any i ≤ n we want to show<br />
B(Ψ ◦ α 1 ◦ · · · ◦ α n ) ≡ B(Ψ ◦ β 1 ◦ · · · ◦ β n ). By equation<br />
(4), this is equival<strong>en</strong>t to prove min([[α n ]], ≤ Ψ◦α1◦···◦αn−1<br />
) = min([[β n ]], ≤ Ψ◦β1◦···◦βn−1). But this is clear because,<br />
by S6, we have ≤ Ψ◦α1◦···◦αn−1=≤ Ψ◦β1◦···◦βn−1 and by hypothesis<br />
[[α n ]] = [[β n ]].<br />
(I5 and I6) By equation (4) we have [[B(Ψ ⋆ (α ∧ β))]] =<br />
min([[α ∧ β]], ≤ Ψ ) and [[B(Ψ ⋆ α) ∧ β]] = min([[α]], ≤ Ψ<br />
) ∩ [[β]]. Thus, it is <strong>en</strong>ough to see that<br />
min([[α]], ≤ Ψ ) ∩ [[β]] = min([[α ∧ β]], ≤ Ψ )<br />
un<strong>de</strong>r the hypothesis min([[α]], ≤ Ψ ) ∩ [[β]] ≠ ∅. It is quite<br />
clear that min([[α]], ≤ Ψ ) ∩ [[β]] ⊆ min([[α ∧ β]], ≤ Ψ ).<br />
For the other inclusion take w ∈ min([[α ∧ β]], ≤ Ψ ). As<br />
w is in [[β]] it remains to see that w ∈ min([[α]], ≤ Ψ ). We<br />
know that w ∈ [[α]]. We claim that it is minimal in [[α]]<br />
with respect to ≤ Ψ . Towards a contradiction, suppose that<br />
ir is not the case. As ≤ Ψ is a total pre-or<strong>de</strong>r there exists<br />
w ′ ∈ min([[α]], ≤ Ψ ) such that w ′ < Ψ w. By hypothesis,<br />
there exists w ′′ ∈ min([[α]], ≤ Ψ ) ∩ [[β]]. Again as≤ Ψ<br />
is a total pre-or<strong>de</strong>r, w ′ ∼ Ψ w ′′ , therefore w ′′ < Ψ w<br />
contradicting the minimality of w in [[α ∧ β]] with respect<br />
to ≤ Ψ .<br />
Proof of Proposition 1: If min([[α]], ≤ Ψ ) and<br />
min([[β]], ≤ Ψ ) are in the same level with respect to ≤ Ψ th<strong>en</strong><br />
min([[(]]α ∨ β), ≤ Ψ ) = min([[α]], ≤ Ψ ) ∪ min([[β]], ≤ Ψ ).<br />
Thus, by Theorem 1, [[B(Ψ ⋆ (α ∨ β))]] = [[B(Ψ ⋆ α)]] ∪<br />
[[B(Ψ ⋆ β)]]. Otherwise, min([[α]], ≤ Ψ ) is in a lower<br />
level than min([[β]], ≤ Ψ ) or min([[β]], ≤ Ψ ) is in a<br />
lower level than min([[α]], ≤ Ψ ). In the first case,<br />
min([[(]]α ∨ β), ≤ Ψ ) = min([[α]], ≤ Ψ ). Thus, by the<br />
Theorem 1, [[B(Ψ ⋆ (α ∨ β))]] = [[B(Ψ ⋆ α)]]. In the<br />
second case, min([[(]]α ∨ β), ≤ Ψ ) = min([[β]], ≤ Ψ ). Thus,<br />
by the Theorem 1, [[B(Ψ ⋆ (α ∨ β))]] = [[B(Ψ ⋆ β)]].<br />
Proof of Corollary 2: (only if) Assume that α ≺ Ψ β, that<br />
is B(Ψ⋆α) ⊢ B(Ψ⋆(α∨β)) and B(Ψ⋆β) ⊬ B(Ψ⋆(α∨β)).<br />
By Proposition 1 and its proof, necessarily min([[α]], ≤ Ψ )<br />
is in a lower level than min([[β]], ≤ Ψ ). Thus, by Theorem<br />
1, it is <strong>en</strong>ough to take w ∈ min([[α]], ≤ Ψ ) and w ′ ∈<br />
min([[β]], ≤ Ψ ) to get w ∈ [[B(Ψ ⋆ α)]], w ′ ∈ [[B(Ψ ⋆ β)]],<br />
w < Ψ w ′ .<br />
(if) Assume that there exist w, w ′ such that w ∈<br />
[[B(Ψ ⋆ α)]], w ′ ∈ [[B(Ψ ⋆ β)]], w < Ψ w ′ . Th<strong>en</strong>, by Theorem<br />
1, min([[α]], ≤ Ψ ) is in a lower level than min([[β]], ≤ Ψ<br />
). Th<strong>en</strong>, by Proposition 1 and its proof, B(Ψ ⋆ (α ∨ β)) ≡<br />
B(Ψ ⋆ α). On the other hand B(Ψ ⋆ β) ⊬ B(Ψ ⋆ (α ∨ β))<br />
because min([[α]], ≤ Ψ ) and min([[β]], ≤ Ψ ) are not in the<br />
same level. Therefore α ≺ Ψ β<br />
Proof of Corollary 3: (only if) Assume α ≺ Ψ β. By<br />
Corollary 2, we get w, w ′ such that w ∈ [[B(Ψ ⋆ α)]],<br />
w ′ ∈ [[B(Ψ ⋆ β)]], w < Ψ w ′ . Towards a contradiction,<br />
suppose that there exists w ′′ such that w < Ψ w ′′ < Ψ w ′ .<br />
But it is clear, using Corollary 2, that α ≺ Ψ ϕ w ′′ ≺ Ψ β<br />
contradicting the fact α ≺ Ψ β.<br />
(if) Assume there exist w, w ′ such that w ∈ [[B(Ψ ⋆ α)]],<br />
114
w ′ ∈ [[B(Ψ ⋆ β)]], w < Ψ w ′ and there is no w ′′ such<br />
that w < Ψ w ′′ < Ψ w ′ . By Corollary 2, α ≺ Ψ β. Thus,<br />
the only possibility for α ⊀≺ Ψ β, is the exist<strong>en</strong>ce of γ<br />
such that α ≺ Ψ γ ≺ Ψ β. Again, by Corollary 2, taking<br />
w ′′ ∈ [[B(Ψ ⋆ γ)]] we have w < Ψ w ′′ < Ψ w ′ , a contradiction.<br />
Proof of Theorem 2: (only if) By Theorem 1 we know<br />
that there exists an epistemic assignm<strong>en</strong>t Ψ ↦→≤ Ψ such that<br />
the equation (4) holds. Thus, it remains to prove that the assignm<strong>en</strong>t<br />
is in<strong>de</strong>ed a gradual assignm<strong>en</strong>t, i.e. it satisfies S1,<br />
S2, S3, S4 and S5.<br />
(S1) Suppose w, w ′ ∈ [[α]]. Thus, ϕ w,w ′ ⊢ α. By I7,<br />
B((Ψ ◦ α) ⋆ ϕ w,w ′) ≡ B(Ψ ⋆ ϕ w,w ′). Th<strong>en</strong> by equation (4)<br />
we have w ≤ Ψ◦α w ′ ⇔ w ∈ min({w, w ′ }, ≤ Ψ◦α )<br />
⇔ w ∈ [[B((Ψ ◦ α) ⋆ ϕ w,w ′)]]<br />
⇔ w ∈ [[B(Ψ ⋆ ϕ w,w ′)]]<br />
⇔ w ∈ min({w, w ′ }, ≤ Ψ )<br />
⇔ w ≤ Ψ w ′<br />
(S2) The proof is analogous to the one of S1 but using I8<br />
instead of I7.<br />
(S3) Suppose that w ∈ [[α]], w ′ ∈ [[¬α]] and w ≤ Ψ w ′ .<br />
We want to show that w < Ψ◦α w ′ . As w ≤ Ψ w ′ , necessarily<br />
w ∈ min({w, w ′ }, ≤ Ψ ) what, by Equation (4),<br />
means w ∈ [[B(Ψ ⋆ ϕ w,w ′)]]. Th<strong>en</strong> B(Ψ ⋆ ϕ w,w ′) ⊬ ¬α,<br />
so by I9, B((Ψ ◦ α) ⋆ ϕ w,w ′) ⊢ α. Th<strong>en</strong>, by I1 and I3,<br />
[[B((Ψ ◦ α) ⋆ ϕ w,w ′)]] = {w}. From this, using Equation<br />
(4), we get w < Ψ◦α w ′ .<br />
(S4) Suppose that w ∈ [[α]], w ′ ∈ [[¬α]] and w ′ < Ψ w.<br />
We want to show that w ′ ≤ Ψ◦α w. From the hypothesis<br />
w ′ < Ψ w we get min({w, w ′ }, ≤ Ψ ) = {w ′ }. Th<strong>en</strong>, by<br />
Equation (4), [[B(Ψ ⋆ ϕ w,w ′)]] = {w ′ }. Therefore B(Ψ ⋆<br />
ϕ w,w ′) ⊢ ¬α. Thus, by I10, B((Ψ ◦ α) ⋆ ϕ w,w ′) ⊬ α. Th<strong>en</strong><br />
w ′ ∈ [[B((Ψ ◦ α) ⋆ ϕ w,w ′)]], and by Equation (4), w ′ ∈<br />
min({w, w ′ }, ≤ Ψ◦α ), i.e. w ′ ≤ Ψ◦α w.<br />
(S5) Suppose that w ∈ [[α]], w ′ ∈ [[¬α]], w ′ < Ψ w and<br />
that there is no w ′′ such that w ′ < Ψ w ′′ < Ψ w. We<br />
want to show that w ≤ Ψ◦α w ′ . From w ′ < Ψ w we have<br />
min({w, w ′ }, ≤ Ψ ) = {w ′ }, so from Theorem 1 we have<br />
[[B(Ψ ⋆ ϕ w,w ′)]] = {w ′ }. So B(Ψ ⋆ ϕ w,w ′) ⊢ ¬α. On the<br />
other hand, the assumptions with the Corollary 3 gives us<br />
ϕ w,w ′ ≺ Ψ ϕ w,w ′ ∧ α. Th<strong>en</strong>, by I11, B((Ψ ◦ α) ⋆ ϕ w,w ′) ⊬<br />
¬α, that means by Theorem 1, w ≤ Ψ◦α w ′ .<br />
(if) By Theorem 1 we know that ◦ is a weak improvem<strong>en</strong>t<br />
operator. Thus, it remains to check that I7-I11 hold.<br />
(I7) Suppose that α ⊢ µ, i.e. [[α]] ⊆ [[µ]]. We want to show<br />
B((Ψ ◦ µ) ⋆ α) ≡ B(Ψ ⋆ α). By Equation (4) this is equival<strong>en</strong>t<br />
to prove that min([[α]], ≤ Ψ◦µ ) = min([[α]], ≤ Ψ ). That<br />
is a straightforward consequ<strong>en</strong>ce of S1 that gives (since<br />
[[α]] ⊆ [[µ]]) ∀w, w ′ |= α, w ≤ Ψ w ′ iff w ≤ Ψ◦µ w ′ .<br />
(I8) Suppose that α ⊢ ¬µ, i.e. [[α]] ⊆ [[¬µ]]. We want to<br />
show B((Ψ ◦ µ) ⋆ α) ≡ B(Ψ ⋆ α). By Equation (4) this is<br />
equival<strong>en</strong>t to prove that min([[α]], ≤ Ψ◦µ ) = min([[α]], ≤ Ψ<br />
). Like for (I7), this is a straightforward consequ<strong>en</strong>ce of S2<br />
that gives (since [[α]] ⊆ [[¬µ]]) ∀w, w ′ |= α, w ≤ Ψ w ′ iff<br />
w ≤ Ψ◦µ w ′ .<br />
(I9) Let us remark from the fact that ≤ Ψ and ≤ Ψ◦µ are total<br />
pre-or<strong>de</strong>rs, that postulate S3 is equival<strong>en</strong>t to the following<br />
one:<br />
(S3’) If w ∈ [[µ]], w ′ ∈ [[¬µ]] th<strong>en</strong> w ′ ≤ Ψ◦µ w ⇒<br />
w ′ < Ψ w<br />
Now suppose that B(Ψ ⋆ α) ⊬ ¬µ. We want to show<br />
B((Ψ ◦ µ) ⋆ α) ⊢ µ. Towards a contradiction, suppose that<br />
B((Ψ ◦ µ) ⋆ α) ⊬ µ, i.e. there exists w ∈ [[B((Ψ ◦ µ) ⋆ α)]]<br />
such that w ∉ [[µ]]. By Equation (4) w ∈ min([[α]], ≤ Ψ◦µ ),<br />
so for any w ′ ∈ [[α]], w ≤ Ψ◦µ w ′ . By the assumption,<br />
there exists w ′′ ∈ [[B(Ψ ⋆ α)]] ∩ [[µ]]. In particular, by<br />
I1, w ′′ ∈ [[α]]. Thus, w ≤ Ψ◦µ w ′′ . On the other hand,<br />
by Equation (4) w ′′ ∈ min([[α]], ≤ Ψ ). As w ∈ [[¬µ]] and<br />
w ′′ ∈ [[µ]], and w ≤ Ψ◦µ w ′′ , by S3’, w < Ψ w ′′ . But, since<br />
w ∈ [[α]], this contradicts the minimality of w ′′ in [[α]] with<br />
respect to ≤ Ψ .<br />
(I10) Let us remark from the fact that ≤ Ψ and ≤ Ψ◦µ are total<br />
pre-or<strong>de</strong>rs, the postulate S4 is equival<strong>en</strong>t to the following<br />
one:<br />
(S4’) If w ∈ [[µ]], w ′ ∈ [[¬µ]] th<strong>en</strong> w < Ψ◦µ w ′ ⇒<br />
w ≤ Ψ w ′<br />
Suppose that B(Ψ ⋆ α) ⊢ ¬µ. We want to show B((Ψ ◦<br />
µ) ⋆ α) ⊬ µ. Towards a contradiction suppose that B((Ψ ◦<br />
µ) ⋆ α) ⊢ µ. Let w, w ′ be such that w |= B((Ψ ◦ µ) ⋆ α)<br />
and w ′ |= B(Ψ ⋆ α). By the assumptions w ∈ [[µ]] and<br />
w ′ ∈ [[¬µ]]. By Equation (4), w ∈ min([[α]], ≤ Ψ◦µ ) and<br />
w ′ ∈ min([[α]], ≤ Ψ ). By the assumptions, w ≄ Ψ◦µ w ′<br />
and w ≄ Ψ w ′ , because if not w ′ ∈ min([[α]], ≤ Ψ◦µ ) or<br />
w ∈ min([[α]], ≤ Ψ ). But this is impossible because in the<br />
first case w ′ ∈ [[µ]], a contradiction and in he second case<br />
w ∈ [[¬µ]], a contradiction. Thus, necessarily w < Ψ◦µ w ′<br />
and w ′ < Ψ w. As we have w ∈ [[µ]], w ′ ∈ [[¬µ]] and<br />
w < Ψ◦µ w ′ , by S4’, w ≤ Ψ w ′ , a contradiction.<br />
(I11) Assume B(Ψ ⋆ α) ⊢ ¬µ, α ∧ µ ⊬ ⊥ and α ≺ Ψ α ∧ µ.<br />
We want to show that B((Ψ ◦ µ) ⋆ α) ⊬ ¬µ. Towards a<br />
contradiction, suppose that B((Ψ ◦ µ) ⋆ α) ⊢ ¬µ. Let w, w ′<br />
such that w ′ ∈ [[B(Ψ ⋆ α)]] and w ∈ [[B(Ψ ⋆ (α ∧ µ))]].<br />
By the assumptions we have w ′ ∈ [[¬µ]] and w ∈ [[µ]].<br />
By Corollary 3, w ′ < Ψ w and there is no w ′′ such that<br />
w ′ < Ψ w ′′ < Ψ w. By S5, w ≤ Ψ◦µ w ′ . By S4,<br />
w ′ ≤ Ψ◦µ w. Therefore, w ≃ Ψ◦µ w ′ . That means that<br />
[[B(Ψ ⋆ α)]] and [[B(Ψ ⋆ α ∧ µ)]] are in the same level<br />
with respect to ≤ Ψ◦µ . We claim that this level is the<br />
level of min([[α]], ≤ Ψ◦µ ). But this is a contradiction<br />
because we have w ∈ min(min([[α]], ≤ Ψ◦µ )) and therefore<br />
w |= ¬µ which contradicts the fact that w |= µ. Now we<br />
turn to the proof of our claim. Towards a contradiction,<br />
suppose the claim is not true. Th<strong>en</strong>, necessarily there<br />
is w ′′ ∈ min([[α]], ≤ Ψ◦µ ) such that w ′′ < Ψ◦µ w. We<br />
consi<strong>de</strong>r two cases: w ′′ ∈ [[µ]] and w ′′ ∈ [[¬µ]]. In<br />
the case w ′′ ∈ [[µ]], we don’t have w ′′ < Ψ w because<br />
w ∈ min([[α ∧ µ]], ≤ Ψ ). Therefore w ≤ Ψ w ′′ . Th<strong>en</strong>, by<br />
S1, w ≤ Ψ◦µ w ′′ , a contradiction. In the case w ′′ ∈ [[¬µ]],<br />
we don’t have w ′′ < Ψ w ′ because w ′ ∈ min([[α]], ≤ Ψ ).<br />
Therefore w ′ ≤ Ψ w ′′ . Th<strong>en</strong>, by S2, w ′ ≤ Ψ◦µ w ′′ , that is<br />
w ≤ Ψ◦µ w ′′ , a contradiction.<br />
115
Proof of Lemma 1: Define A =<br />
{w ′′ ∈ W : w < Ψ w ′′ < Ψ w ′ }. By the assumptions<br />
w < Ψ w ′ and w ̸≪ Ψ w ′ , the set A is nonempty. Thus<br />
A ∩ [[¬α]] ≠ ∅ or A ∩ [[α]] ≠ ∅. We consi<strong>de</strong>r first the case<br />
A ∩ [[¬α]] ≠ ∅. Take w ′′ ∈ max(A ∩ [[¬α]], ≤ Ψ ). By<br />
<strong>de</strong>finition of A, w < Ψ w ′′ and w ′′ < Ψ w ′ . We consi<strong>de</strong>r two<br />
subcases:<br />
• w ′′ ≪ Ψ w ′ . In this situation, we conclu<strong>de</strong> by S4 and S5<br />
w ′′ ∼ Ψ◦α w ′ . By S2, w < Ψ◦α w ′′ . Therefore by transitivity<br />
w < Ψ◦α w ′ .<br />
• w ′′ ̸≪ Ψ w ′ . In this situation we take w ′′′ such that<br />
w ′′ ≪ Ψ w ′′′ . Its clear that w ′′′ < Ψ w ′ and by <strong>de</strong>finition<br />
of w ′′ , w ′′′ ∈ [[α]]. By S4 and S5, w ′′ ≃ Ψ◦α w ′′′ . Thus<br />
w < Ψ◦α w ′′′ . By S1, w ′′′ < Ψ◦α w ′ . Th<strong>en</strong>, by transitivity,<br />
w < Ψ◦α w ′ .<br />
For the second case, A ∩ [[α]] ≠ ∅, we proceed<br />
with an analogous reasoning, but this time taking<br />
w ′′ ∈ min(A ∩ [[α]], ≤ Ψ ).<br />
Proof of Proposition 2: Towards a contradiction,<br />
suppose that we have ≤ 1 Ψ◦α≠≤2 Ψ◦α and both pre-or<strong>de</strong>rs<br />
obey to (S1-S5). Let w, w ′ be witness of this inequality.<br />
Thus, without lost of g<strong>en</strong>erality, we can suppose w < 1 Ψ◦α w′<br />
and w ′ ≤ 2 Ψ◦α w. By S1, it is not the case w, w′ ∈ [[α]],<br />
since otherwise by w < 1 Ψ◦α w′ we obtain w < Ψ w ′ and<br />
by w ′ ≤ 2 Ψ◦α w we obtain w′ ≤ Ψ w and a contradiction.<br />
Similarly by S2, it is not the case w, w ′ ∈ [[¬α]]. Thus,<br />
the only possibilities are w ∈ [[α]] and w ′ ∈ [[¬α]] or<br />
w ∈ [[¬α]] and w ′ ∈ [[α]].<br />
We consi<strong>de</strong>r the first case, i.e. w ∈ [[α]] and w ′ ∈ [[¬α]]. As<br />
w ′ ≤ 2 Ψ◦α w, by S3, w ≰ Ψ w ′ , i.e. w ′ < Ψ w. If w ′ ̸≪ Ψ w<br />
th<strong>en</strong>, by the Lemma 1 w ′ < 1 Ψ◦α w, a contradiction. If<br />
w ′ ≪ Ψ w, by S4 and S5, w ′ ≃ 1 Ψ◦α w, again a contradiction.<br />
Now, we consi<strong>de</strong>r the second case, i.e. w ∈ [[¬α]] and<br />
w ′ ∈ [[α]]. As w < 1 Ψ◦α w′ , by S3, w ′ ≰ Ψ w, i.e. w < Ψ w ′ .<br />
Suppose w ≪ Ψ w ′ . Th<strong>en</strong>, by S5, w ′ ≤ 1 Ψ◦α w a contradiction.<br />
So w ̸≪ Ψ w ′ , and by Lemma 1, w < 2 Ψ◦α w′ , a<br />
contradiction.<br />
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117
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130
DA 2 MERGING OPERATORS<br />
Sébasti<strong>en</strong> Konieczny, Jérôme Lang, Pierre Marquis.<br />
Artificial Intellig<strong>en</strong>ce. 157(1-2).<br />
pages 49-79.<br />
2004.<br />
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ON THE MERGING OF DUNG’S ARGUMENTATION<br />
SYSTEMS<br />
Sylvie Coste-Marquis, Caroline Devred, Sébasti<strong>en</strong> Konieczny, Marie-<br />
Christine Lagasquie-Schiex, Pierre Marquis.<br />
Artificial Intellig<strong>en</strong>ce. 171.<br />
pages 740-753.<br />
2007.<br />
171
On the Merging of Dung’s Argum<strong>en</strong>tation<br />
Systems<br />
Sylvie Coste-Marquis a , Caroline Devred a ,<br />
Sébasti<strong>en</strong> Konieczny a , Marie-Christine Lagasquie-Schiex b ,<br />
Pierre Marquis a<br />
a CRIL-CNRS, Université d’Artois, L<strong>en</strong>s, France<br />
b IRIT-CNRS, UPS, Toulouse, France<br />
Abstract<br />
In this paper, the problem of <strong>de</strong>riving s<strong>en</strong>sible information from a collection of<br />
argum<strong>en</strong>tation systems coming from differ<strong>en</strong>t ag<strong>en</strong>ts is addressed. The un<strong>de</strong>rlying<br />
argum<strong>en</strong>tation theory is Dung’s one: each argum<strong>en</strong>tation system gives both a set of<br />
argum<strong>en</strong>ts and the way they interact (i.e., attack or non-attack) according to the<br />
corresponding ag<strong>en</strong>t. The ina<strong>de</strong>quacy of the simple, yet appealing, method which<br />
consists in voting on the ag<strong>en</strong>ts’ selected ext<strong>en</strong>sions calls for a new approach. To<br />
this purpose, a g<strong>en</strong>eral framework for merging argum<strong>en</strong>tation systems from Dung’s<br />
theory of argum<strong>en</strong>tation is pres<strong>en</strong>ted. The objective is achieved through a threestep<br />
process: first, each argum<strong>en</strong>tation system is expan<strong>de</strong>d into a partial system<br />
over the set of all argum<strong>en</strong>ts consi<strong>de</strong>red by the group of ag<strong>en</strong>ts (reflecting that<br />
some ag<strong>en</strong>ts may easily ignore argum<strong>en</strong>ts pointed out by other ag<strong>en</strong>ts, as well as<br />
how such argum<strong>en</strong>ts interact with her own ones); th<strong>en</strong>, merging is used on the<br />
expan<strong>de</strong>d systems as a way to solve the possible conflicts betwe<strong>en</strong> them, and a<br />
set of argum<strong>en</strong>tation systems which are as close as possible to the whole profile is<br />
g<strong>en</strong>erated; finally, voting is used on the selected ext<strong>en</strong>sions of the resulting systems<br />
so as to characterize the acceptable argum<strong>en</strong>ts at the group level.<br />
Key words: Argum<strong>en</strong>tation frameworks, Argum<strong>en</strong>t in ag<strong>en</strong>t system<br />
1 Introduction<br />
Argum<strong>en</strong>tation is based on the exchange and the evaluation of interacting<br />
argum<strong>en</strong>ts which may repres<strong>en</strong>t information of various kinds, especially beliefs<br />
⋆ This paper is an ext<strong>en</strong><strong>de</strong>d and revised version of a paper <strong>en</strong>titled “Merging Argum<strong>en</strong>tation<br />
Systems” that appeared in the Proceedings of AAAI’05, pages 614–619.<br />
Preprint submitted to Artificial Intellig<strong>en</strong>ce 2 April 2007<br />
173
or goals. Argum<strong>en</strong>tation can be used for mo<strong>de</strong>lling some aspects of reasoning,<br />
<strong>de</strong>cision making, and dialogue; as such, it has be<strong>en</strong> applied to several domains,<br />
including law. For instance, wh<strong>en</strong> an ag<strong>en</strong>t has conflicting beliefs (viewed as<br />
argum<strong>en</strong>ts), a (nontrivial) set of plausible consequ<strong>en</strong>ces can be <strong>de</strong>rived through<br />
argum<strong>en</strong>tation from the most acceptable argum<strong>en</strong>ts for the ag<strong>en</strong>t (additional<br />
information like a plausibility or<strong>de</strong>ring are oft<strong>en</strong> tak<strong>en</strong> into account in the<br />
evaluation phase). Much work has be<strong>en</strong> <strong>de</strong>voted to this reasoning issue (see<br />
for example (13; 21; 26; 25; 1; 27)).<br />
Several theories of argum<strong>en</strong>tation exist; many of them make explicit the nature<br />
of argum<strong>en</strong>ts, the way argum<strong>en</strong>ts are g<strong>en</strong>erated, how they interact and<br />
how to evaluate them, and finally a characterization of the most acceptable<br />
argum<strong>en</strong>ts. A key issue is the interaction betwe<strong>en</strong> argum<strong>en</strong>ts which is typically<br />
based on a notion of attack; for example, wh<strong>en</strong> an argum<strong>en</strong>t takes the form<br />
of a logical proof, argum<strong>en</strong>ts for a statem<strong>en</strong>t and argum<strong>en</strong>ts against it can be<br />
put forward. In that case, the attack relation relies on logical inconsist<strong>en</strong>cy.<br />
Dung’s theory of argum<strong>en</strong>tation inclu<strong><strong>de</strong>s</strong> several formal systems <strong>de</strong>veloped<br />
so far for commons<strong>en</strong>se reasoning or logic programming (13). It is abstract<br />
<strong>en</strong>ough to manage without any assumptions on the nature of argum<strong>en</strong>ts or<br />
the attack relation. In<strong>de</strong>ed, an argum<strong>en</strong>tation system <strong>à</strong> la Dung consists of a<br />
set of (abstract) argum<strong>en</strong>ts, together with a binary relation on it (the attack<br />
relation). Several semantics can be used for <strong>de</strong>fining interesting sets of argum<strong>en</strong>ts<br />
(so-called ext<strong>en</strong>sions) from which acceptable sets of argum<strong>en</strong>ts (i.e., the<br />
<strong>de</strong>rivable sets) can be characterized.<br />
In a multi-ag<strong>en</strong>t setting, argum<strong>en</strong>tation can also be used to repres<strong>en</strong>t (part of)<br />
some information exchange processes, like negotiation, or persuasion (see for<br />
example (22; 28; 18; 24; 3; 4; 5)). For instance, a negotiation process betwe<strong>en</strong><br />
two ag<strong>en</strong>ts about whether some belief must be consi<strong>de</strong>red as true giv<strong>en</strong> some<br />
evi<strong>de</strong>nce can be mo<strong>de</strong>lled as a two-player game where each move consists in<br />
reporting an argum<strong>en</strong>t which attacks argum<strong>en</strong>ts giv<strong>en</strong> by the oppon<strong>en</strong>t.<br />
In this paper, we also consi<strong>de</strong>r argum<strong>en</strong>tation in a multi-ag<strong>en</strong>t setting, but<br />
from a very differ<strong>en</strong>t perspective. Basically, our purpose is to characterize the<br />
set of argum<strong>en</strong>ts acceptable by a group of ag<strong>en</strong>ts, wh<strong>en</strong> the data furnished by<br />
each ag<strong>en</strong>t consist solely of an (abstract) argum<strong>en</strong>tation system from Dung’s<br />
theory.<br />
At a first glance, a simple approach for achieving this goal consists in voting<br />
on the acceptable sets provi<strong>de</strong>d by each ag<strong>en</strong>t: a set of argum<strong>en</strong>ts is consi<strong>de</strong>red<br />
acceptable by the group if and only if it is acceptable for “suffici<strong>en</strong>tly many”<br />
ag<strong>en</strong>ts from the group (where the meaning of “suffici<strong>en</strong>tly many” refers to<br />
differ<strong>en</strong>t voting methods). No merging at all is required here. By means of<br />
example, we show that our merging-based approach leads to results which are<br />
2 174
much more expected than those furnished by a direct vote on the (sets of)<br />
argum<strong>en</strong>ts acceptable by each ag<strong>en</strong>t.<br />
Our approach is more sophisticated. It follows a three-step process: first, each<br />
argum<strong>en</strong>tation system is expan<strong>de</strong>d into a partial system over the set of all<br />
argum<strong>en</strong>ts consi<strong>de</strong>red by the group of ag<strong>en</strong>ts (reflecting that some ag<strong>en</strong>ts<br />
may easily ignore argum<strong>en</strong>ts pointed out by other ag<strong>en</strong>ts, as well as how such<br />
argum<strong>en</strong>ts interact with her own ones); th<strong>en</strong>, merging is used on the expan<strong>de</strong>d<br />
systems as a way to solve the possible conflicts betwe<strong>en</strong> them, and a set of<br />
argum<strong>en</strong>tation systems which are as close as possible to the whole profile is<br />
g<strong>en</strong>erated; finally, the last step consists in selecting the acceptable argum<strong>en</strong>ts<br />
at the group levels from the set of argum<strong>en</strong>tation systems.<br />
In or<strong>de</strong>r to reach this goal, we first introduce a notion of partial argum<strong>en</strong>tation<br />
system, which ext<strong>en</strong>ds Dung’s argum<strong>en</strong>tation system so as to repres<strong>en</strong>t<br />
ignorance concerning the attack relation. This is necessary in our setting since<br />
all the ag<strong>en</strong>ts participating in the merging process are not assumed to share<br />
the same global set of argum<strong>en</strong>ts. Accordingly, the argum<strong>en</strong>tation system furnished<br />
by each ag<strong>en</strong>t is first expan<strong>de</strong>d into a partial argum<strong>en</strong>tation system,<br />
and all such partial systems are built over the same set of argum<strong>en</strong>ts, those<br />
pointed out by at least one ag<strong>en</strong>t. Of course, there exist many differ<strong>en</strong>t ways<br />
to incorporate a new argum<strong>en</strong>t into an argum<strong>en</strong>tation system. Each ag<strong>en</strong>t can<br />
have her own expansion policy. We m<strong>en</strong>tion some possible policies, and focus<br />
on one of them, called the cons<strong>en</strong>sual expansion: wh<strong>en</strong> incorporating a new<br />
argum<strong>en</strong>t into her own system, an ag<strong>en</strong>t is ready to conclu<strong>de</strong> that this argum<strong>en</strong>t<br />
attacks (resp. is attacked by) another argum<strong>en</strong>t wh<strong>en</strong>ever all the other<br />
ag<strong>en</strong>ts who are aware of both argum<strong>en</strong>ts agree with this attack; otherwise, she<br />
conclu<strong><strong>de</strong>s</strong> that she ignores whether an attack takes place or not.<br />
Once all the expansions of the input argum<strong>en</strong>tation systems have be<strong>en</strong> computed,<br />
the proper merging step can be achieved; it consists in computing all<br />
the argum<strong>en</strong>tation systems over the global set of argum<strong>en</strong>ts which are “as close<br />
as possible” to the partial systems g<strong>en</strong>erated during the last stage. Clos<strong>en</strong>ess is<br />
characterized by a notion of distance betwe<strong>en</strong> an argum<strong>en</strong>tation system and a<br />
profile of partial systems, induced from a primitive notion of distance betwe<strong>en</strong><br />
partial systems and an aggregation function. Several primitive distances and<br />
aggregation functions can be used; we mainly focus on the edit distance (which<br />
is, roughly speaking, the number of insertions/<strong>de</strong>letions of attacks nee<strong>de</strong>d to<br />
turn a giv<strong>en</strong> system into another one), and consi<strong>de</strong>r sum, max and leximax as<br />
aggregation functions.<br />
Like the input of the overall merging process, the result of the merging step is<br />
a set of argum<strong>en</strong>tation systems. However, while the first one reflects differ<strong>en</strong>t<br />
points of view (since each system is provi<strong>de</strong>d by a specific ag<strong>en</strong>t), the second<br />
set expresses some uncertainty on the merging due to the pres<strong>en</strong>ce of conflicts.<br />
3 175
The last step of the process consists in <strong>de</strong>fining the acceptable argum<strong>en</strong>ts for<br />
the group un<strong>de</strong>r the uncertainty provi<strong>de</strong>d by this set of argum<strong>en</strong>tation systems.<br />
Once again, several s<strong>en</strong>sible <strong>de</strong>finitions are giv<strong>en</strong>. We show that the sets of<br />
argum<strong>en</strong>ts consi<strong>de</strong>red acceptable wh<strong>en</strong> the input is the set of argum<strong>en</strong>tation<br />
systems primarily furnished by the ag<strong>en</strong>ts may drastically differ from the sets<br />
of argum<strong>en</strong>ts consi<strong>de</strong>red acceptable after the merging step, and by means<br />
of example, we show that the latter ones are more in accordance with the<br />
intuition.<br />
The rest of the paper is organized as follows. After a refresher on Dung’s<br />
theory of argum<strong>en</strong>tation (in which our approach takes place), we give a simple<br />
motivating example (Section 3) which shows that voting on the argum<strong>en</strong>ts<br />
accepted by each ag<strong>en</strong>t is not a<strong>de</strong>quate for <strong>de</strong>fining the argum<strong>en</strong>ts accepted<br />
by the group. Th<strong>en</strong> we introduce a notion of partial argum<strong>en</strong>tation system<br />
(Section 4) which ext<strong>en</strong>ds the notion of argum<strong>en</strong>tation system and <strong>en</strong>ables to<br />
handle the case wh<strong>en</strong> ag<strong>en</strong>ts do not share the same set of argum<strong>en</strong>ts. On this<br />
ground, we <strong>de</strong>fine a family of merging operators for argum<strong>en</strong>tation systems<br />
(Section 5) and we study the properties of some of them (especially, those<br />
based on the edit distance) (Section 6). Th<strong>en</strong>, we focus on acceptability for<br />
partial argum<strong>en</strong>tation systems (Section 7). Finally, we conclu<strong>de</strong> the paper<br />
and give a short pres<strong>en</strong>tation of some possible refinem<strong>en</strong>ts of our framework<br />
(Section 8).<br />
2 Dung’s Theory of Argum<strong>en</strong>tation<br />
Let us pres<strong>en</strong>t some basic <strong>de</strong>finitions at work in Dung’s theory of argum<strong>en</strong>tation<br />
(13). We restrict them to finite argum<strong>en</strong>tation frameworks.<br />
Definition 1 (Argum<strong>en</strong>tation system (AF))<br />
A (finite) argum<strong>en</strong>tation system AF = 〈A, R〉 over A is giv<strong>en</strong> by a finite set<br />
A of argum<strong>en</strong>ts and a binary relation R on A called an attack relation. a i Ra j<br />
means that a i attacks a j (also <strong>de</strong>noted by (a i , a j ) ∈ R).<br />
For our study, we are not interested in the structure of argum<strong>en</strong>ts and we<br />
consi<strong>de</strong>r an arbitrary attack relation.<br />
〈A, R〉 <strong>de</strong>fines a directed graph G called the attack graph.<br />
Example 2 The argum<strong>en</strong>tation system AF = 〈A = {a 1 , a 2 , a 3 , a 4 }, R =<br />
{(a 2 , a 3 ), (a 4 , a 3 ), (a 1 , a 2 )}〉 <strong>de</strong>fines the following graph G:<br />
Acceptability is about the selection of the most acceptable argum<strong>en</strong>ts. Two<br />
mainstream approaches exist:<br />
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AF<br />
a 1 a 2<br />
a 3<br />
a 4<br />
• Individual acceptability: acceptability of an argum<strong>en</strong>t <strong>de</strong>p<strong>en</strong>ds only on its<br />
properties (see (16; 2));<br />
• Collective acceptability: an argum<strong>en</strong>t can be <strong>de</strong>f<strong>en</strong><strong>de</strong>d by other argum<strong>en</strong>ts;<br />
in this case, the acceptability of a set of argum<strong>en</strong>ts is consi<strong>de</strong>red (see (13)).<br />
Dung’s theory is concerned with the second approach. Whether an argum<strong>en</strong>t<br />
can be accepted <strong>de</strong>p<strong>en</strong>ds on the way argum<strong>en</strong>ts interact. Collective acceptability<br />
is based on two key notions: lack of conflict betwe<strong>en</strong> argum<strong>en</strong>ts and<br />
collective <strong>de</strong>f<strong>en</strong>se.<br />
Definition 3 ((13)) Let 〈A, R〉 be an argum<strong>en</strong>tation system.<br />
Conflict-free set A set E ⊆ A is conflict-free if and only if ∄a, b ∈ E such<br />
that aRb.<br />
Collective <strong>de</strong>f<strong>en</strong>se Consi<strong>de</strong>r E ⊆ A, a ∈ A. E (collectively) <strong>de</strong>f<strong>en</strong>ds a if<br />
and only if ∀b ∈ A, if bRa, th<strong>en</strong> ∃c ∈ E such that cRb (a is said acceptable<br />
w.r.t. E). E <strong>de</strong>f<strong>en</strong>ds all its elem<strong>en</strong>ts if and only if ∀a ∈ E, E collectively<br />
<strong>de</strong>f<strong>en</strong>ds a.<br />
Dung <strong>de</strong>fines several semantics for collective acceptability based on those two<br />
notions (13). Among them the admissible semantics, the preferred semantics,<br />
the stable semantics and the groun<strong>de</strong>d semantics.<br />
Definition 4 ((13)) Let 〈A, R〉 be an argum<strong>en</strong>tation system.<br />
Admissible semantics A set E ⊆ A is admissible if and only if E is<br />
conflict-free and E <strong>de</strong>f<strong>en</strong>ds all its elem<strong>en</strong>ts.<br />
Preferred semantics A set E ⊆ A is a preferred ext<strong>en</strong>sion if and only if E<br />
is maximal for set inclusion among the admissible sets.<br />
Stable semantics A set E ⊆ A is a stable ext<strong>en</strong>sion if and only if E is<br />
conflict-free and every a ∈ A \ E is attacked by an elem<strong>en</strong>t of E.<br />
Groun<strong>de</strong>d semantics The groun<strong>de</strong>d ext<strong>en</strong>sion of 〈A, R〉 is the smallest subset<br />
of A with respect to set inclusion among the subsets of A which are<br />
admissible and coinci<strong>de</strong> with the set of argum<strong>en</strong>ts acceptable w.r.t. itself.<br />
Note that in all the above <strong>de</strong>finitions, each attacker of a giv<strong>en</strong> argum<strong>en</strong>t is<br />
consi<strong>de</strong>red in<strong>de</strong>p<strong>en</strong><strong>de</strong>ntly of the other attackers (there is no way to repres<strong>en</strong>t<br />
synergetic effects and the possibility to quantify all attackers as a whole is<br />
not consi<strong>de</strong>red – there exist other works which are concerned with this aspect,<br />
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see (19; 8; 6; 9; 10; 23)).<br />
Definition 5 (Well-foun<strong>de</strong>d argum<strong>en</strong>tation system (13))<br />
An argum<strong>en</strong>tation framework AF = 〈A, R〉 is well-foun<strong>de</strong>d if and only if there<br />
does not exist an infinite sequ<strong>en</strong>ce a 0 , a 1 , . . .,a n . . . of argum<strong>en</strong>ts from A, such<br />
that for each i, a i+1 Ra i .<br />
Among other things, It is shown in (13) that:<br />
• Any admissible set of 〈A, R〉 is inclu<strong>de</strong>d in a preferred ext<strong>en</strong>sion of 〈A, R〉.<br />
• Each 〈A, R〉 has at least one preferred ext<strong>en</strong>sion.<br />
• Each 〈A, R〉 has exactly one groun<strong>de</strong>d ext<strong>en</strong>sion of 〈A, R〉 and this ext<strong>en</strong>sion<br />
is inclu<strong>de</strong>d in each preferred ext<strong>en</strong>sion.<br />
• If 〈A, R〉 is well-foun<strong>de</strong>d th<strong>en</strong> it has a unique preferred ext<strong>en</strong>sion which is<br />
also the only stable ext<strong>en</strong>sion and the groun<strong>de</strong>d ext<strong>en</strong>sion.<br />
• Any stable ext<strong>en</strong>sion of 〈A, R〉 is also a preferred ext<strong>en</strong>sion (the converse is<br />
false).<br />
• Some 〈A, R〉 do not have a stable ext<strong>en</strong>sion.<br />
The acceptability status of each subset of argum<strong>en</strong>ts can now be <strong>de</strong>fined by<br />
the following relation:<br />
Definition 6 (Acceptability relation) An acceptability relation, <strong>de</strong>noted<br />
by Acc AF , for a giv<strong>en</strong> argum<strong>en</strong>tation system AF = 〈A, R〉, is a total function<br />
from 2 A to {true,false} which associates each subset E of A with true if E is<br />
an acceptable set for AF and with false otherwise.<br />
Usually, an acceptability relation is based on a specific semantics (plus a selection<br />
principle). For instance, a set of argum<strong>en</strong>ts can be consi<strong>de</strong>red acceptable<br />
if and only if it is inclu<strong>de</strong>d in one ext<strong>en</strong>sion (credulous selection) or in every<br />
ext<strong>en</strong>sion (skeptical selection). Alternatively, a set of argum<strong>en</strong>ts can be consi<strong>de</strong>red<br />
acceptable if and only if it coinci<strong><strong>de</strong>s</strong> with one ext<strong>en</strong>sion for the chos<strong>en</strong><br />
semantics. Whatever the way it is <strong>de</strong>fined, an acceptability relation can be<br />
viewed as a choice function among the elem<strong>en</strong>ts of 2 A . In this context, the<br />
“acceptability of an argum<strong>en</strong>t” a can correspond either to the acceptability of<br />
the singleton {a}, or to the membership of a to an acceptable set (see (12)).<br />
3 Simple is not so Beautiful<br />
Giv<strong>en</strong> a profile (i.e., a vector) P = 〈AF 1 , . . .,AF n 〉 of n AFs (with n ≥ 1)<br />
where each AF i = 〈A i , R i 〉 repres<strong>en</strong>ts the data giv<strong>en</strong> by Ag<strong>en</strong>t i, our purpose<br />
is to <strong>de</strong>termine the subsets of ⋃ i A i which are acceptable by the group of n<br />
ag<strong>en</strong>ts. Voting is one way to achieve this goal.<br />
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3.1 Voting is not <strong>en</strong>ough<br />
In<strong>de</strong>ed, a simple approach to address the problem consists in consi<strong>de</strong>ring a<br />
set of argum<strong>en</strong>ts acceptable for the group wh<strong>en</strong> it is acceptable for “suffici<strong>en</strong>tly<br />
many” ag<strong>en</strong>ts of the group. The voting method un<strong>de</strong>r consi<strong>de</strong>ration<br />
makes precise what “suffici<strong>en</strong>tly many” means: it can be, for instance, simple<br />
majority. Let us illustrate such an approach on an example:<br />
Example 7 Consi<strong>de</strong>r the three following argum<strong>en</strong>tation systems:<br />
• AF 1 = 〈{a, b, e, f}, {(a, b), (b, a), (e, f)}〉,<br />
• AF 2 = 〈{b, c, d, e, f}, {(b, c), (c, d), (f, e)}〉,<br />
• AF 3 = 〈{e, f}, {(e, f)}〉.<br />
AF 1<br />
AF 2<br />
AF 3<br />
b<br />
b<br />
a<br />
e<br />
c<br />
e<br />
e<br />
f<br />
d<br />
f<br />
f<br />
Whatever the chos<strong>en</strong> semantics (among Dung’s ones), c does not belong to<br />
any ext<strong>en</strong>sion of AF 2 . As c is not known by the two other ag<strong>en</strong>ts, it cannot<br />
be consi<strong>de</strong>red as acceptable by the group whatever the voting method (un<strong>de</strong>r<br />
the reasonable assumption that it is a choice function based on ext<strong>en</strong>sions, i.e.,<br />
only subsets of an ext<strong>en</strong>sion of an AF i are eligible as acceptable sets). However<br />
since c (resp. a) is not among the argum<strong>en</strong>ts reported by the first ag<strong>en</strong>t and<br />
the third one (resp. the second and the third ones), it can be s<strong>en</strong>sible to assume<br />
that the three ag<strong>en</strong>ts agree on the fact that a attacks b, b attacks a and b<br />
attacks c. In<strong>de</strong>ed, this assumption is compatible with any of the three argum<strong>en</strong>tation<br />
systems reported by the ag<strong>en</strong>ts. Un<strong>de</strong>r this assumption, it makes s<strong>en</strong>se<br />
to consi<strong>de</strong>r {c} credulously acceptable for the group giv<strong>en</strong> that c is consi<strong>de</strong>red<br />
<strong>de</strong>f<strong>en</strong><strong>de</strong>d by a against b by Ag<strong>en</strong>t 1 and there is no conflicting evi<strong>de</strong>nce about<br />
it in the AFs provi<strong>de</strong>d by the two other ag<strong>en</strong>ts.<br />
As this example illustrates it, our claim is that, in g<strong>en</strong>eral, voting is not a<br />
satisfying way to aggregate the data furnished by the differ<strong>en</strong>t ag<strong>en</strong>ts un<strong>de</strong>r<br />
the form of argum<strong>en</strong>tation systems. Two problems arise:<br />
Problem 1 Voting makes s<strong>en</strong>se only if all ag<strong>en</strong>ts consi<strong>de</strong>r the same set of<br />
argum<strong>en</strong>ts A at start (otherwise, the set 2 A of alternatives is not common to<br />
all ag<strong>en</strong>ts). However, it can be the case that the sets of argum<strong>en</strong>ts reported<br />
by the ag<strong>en</strong>ts differ from one another.<br />
Problem 2 Voting relies only on the selected ext<strong>en</strong>sions: the attack relations<br />
(from which ext<strong>en</strong>sions are characterized) are not tak<strong>en</strong> into consi<strong>de</strong>ration<br />
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any more once ext<strong>en</strong>sions have be<strong>en</strong> computed. This leads to much significant<br />
information being set asi<strong>de</strong> which could be exploited to <strong>de</strong>fine the sets<br />
of acceptable argum<strong>en</strong>ts at the group level.<br />
3.2 Union is not merging (in g<strong>en</strong>eral)<br />
In or<strong>de</strong>r to solve both problems, a simple approach (at a first glance) consists in<br />
forming the union of the argum<strong>en</strong>tation systems AF 1 , ..., AF n , i.e., consi<strong>de</strong>ring<br />
the argum<strong>en</strong>tation system <strong>de</strong>noted AF = ⋃ n<br />
i=1 〈A i , R i 〉 and <strong>de</strong>fined by AF =<br />
〈 ⋃ n<br />
i=1 A i , ⋃ n<br />
i=1 R i 〉. Unfortunately, such a merging approach to argum<strong>en</strong>tation<br />
systems cannot be tak<strong>en</strong> seriously. Let us illustrate it on our running example:<br />
Example 7 (continued) The resulting AF is ⋃ 3<br />
i=1 AF i = 〈{a, b, c, d, e, f}, {(a,<br />
b), (b, a), (b, c), (c, d), (e, f), (f, e)}〉.<br />
Example 7 shows that the union approach to merging argum<strong>en</strong>tation systems<br />
suffers from a major problem: it solves conflicts by giving to the explicit attack<br />
information some undue promin<strong>en</strong>ce to implicit non-attack information. Thus,<br />
wh<strong>en</strong> a pair of argum<strong>en</strong>ts (like, say (f, e)) does not belong to the attack relation<br />
furnished by an ag<strong>en</strong>t (say, Ag<strong>en</strong>t 1) while both argum<strong>en</strong>ts (f and e) belong to<br />
the set of argum<strong>en</strong>ts she points out, the meaning is that for Ag<strong>en</strong>t 1, argum<strong>en</strong>t<br />
f does not attack argum<strong>en</strong>t e. Imagine now that in the consi<strong>de</strong>red profile of<br />
argum<strong>en</strong>tation systems, 999 ag<strong>en</strong>ts report the same system as Ag<strong>en</strong>t 1, and<br />
the 1000 th ag<strong>en</strong>t is Ag<strong>en</strong>t 2. In the resulting argum<strong>en</strong>tation system consi<strong>de</strong>red<br />
at the group level, assuming that union is used as a merging operator, it will<br />
be the case that f attacks e while 999 ag<strong>en</strong>ts over 1000 believes that it is not<br />
the case!<br />
4 Partial Argum<strong>en</strong>tation Systems<br />
The example introduced in the previous section has illustrated that differ<strong>en</strong>t<br />
cases must be tak<strong>en</strong> into account:<br />
• an argum<strong>en</strong>t exists in the argum<strong>en</strong>tation system AF 1 of one of the ag<strong>en</strong>ts<br />
and does not exist in the argum<strong>en</strong>tation system AF 2 of at least another<br />
ag<strong>en</strong>t;<br />
• an interaction betwe<strong>en</strong> two argum<strong>en</strong>ts exists in the argum<strong>en</strong>tation system<br />
AF 1 of one ag<strong>en</strong>t and does not exist in the argum<strong>en</strong>tation system AF 2 of at<br />
least another ag<strong>en</strong>t.<br />
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In the first case, the new argum<strong>en</strong>t can be ad<strong>de</strong>d to AF 2 but the question<br />
is what to do for the interactions betwe<strong>en</strong> this new argum<strong>en</strong>t and the other<br />
argum<strong>en</strong>ts of AF 2 .<br />
In the second case, things are differ<strong>en</strong>t: if an interaction betwe<strong>en</strong> two argum<strong>en</strong>ts<br />
a and b exists in a system AF 1 and not in another system AF 2 , ev<strong>en</strong><br />
wh<strong>en</strong> a and b are in AF 2 , we cannot add the interaction in AF 2 (that Ag<strong>en</strong>t<br />
2 did not inclu<strong>de</strong> this attack in AF 2 is on purpose). In<strong>de</strong>ed, if an interaction<br />
is not pres<strong>en</strong>t in an AF, it means that this interaction does not exist for the<br />
corresponding ag<strong>en</strong>t. The consequ<strong>en</strong>ce of this is the necessity to discriminate<br />
among several cases wh<strong>en</strong>ever an argum<strong>en</strong>t a has to be ad<strong>de</strong>d to an AF. Let b<br />
be an argum<strong>en</strong>t of the AF un<strong>de</strong>r consi<strong>de</strong>ration, three cases must be consi<strong>de</strong>red:<br />
• the ag<strong>en</strong>t believes that the interaction (a, b) exists (attack);<br />
• the ag<strong>en</strong>t believes that the interaction (a, b) does not exist (non-attack);<br />
• the ag<strong>en</strong>t does not know whether the interaction (a, b) exists (ignorance).<br />
The first two cases express the fact that the knowledge of the ag<strong>en</strong>t is suffici<strong>en</strong>t<br />
for computing the new interaction concerning a. The third case expresses that<br />
the ag<strong>en</strong>t is not able to compute the new interaction concerning a and the<br />
argum<strong>en</strong>ts she pointed out (several reasons can explain it, especially a lack of<br />
information, or a lack of computational resources).<br />
Handling these differ<strong>en</strong>t kinds of information within a uniform setting calls<br />
for an ext<strong>en</strong>sion 1 of the notion of argum<strong>en</strong>tation systems, that we call partial<br />
argum<strong>en</strong>tation systems.<br />
Definition 8 (Partial argum<strong>en</strong>tation system (PAF)) A (finite) partial<br />
argum<strong>en</strong>tation system over A is a quadruple PAF = 〈A, R, I, N〉 where<br />
• A is a finite set of argum<strong>en</strong>ts,<br />
• R, I, N are binary relations on A:<br />
· R is the attack relation,<br />
· I is called the ignorance relation and is such that R ∩ I = ∅,<br />
· and N = (A × A) \ (R ∪ I) is called the non-attack relation.<br />
N is <strong>de</strong>duced from A, R and I, so a partial argum<strong>en</strong>tation system can be fully<br />
specified by 〈A, R, I〉. We use both notations in the following.<br />
Each AF is a particular PAF for which the set I is empty (we say that such<br />
an AF is equival<strong>en</strong>t to the associated PAF). In an AF, the N relation also<br />
1 In (11), a new binary relation on the argum<strong>en</strong>ts is also introduced in Dung’s<br />
argum<strong>en</strong>tation framework : however, this new relation repres<strong>en</strong>ts a notion of support<br />
betwe<strong>en</strong> argum<strong>en</strong>ts. Clearly <strong>en</strong>ough, this is unrelated with the relation introduced<br />
here repres<strong>en</strong>ting the ignorance about the attack betwe<strong>en</strong> argum<strong>en</strong>ts.<br />
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exists ev<strong>en</strong> if it is not giv<strong>en</strong> explicitly (I = ∅ and N = A × A \ R). So, an AF<br />
could also be <strong>de</strong>noted by 〈A, R, N〉.<br />
Each PAF over A can be viewed as a compact repres<strong>en</strong>tation of a set of AFs<br />
over A, called its completions:<br />
Definition 9 (Completion of a PAF) Let PAF = 〈A, R, I〉. Let AF = 〈A,<br />
S〉. AF is a completion of PAF if and only if R ⊆ S ⊆ R ∪ I.<br />
The set of all completions of PAF is <strong>de</strong>noted C(PAF).<br />
Example 10 The partial argum<strong>en</strong>tation system PAF = 〈A = {a, b, c, d}, R =<br />
{(a, b), (a, c)}, I = {(c, a), (b, d)}, N = {(a, a), (b, b), (c, c), (d, d), (b, a), (b, c),<br />
(c, b), (a, d), (d, a), (d, b), (c, d), (d, c)}〉 is illustrated on the following figure (solid<br />
arrows repres<strong>en</strong>t the attack relation and dotted arrows repres<strong>en</strong>t the ignorance<br />
relation; non-attack relations are not repres<strong>en</strong>ted explicitly as in the AF case):<br />
b<br />
PAF<br />
d<br />
a<br />
c<br />
The completions of this PAF are:<br />
b<br />
d<br />
b<br />
d<br />
b<br />
d<br />
b<br />
d<br />
a<br />
c<br />
a<br />
c<br />
a<br />
c<br />
a<br />
c<br />
Now, Problem 1 can be addressed by first associating each argum<strong>en</strong>tation<br />
system AF i with a corresponding PAF i so that all PAF i are about the same set<br />
of argum<strong>en</strong>ts ⋃ n<br />
i=1 A i . To this <strong>en</strong>d, we introduce the notion of expansion of an<br />
AF:<br />
Definition 11 (Expansion of an AF) Let P = 〈AF 1 , . . .,AF n 〉 be a profile<br />
of n AFs such that AF i = 〈A i , R i , N i 〉. Let AF = 〈A, R〉 be an argum<strong>en</strong>tation<br />
system. An expansion of AF giv<strong>en</strong> P is any PAF exp(AF, P) <strong>de</strong>fined by 〈A∪<br />
⋃<br />
i A i , R ′ , I ′ , N ′ 〉 such that R ⊆ R ′ and (A × A) \ R ⊆ N ′ . exp is referred to<br />
as an expansion function.<br />
In or<strong>de</strong>r to be g<strong>en</strong>eral <strong>en</strong>ough, this <strong>de</strong>finition does not impose many constraints<br />
on the resulting PAF: what is important is to preserve the attack and nonattack<br />
relations from the initial AF while ext<strong>en</strong>ding its set of argum<strong>en</strong>ts. Many<br />
policies can be used to give rise to expansions of differ<strong>en</strong>t kinds, reflecting the<br />
various attitu<strong><strong>de</strong>s</strong> of ag<strong>en</strong>ts in light of “new” argum<strong>en</strong>ts; for instance, if a is<br />
any argum<strong>en</strong>t consi<strong>de</strong>red by Ag<strong>en</strong>t i at the start and a “new” argum<strong>en</strong>t b has<br />
to be incorporated, Ag<strong>en</strong>t i can (among other things):<br />
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• always reject b (e.g., adding (b, b) to her relation R ′ i ),<br />
• always accept b (adding (a, b), (b, a) and (b, b) to her non-attack relation<br />
N ′ i),<br />
• just express her ignorance about b (adding (a, b), (b, a) and (b, b) to her<br />
ignorance relation I ′ i).<br />
Each ag<strong>en</strong>t may also compute the exact interaction betwe<strong>en</strong> a and b wh<strong>en</strong> the<br />
attack relation is not primitive but <strong>de</strong>fined from more basic notions (as in the<br />
approach by Elvang-Gøransson et al., see e.g., (15; 16; 17)). Note that if she<br />
has limited computational resources, Ag<strong>en</strong>t i can compute exact interactions<br />
as far as she can, th<strong>en</strong> express ignorance for the remaining ones.<br />
In the following, we specifically focus on cons<strong>en</strong>sual expansions. Intuitively,<br />
the cons<strong>en</strong>sual expansion of an argum<strong>en</strong>tation system AF = 〈A, R〉 giv<strong>en</strong> a<br />
profile of such systems is obtained by adding a pair of argum<strong>en</strong>ts (a, b) (where<br />
at least one of a, b is not in A) into the attack (resp. the non-attack relation)<br />
provi<strong>de</strong>d that all other ag<strong>en</strong>ts of the profile who know the two argum<strong>en</strong>ts agree<br />
on the exist<strong>en</strong>ce of the attack 2 (resp. the non-attack); otherwise, it is ad<strong>de</strong>d<br />
to the ignorance relation.<br />
This expansion policy is s<strong>en</strong>sible as soon as each ag<strong>en</strong>t has a minimum level<br />
of confi<strong>de</strong>nce in the other ag<strong>en</strong>ts: if a piece of information conveyed by one<br />
ag<strong>en</strong>t is not conflicting with the information stemming from the other ag<strong>en</strong>ts,<br />
every ag<strong>en</strong>t of the group is ready to accept it.<br />
Definition 12 (Cons<strong>en</strong>sual expansion) Let P = 〈AF 1 , . . .,AF n 〉 be a profile<br />
of n AFs such that AF i = 〈A i , R i 〉. Let AF = 〈A, R, N〉 be an argum<strong>en</strong>tation<br />
system. Let conf(P) = ( ⋃ i R i ) ∩ ( ⋃ i N i ) be the set of interactions for which<br />
a conflict exists within the profile. The cons<strong>en</strong>sual expansion of AF over P is<br />
the tuple <strong>de</strong>noted by exp C = 〈A ′ , R ′ , I ′ , N ′ 〉 with:<br />
• A ′ = A ∪ ⋃ i A i ,<br />
• R ′ = R ∪ (( ⋃ i R i \ conf(P)) \ N),<br />
• I ′ = conf(P) \ (R ∪ N),<br />
• N ′ = (A ′ × A ′ ) \ (R ′ ∪ I ′ ).<br />
The next proposition states that, as expected, the cons<strong>en</strong>sual expansion of an<br />
argum<strong>en</strong>tation system over a profile is an expansion:<br />
Proposition 13 Let P = 〈AF 1 , . . .,AF n 〉 be a profile of n AFs such that<br />
AF i = 〈A i , R i 〉. Let AF = 〈A, R, N〉 be an argum<strong>en</strong>tation system. The cons<strong>en</strong>sual<br />
expansion exp C<br />
of AF over P is an expansion of AF over P in the s<strong>en</strong>se<br />
of Definition 11.<br />
2 i.e., if a,b ∈ A i , th<strong>en</strong> (a,b) ∈ R i .<br />
11 183
Proof :<br />
Consi<strong>de</strong>r (a, b) ∈ A ′ × A ′ . There are several cases:<br />
• if (a, b) ∈ R th<strong>en</strong> (a, b) ∈ R ′ and (a, b) ∉ I ′ ∪ N ′ (so, R ⊆ R ′ );<br />
• if (a, b) ∉ R and (a, b) ∈ N th<strong>en</strong> (a, b) ∉ I ′ ∪R ′ and (a, b) ∈ N ′ (so, N ⊆ N ′ );<br />
• if (a, b) ∉ R ∪ N th<strong>en</strong> there are two cases:<br />
· if ∄AF i ∈ P such that a, b ∈ A i th<strong>en</strong> (a, b) ∉ conf(P); so, (a, b) ∈ N ′ and<br />
(a, b) ∉ R ′ ∪ I ′ ;<br />
· if ∃AF i ∈ P such that a, b ∈ A i th<strong>en</strong> we have 4 possible cases:<br />
if (a, b) ∈ R i and ∄AF j ∈ P such that (a, b) ∈ N j th<strong>en</strong> (a, b) ∉<br />
conf(P); so, (a, b) ∈ R ′ and (a, b) ∉ N ′ ∪ I ′ ;<br />
if (a, b) ∈ R i and ∃AF j ∈ P such that (a, b) ∈ N j th<strong>en</strong> (a, b) ∈<br />
conf(P); so, (a, b) ∈ I ′ and (a, b) ∉ R ′ ∪ N ′ ;<br />
if (a, b) ∈ N i and ∄AF j ∈ P such that (a, b) ∈ R j th<strong>en</strong> (a, b) ∉<br />
conf(P); so, (a, b) ∈ N ′ and (a, b) ∉ R ′ ∪ I ′ ;<br />
if (a, b) ∈ N i and ∃AF j ∈ P such that (a, b) ∈ R j th<strong>en</strong> (a, b) ∈<br />
conf(P); so, (a, b) ∈ I ′ and (a, b) ∉ R ′ ∪ N ′ .<br />
So, R ′ , I ′ and N ′ form a partition of A ′ × A ′ which satisfies R ⊆ R ′ and<br />
N ⊆ N ′ .<br />
✷<br />
The cons<strong>en</strong>sual expansion is among the most cautious expansions one can<br />
<strong>de</strong>fine since it leads to adding a pair of argum<strong>en</strong>ts in the attack (or the nonattack<br />
relation) associated with an ag<strong>en</strong>t only wh<strong>en</strong> all the other ag<strong>en</strong>ts agree<br />
on it.<br />
Example 14 Consi<strong>de</strong>r the profile consisting of the following four argum<strong>en</strong>tation<br />
systems:<br />
• AF 1 = 〈A 1 = {a, b}, R 1 = {(a, b), (b, a)}〉,<br />
• AF 2 = 〈A 2 = {b, c, d}, R 2 = {(b, c), (c, d)}〉,<br />
• AF 3 = 〈A 3 = {a, b, d}, R 3 = {(a, b), (a, d)}〉,<br />
• AF 4 = 〈A 4 = {a, b, d}, R 4 = {(b, d), (b, a)}〉.<br />
AF 1<br />
AF 2<br />
AF 3<br />
AF 4<br />
b<br />
b<br />
b<br />
b<br />
a<br />
c<br />
a<br />
a<br />
d<br />
d<br />
d<br />
For each i, the cons<strong>en</strong>sual expansion PAF i of AF i is giv<strong>en</strong> by:<br />
• PAF 1 = 〈{a, b, c, d}, {(a, b), (b, a), (b, c), (c, d)}, {(a, d), (b, d)}〉,<br />
• PAF 2 = 〈{a, b, c, d}, {(b, c), (c, d)}, {(a, b), (b, a), (a, d)}〉,<br />
• PAF 3 = 〈{a, b, c, d}, {(a, b), (a, d), (b, c), (c, d), {}}〉,<br />
• PAF 4 = 〈{a, b, c, d}, {(b, d), (b, a), (b, c), (c, d), {}}〉.<br />
12 184
PAF 1<br />
PAF 2<br />
PAF 3<br />
PAF 4<br />
b<br />
b<br />
b<br />
b<br />
a<br />
c<br />
a<br />
c<br />
a<br />
c<br />
a<br />
c<br />
d<br />
d<br />
d<br />
d<br />
Wh<strong>en</strong> the expansion policies consi<strong>de</strong>red by each ag<strong>en</strong>t are the same one exp,<br />
for any profile P = 〈AF 1 , . . .,AF n 〉 we shall oft<strong>en</strong> note exp(P) the profile of<br />
PAFs 〈exp(AF 1 , P), . . .,exp(AF n , P)〉.<br />
5 Merging Operators<br />
In or<strong>de</strong>r to <strong>de</strong>al with Problem 2, we propose to merge interactions instead of<br />
sets of acceptable argum<strong>en</strong>ts. The goal is to characterize the argum<strong>en</strong>tation<br />
systems which are as close as possible to the giv<strong>en</strong> profile of argum<strong>en</strong>tation<br />
systems, tak<strong>en</strong> as a whole.<br />
A way to achieve this consists in <strong>de</strong>fining a notion of “distance” betwe<strong>en</strong> an<br />
AF and a profile of AFs, or more g<strong>en</strong>erally betwe<strong>en</strong> a PAF and a profile of<br />
PAFs. This calls for a notion of pseudo-distance betwe<strong>en</strong> two PAFs, and a<br />
way to combine such pseudo-distances:<br />
Definition 15 (Pseudo-distance) A pseudo-distance d betwe<strong>en</strong> PAFs over<br />
A is a mapping which associates a non-negative real number to each pair of<br />
PAFs over A and satisfies the properties of symmetry (d(x, y) = d(y, x)) and<br />
minimality (d(x, y) = 0 if and only if x = y).<br />
d is a distance if it satisfies also the triangular inequality (d(x, z) ≤ d(x, y) +<br />
d(y, z)).<br />
Definition 16 (Aggregation function) An aggregation function is a mapping<br />
⊗ from (R+) n to (R+) (strictly speaking, it is a family of mappings, one<br />
for each n), that satisfies<br />
• if x i ≥ x ′ i , th<strong>en</strong> ⊗(x 1, . . .,x i , . . .,x n ) ≥ ⊗(x 1 , . . .,x ′ i , . . .,x n)<br />
(non-<strong>de</strong>creasingness)<br />
• ⊗(x 1 , . . .,x n ) = 0 if ∀i, x i = 0<br />
(minimality)<br />
• ⊗(x) = x<br />
(i<strong>de</strong>ntity)<br />
The merging of a profile of AFs is <strong>de</strong>fined as a set of AFs:<br />
13 185
Definition 17 (Merging of n AFs) Let P = 〈AF 1 , ..., AF n 〉 be a profile<br />
of n AFs. Let d be any pseudo-distance betwe<strong>en</strong> PAFs, let ⊗ be an aggregation<br />
function, and let exp 1 , . . .,exp n be n expansion functions. The merging of P<br />
is the set of AFs<br />
∆ ⊗ d (〈AF 1, . . .,AF n 〉, 〈exp 1<br />
, . . .,exp n<br />
〉) =<br />
{AF over ⋃ i<br />
A i | AF minimizes ⊗ n i=1 d(AF,exp i (AF i , P))}.<br />
In or<strong>de</strong>r to avoid heavy notations, we shall sometimes i<strong>de</strong>ntify the resulting<br />
set of AFs {AF ′ 1 , . . .,AF′ k } with the profile 〈AF′ 1 , . . .,AF′ k 〉 (or any other permutation<br />
of it).<br />
Thus, merging a profile of AFs P = 〈AF 1 , . . .,AF n 〉 is a two-step process:<br />
expansion: An expansion of each AF i over P is first computed. Note that<br />
consi<strong>de</strong>ring expansion functions specific to each ag<strong>en</strong>t is possible. What is<br />
important is that exp i<br />
(AF i , P) is a PAF over A = ⋃ i A i .<br />
fusion: The AFs over A that are selected as the result of the merging process<br />
are the ones that best repres<strong>en</strong>t P (i.e., that are the “closest” to P w.r.t.<br />
the aggregated distances).<br />
In the following, we assume that each ag<strong>en</strong>t uses cons<strong>en</strong>sual expansion. In<br />
or<strong>de</strong>r to light<strong>en</strong> the notations, we remove 〈exp 1 , . . .,exp n 〉 from the list of<br />
parameters of merging operators.<br />
Note that it would be possible to refine Definition 17 so as to inclu<strong>de</strong> integrity<br />
constraints into the picture. This can be useful if there exists some<br />
(unquestionable) knowledge about the expected result (some attacks betwe<strong>en</strong><br />
argum<strong>en</strong>ts which have to hold for the group). It is th<strong>en</strong> <strong>en</strong>ough to look only<br />
to the AFs which satisfy the constraints, similarly to what is done in propositional<br />
belief base merging (see e.g., (20)). In contrast to the belief base merging<br />
sc<strong>en</strong>ario, constraints on the structure of the candidate AFs can also be set. In<br />
particular, consi<strong>de</strong>ring only acyclic AFs can prove valuable since (1) such AFs<br />
are well-foun<strong>de</strong>d, (which implies that only one ext<strong>en</strong>sion has to be consi<strong>de</strong>red<br />
whatever the un<strong>de</strong>rlying semantics – among Dung’s ones), and (2) this ext<strong>en</strong>sion<br />
(which turns out to be the groun<strong>de</strong>d one, see (13)) can be computed in<br />
time polynomial in the size of the AF (while computing a single ext<strong>en</strong>sion is<br />
intractable for the other semantics in the g<strong>en</strong>eral case – un<strong>de</strong>r the standard<br />
assumptions of complexity theory – see (14)).<br />
Now, many pseudo-distances betwe<strong>en</strong> PAFs and many aggregation functions<br />
can be used, giving rise to many merging operators. Usual aggregation functions<br />
inclu<strong>de</strong> the sum Σ, the max Max and the leximax Leximax 3 but using<br />
3 Wh<strong>en</strong> applied to a vector of n real numbers, the leximax function Leximax gives<br />
14 186
non-symmetric functions is also possible (this may be particularly valuable<br />
if some ag<strong>en</strong>ts are more important than others). Some aggregation functions<br />
(like the sum) <strong>en</strong>able the merging process to take into account the number of<br />
ag<strong>en</strong>ts believing that an argum<strong>en</strong>t attacks or not another argum<strong>en</strong>t:<br />
Example 7 (continued) Two ag<strong>en</strong>ts over three agree with the fact that e<br />
attacks f and f does not attack e. It may prove s<strong>en</strong>sible that the group agrees<br />
with the majority.<br />
The choice of the aggregation function is very important for tuning the operator<br />
behaviour with the expected one. For example, sum is a possible choice<br />
in or<strong>de</strong>r to solve conflicts using majority. Otherwise, the leximax function can<br />
prove more valuable if the aim is to behave in a more cons<strong>en</strong>sual way, trying to<br />
<strong>de</strong>fine a result close to the AF of each ag<strong>en</strong>t of the group. The distinction betwe<strong>en</strong><br />
majority and arbitration operators as consi<strong>de</strong>red in propositional belief<br />
base merging (20) also applies here.<br />
In the following, we focus on the edit distance betwe<strong>en</strong> PAFs:<br />
Definition 18 (Edit distance) Let PAF 1 = 〈A, R 1 , I 1 , N 1 〉 and PAF 2 = 〈A,<br />
R 2 , I 2 , N 2 〉 be two PAFs over A.<br />
• Let a, b be two argum<strong>en</strong>ts ∈ A. The edit distance betwe<strong>en</strong> PAF 1 and PAF 2<br />
over a, b is the mapping <strong>de</strong> a,b such that:<br />
· <strong>de</strong> a,b (PAF1, PAF2) = 0 if and only if (a, b) ∈ R 1 ∩R 2 or I 1 ∩I 2 or N 1 ∩N 2 ,<br />
· <strong>de</strong> a,b (PAF1, PAF2) = 1 if and only if (a, b) ∈ R 1 ∩ N 2 or N 1 ∩ R 2 ,<br />
· <strong>de</strong> a,b (PAF1, PAF2) = 0.5 otherwise.<br />
• The edit distance betwe<strong>en</strong> PAF 1 and PAF 2 is giv<strong>en</strong> by<br />
<strong>de</strong>(PAF1, PAF2) = Σ (a,b)∈A×A <strong>de</strong> a,b (PAF1, PAF2).<br />
The edit distance betwe<strong>en</strong> two PAFs is the (minimum) number of additions/<strong>de</strong>letions<br />
which must be ma<strong>de</strong> to r<strong>en</strong><strong>de</strong>r them i<strong>de</strong>ntical. Ignorance is<br />
treated as halfway betwe<strong>en</strong> attack and non-attack.<br />
It is easy to show that:<br />
Proposition 19 The edit distance <strong>de</strong> betwe<strong>en</strong> PAFs is a distance.<br />
Proof : We show that <strong>de</strong> and <strong>de</strong> a,b , ∀(a, b) ∈ A × A are distances, i.e. they<br />
are (1) symmetric, they satisfy (2) the minimality requirem<strong>en</strong>t and (3) the<br />
triangular inequality:<br />
(1) Obvious.<br />
the list of those numbers sorted in a <strong>de</strong>creasing way. Such lists are compared w.r.t.<br />
the lexicographic or<strong>de</strong>ring induced by the standard or<strong>de</strong>ring on real numbers.<br />
15 187
(2) (⇒) Consi<strong>de</strong>r PAF 1 = 〈A, R 1 , I 1 , N 1 〉 and PAF 2 = 〈A, R 2 , I 2 , N 2 〉 such<br />
that PAF 1 = PAF 2 . For all (a, b) ∈ A × A, if PAF 1 = PAF 2 th<strong>en</strong> (a, b) ∈<br />
R 1 ∩ R 2 or (a, b) ∈ I 1 ∩ I 2 or (a, b) ∈ N 1 ∩ N 2 . So, ∀(a, b) ∈ A × A,<br />
<strong>de</strong> a,b (PAF 1 , PAF 2 ) = 0, and <strong>de</strong>(PAF 1 , PAF 2 ) = 0.<br />
(⇐) Suppose <strong>de</strong>(PAF 1 , PAF 2 ) = 0 and make a reductio ad absurdum: if<br />
PAF 1 ≠ PAF 2 th<strong>en</strong> ∃(a, b) ∈ A×A such that (a, b) ∉ R 1 ∩R 2 , (a, b) ∉ I 1 ∩I 2<br />
and (a, b) ∉ N 1 ∩ N 2 ; so, <strong>de</strong> a,b (PAF 1 , PAF 2 ) ≠ 0; so, <strong>de</strong>(PAF 1 , PAF 2 ) ≠ 0<br />
which is a contradiction with the hypothesis; so, PAF 1 = PAF 2 . The same<br />
reasoning can be achieved with <strong>de</strong> a,b (PAF 1 , PAF 2 ) = 0 and the same result<br />
is obtained: PAF 1 = PAF 2 .<br />
(3) Consi<strong>de</strong>r PAF 1 = 〈A, R 1 , I 1 , N 1 〉, PAF 2 = 〈A, R 2 , I 2 , N 2 〉 and PAF 3 =<br />
〈A, R 3 , I 3 , N 3 〉. ∀(a, b) ∈ A×A, we compute and compare <strong>de</strong> a,b (PAF 1 , PAF 2 ),<br />
<strong>de</strong> a,b (PAF 1 , PAF 3 ) and <strong>de</strong> a,b (PAF 3 , PAF 2 ), respectively <strong>de</strong>noted by x, y, z.<br />
We have three possible cases:<br />
• x = 0: ∀y, z, we have x ≤ y + z;<br />
• x = 0.5: x ≤ y + z is false if and only if y = z = 0; however, y =<br />
z = 0 implies that (a, b) ∈ R 1 ∩ R 2 ∩ R 3 or (a, b) ∈ I 1 ∩ I 2 ∩ I 3 or<br />
(a, b) ∈ N 1 ∩ N 2 ∩ N 3 which also implies x = 0 (contradiction with the<br />
hypothesis); so, x ≤ y + z;<br />
• x = 1: we have (a, b) ∈ R 1 ∩ N 2 or (a, b) ∈ N 1 ∩ R 2 ; suppose that<br />
(a, b) ∈ R 1 ∩ N 2 th<strong>en</strong> there are 3 possible cases:<br />
· (a, b) ∈ R 3 : so, y = 0, z = 1 and we have x ≤ y + z;<br />
· (a, b) ∈ I 3 : so, y = 0.5, z = 0.5 and we have x ≤ y + z;<br />
· (a, b) ∈ N 3 : so, y = 1, z = 0 and we have x ≤ y + z.<br />
The same reasoning can be achieved if (a, b) ∈ N 1 ∩ R 2 . So, ∀(a, b) ∈<br />
A × A: <strong>de</strong> a,b (PAF 1 , PAF 2 ) ≤ <strong>de</strong> a,b (PAF 1 , PAF 3 ) + <strong>de</strong> a,b (PAF 3 , PAF 2 );<br />
summing over all (a, b) ∈ A × A, we get:<br />
<strong>de</strong>(PAF 1 , PAF 2 ) ≤ <strong>de</strong>(PAF 1 , PAF 3 ) + <strong>de</strong>(PAF 3 , PAF 2 ).<br />
✷<br />
Let us now illustrate the notion of edit distance as well some associated merging<br />
operators on Example 14.<br />
Example 14 (continued) We consi<strong>de</strong>r the following argum<strong>en</strong>tation system<br />
AF ′ 1 = 〈{a, b, c, d}, {(a, b), (b, a), (b, c), (c, d)}〉.<br />
The edit distance betwe<strong>en</strong> AF ′ 1 and each of the PAFs PAF 1, PAF 2 , PAF 3 , PAF 4<br />
obtained by cons<strong>en</strong>sual expansion from the profile 〈AF 1 , AF 2 , AF 3 , AF 4 〉 is:<br />
• <strong>de</strong>(AF ′ 1, PAF 1 ) = 1,<br />
• <strong>de</strong>(AF ′ 1 , PAF 2) = 1.5,<br />
• <strong>de</strong>(AF ′ 1, PAF 3 ) = 2,<br />
• <strong>de</strong>(AF ′ 1 , PAF 4) = 2.<br />
Taking the sum as the aggregation function, we obtain:<br />
Σ 4 i=1<strong>de</strong>(AF ′ 1, PAF i ) = 6.5.<br />
16 188
Taking the max, we obtain: Max 4 i=1<strong>de</strong>(AF ′ 1, PAF i ) = 2.<br />
Taking the leximax, we obtain: Leximax 4 i=1<strong>de</strong>(AF ′ 1, PAF i ) = (2, 2, 1.5, 1).<br />
By computing such distances for all candidate AFs (i.e., all AFs over {a, b, c, d}),<br />
we can compute the result of the merging:<br />
∆ Σ <strong>de</strong> (〈AF 1, . . ., AF 4 〉) is the set containing the two following AFs:<br />
• AF ′ 1 = 〈{a, b, c, d}, {(a, b), (b, a), (b, c), (c, d)}〉,<br />
• AF ′ 2 = 〈{a, b, c, d}, {(a, b), (b, a), (b, c), (a, d), (c, d)}〉.<br />
AF ′ 1<br />
b<br />
AF ′ 2<br />
b<br />
a<br />
c<br />
a<br />
c<br />
d<br />
d<br />
∆ Max<br />
<strong>de</strong> (〈AF 1 , . . .,AF 4 〉) is the set containing AF ′ 1 and AF ′′<br />
2 = 〈{a, b, c, d}, {(b, a),<br />
(b, c), (a, d), (c, d)}〉.<br />
AF ′ 1<br />
b<br />
AF ′′<br />
2<br />
b<br />
a<br />
c<br />
a<br />
c<br />
d<br />
d<br />
∆ Leximax<br />
<strong>de</strong> (〈AF 1 , . . .,AF 4 〉) is the singleton containing AF ′ 1.<br />
AF ′ 1<br />
b<br />
a<br />
c<br />
d<br />
The discrepancies betwe<strong>en</strong> the merging obtained with the various aggregation<br />
operators can be explained in the following way:<br />
• AF ′ 1 is the most cons<strong>en</strong>sual AF obtained as it is almost equidistant from<br />
each PAF;<br />
• AF ′ 2 is much closer to PAF 1, PAF 2 and PAF 3 than to PAF 4 , thus it is selected<br />
with the sum as an aggregation operator but it is too far from PAF 4 for being<br />
selected with the Max or Leximax operators.<br />
17 189
• AF ′′<br />
2 is nearly equidistant from all four PAFs of the profile but less cons<strong>en</strong>sual<br />
than AF ′ 1 , thus it is selected neither with Σ nor with Leximax but only with<br />
Max as it is not far from any of the giv<strong>en</strong> PAFs.<br />
Having AF ′ 1 in all mergings - whatever the aggregation function chos<strong>en</strong> - seems<br />
very intuitive. In<strong>de</strong>ed, wh<strong>en</strong>ever an attack (or a non-attack) is pres<strong>en</strong>t in the<br />
(weak) majority of the initial AFs, it is also in AF ′ 1. This is not the case for<br />
the two others AFs belonging to the above mergings.<br />
Here is another simple example:<br />
Example 20 Consi<strong>de</strong>r the two following argum<strong>en</strong>tation systems:<br />
• AF 1 = 〈{a, b, c, e}, {(b, a), (c, b), (c, e)}〉<br />
• AF 2 = 〈{a, d, e, c}, {(d, a), (e, d), (e, c)}〉<br />
AF 1<br />
b<br />
c<br />
AF 2<br />
c<br />
a<br />
a<br />
e<br />
d<br />
e<br />
Note that the attack from c to e is known by Ag<strong>en</strong>t 1 but not by Ag<strong>en</strong>t 2 and<br />
the attack from e to c is known by Ag<strong>en</strong>t 2 but not by Ag<strong>en</strong>t 1. This illustrates<br />
the fact that the ag<strong>en</strong>ts do not share the same attack relation.<br />
AF 1 has a unique preferred ext<strong>en</strong>sion: {c, a}. AF 2 has a unique preferred ext<strong>en</strong>sion:<br />
{e, a}.<br />
The cons<strong>en</strong>sual expansions of AF 1 and AF 2 are respectively:<br />
• PAF 1 = 〈{a, b, c, d, e}, {(b, a), (c, b), (c, e), (d, a), (e, d)}, ∅〉,<br />
• PAF 2 = 〈{a, b, c, d, e}, {(d, a), (e, d), (e, c), (b, a), (c, b)}, ∅〉.<br />
The result of merging the profile 〈AF 1 , AF 2 〉 with <strong>de</strong> and ⊗ = Max (or ⊗ =<br />
Leximax) is:<br />
(〈AF 1 , AF 2 〉) = ∆ Leximax (〈AF 1 , AF 2 〉) = {AF ′ 1 , AF′ 2 } with<br />
∆ Max<br />
<strong>de</strong><br />
• AF ′ 1 = 〈{a, b, c, d, e}, {(b, a), (c, b), (c, e), (d, a), (e, d), (e, c)}〉,<br />
• AF ′ 2 = 〈{a, b, c, d, e}, {(b, a), (c, b), (d, a), (e, d)}〉.<br />
<strong>de</strong><br />
Using the sum as an aggregation function, two additional AFs are g<strong>en</strong>erated:<br />
∆ Σ <strong>de</strong> (〈AF 1, AF 2 〉) = {AF ′ 1 , AF′ 2 , AF′ 3 , AF′ 4 }, with<br />
• AF ′ 3 = 〈{a, b, c, d, e}, {(b, a), (c, b), (c, e), (e, d), (d, a)}〉,<br />
• AF ′ 4 = 〈{a, b, c, d, e}, {(b, a), (c, b), (e, c), (e, d), (d, a)}〉.<br />
18 190
AF ′ 1<br />
b<br />
c<br />
AF ′ 2<br />
b<br />
c<br />
a<br />
a<br />
d<br />
e<br />
d<br />
e<br />
AF ′ 3<br />
b<br />
c<br />
AF ′ 4<br />
b<br />
c<br />
a<br />
a<br />
d<br />
e<br />
d<br />
e<br />
Each of the resulting mergings contains an argum<strong>en</strong>tation system from which<br />
argum<strong>en</strong>t a can be <strong>de</strong>rived, as it is the case in AF 1 and AF 2 . Using the sum<br />
as an aggregation function leads to the most cons<strong>en</strong>sual result here since it<br />
preserves the initial AFs of the differ<strong>en</strong>t ag<strong>en</strong>ts. In<strong>de</strong>ed, AF ′ 3 is equival<strong>en</strong>t to<br />
PAF 1 and AF ′ 4 is equival<strong>en</strong>t to PAF 2.<br />
6 Some Properties<br />
Let us now pres<strong>en</strong>t some properties of cons<strong>en</strong>sual expansions and merging<br />
operators based on the edit distance, showing them as interesting choices.<br />
6.1 Properties of PAFs and cons<strong>en</strong>sual expansions<br />
Intuitively speaking, a natural requirem<strong>en</strong>t on any AF resulting from a merging<br />
is that it preserves all the information which are shared by the ag<strong>en</strong>ts<br />
participating in the merging process, and more g<strong>en</strong>erally, all the information<br />
on which the ag<strong>en</strong>ts participating in the merging process do not disagree.<br />
In or<strong>de</strong>r to show that our merging operators satisfy those requirem<strong>en</strong>ts, one<br />
first need the notions of clash-free part and of common part of a profile of<br />
PAFs:<br />
Definition 21 (Clash-free part of a profile of PAFs)<br />
Let P = 〈PAF 1 , . . .,PAF n 〉 be a profile of PAFs. The clash-free part of P is<br />
<strong>de</strong>noted by CFP(P) and is <strong>de</strong>fined by:<br />
CFP(P) = 〈 ⋃ i<br />
A i , ⋃ i<br />
R i \ ⋃ i<br />
N i , I CFP , ⋃ i<br />
N i \ ⋃ i<br />
R i 〉<br />
where I CFP = ( ⋃ i A i × ⋃ i A i ) \ (( ⋃ i R i \ ⋃ i N i ) ∪ ( ⋃ i N i \ ⋃ i R i )).<br />
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The clash-free part of a profile of PAFs repres<strong>en</strong>ts the pieces of information<br />
(attack / non-attack) that are not questioned by any other ag<strong>en</strong>t. As they are<br />
not the source of any disagreem<strong>en</strong>t, they are expected to be inclu<strong>de</strong>d in each<br />
AF resulting from the merging process.<br />
Example 14 (continued) With P = 〈AF 1 , AF 2 , AF 3 , AF 4 〉, CFP(P) = 〈{a, b,<br />
c, d}, {(b, c), (c, d)}, {(a, b), (b, a), (a, d), (b, d), (a, c), (c, a)}〉.<br />
Note that withexp C (P) = 〈exp C (AF 1 , P), . . .,exp C (AF 4 , P)〉, CFP(exp C (P))<br />
= 〈{a, b, c, d}, {(b, c), (c, d)}, {(a, b), (b, a), (a, d), (b, d)}〉 (now (a, c) and (c, a)<br />
are non-attacks); so CFP(P) ≠ CFP(exp C (P)).<br />
Definition 22 (Common part of a profile of PAFs)<br />
Let P = 〈PAF 1 , . . .,PAF n 〉 be a profile of PAFs. The common part of P is<br />
<strong>de</strong>noted by CP(P) and is <strong>de</strong>fined by: CP(P) = 〈 ⋂ i A i , ⋂ i R i , ⋂ i I i , ⋂ i N i 〉.<br />
The common part of a profile of PAFs is a much more <strong>de</strong>manding notion than<br />
the clash-free one. It repres<strong>en</strong>ts the pieces of information on which all the<br />
ag<strong>en</strong>ts agree. There is no doubt that those pieces of information must hold in<br />
any cons<strong>en</strong>sual view of the group’s opinion, so the common part of the profile<br />
must be inclu<strong>de</strong>d in each AF of the result of the merging process.<br />
Example 14 (continued) With P = 〈AF 1 , AF 2 , AF 3 , AF 4 〉, CP(P) = 〈{b},<br />
∅, ∅, {(b, b)}〉.<br />
We have the following easy property:<br />
Proposition 23 Let P = 〈PAF 1 , . . .,PAF n 〉 be a profile of PAFs. The common<br />
part of P is pointwise inclu<strong>de</strong>d into the clash-free part of P, i.e.:<br />
• ⋂ i R i ⊆ ⋃ i R i \ ⋃ i N i ;<br />
• ⋂ i I i ⊆ I CFP ;<br />
• ⋂ i N i ⊆ ⋃ i N i \ ⋃ i R i .<br />
Proof :<br />
The proof is straightforward:<br />
• ⋂ i R i ⊆ ⋃ i R i is obvious; and we also have ∀(a, b) ∈ ⋂ i R i , (a, b) ∉ N j for<br />
all j (otherwise, ∃PAF k such that (a, b) ∈ R k ∩ N k that is impossible by<br />
<strong>de</strong>finition), so ⋂ i R i ⊆ ⋃ i R i \ ⋃ i N i .<br />
• In the same way, we can prove ⋂ i N i ⊆ ⋃ i N i \ ⋃ i R i .<br />
• if ∀(a, b) ∈ ⋂ i I i th<strong>en</strong>, by <strong>de</strong>finition, (a, b) ∉ R i and (a, b) ∉ N i for all i ; so,<br />
(a, b) ∈ ( ⋃ i A i × ⋃ i A i ) \ (( ⋃ i R i \ ⋃ i N i ) ∪ ( ⋃ i N i \ ⋃ i R i )).<br />
✷<br />
The common part of a profile of n PAFs (resp. AFs) is not always a PAF<br />
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(resp. an AF). Contrastingly, the clash-free part of a profile of n PAFs is a<br />
PAF (however, the clash-free part of a profile of n AFs is not always an AF).<br />
There exists an interesting particular case: if the various PAFs of the profile are<br />
based on the same set of argum<strong>en</strong>ts and if for each or<strong>de</strong>red pair of argum<strong>en</strong>ts<br />
(a, b) such that (a, b) belongs to the ignorance relation in one PAF, this pair<br />
belongs to the attack relation for another PAF of the profile and to the nonattack<br />
relation for at least a third PAF of the profile, th<strong>en</strong> the clash-free part<br />
of the profile and its common part are i<strong>de</strong>ntical:<br />
Proposition 24 Let P = 〈PAF 1 , . . .,PAF n 〉 be a profile of n PAFs over the<br />
same set of argum<strong>en</strong>ts A. Consi<strong>de</strong>r the clash-free part of P <strong>de</strong>noted by CFP(P)<br />
= 〈A CFP , R CFP , I CFP , N CFP 〉 and the common part of P <strong>de</strong>noted by CP(P)<br />
= 〈A CP , R CP , I CP , N CP 〉. If ⋃ i I i ⊆ conf(P) = ( ⋃ i R i ) ∩ ( ⋃ i N i ), we have:<br />
• A CFP = A CP ,<br />
• R CFP = R CP ,<br />
• N CFP = N CP .<br />
Proof : All the PAFs are over the same set of argum<strong>en</strong>ts, so we have<br />
A = ⋃ i A i = ⋂ i A i and A CFP = A CP .<br />
First, we prove that R CFP = R CP .<br />
• R CFP ⊆ R CP : consi<strong>de</strong>r (a, b) ∈ R CFP ; so (a, b) ∈ ⋃ i R i \ ⋃ i N i ; suppose that<br />
(a, b) ∉ R CP ; so ∃PAF k such that (a, b) ∉ R k ; so (a, b) ∈ N k or (a, b) ∈ I k ;<br />
In the first case, we have (a, b) ∉ ⋃ i R i \ ⋃ i N i : contradiction with the<br />
hypothesis (a, b) ∈ R CFP ;<br />
In the second case, we retrieve the first case because ⋃ i I i ⊆ conf(P) =<br />
( ⋃ i R i ) ∩ ( ⋃ i N i ).<br />
Thus (a, b) ∈ R CP .<br />
• R CFP ⊇ R CP : giv<strong>en</strong> by Proposition 23.<br />
N CFP = N CP is prov<strong>en</strong> in the same way.<br />
✷<br />
This result is interesting since this situation always holds (by <strong>de</strong>finition) if<br />
cons<strong>en</strong>sual expansion is used as an expansion policy by each ag<strong>en</strong>t.<br />
Example 14 (continued) With P = 〈AF 1 , AF 2 , AF 3 , AF 4 〉 and exp C (P) =<br />
〈exp C (AF 1 , P), exp C (AF 2 , P), exp C (AF 3 , P), exp C (AF 4 , P)〉, we have:<br />
• CFP(exp C (P)) = 〈{a, b, c, d}, {(b, c), (c, d)}, {(a, b), (b, a), (a, d), (b, d)},<br />
{(a, a), (b, b), (c, c), (d, d), (a, c), (c, a), (d, a), (d, b), (d, c), (c,b)〉<br />
• CP(exp C (P)) = 〈{a, b, c, d}, {(b, c), (c, d)}, ∅,<br />
{(a, a), (b, b), (c, c), (d, d), (a, c), (c, a), (d, a), (d, b), (d, c), (c,b)}〉.<br />
A valuable property of any cons<strong>en</strong>sual expansion over a profile of AFs is that<br />
21 193
it preserves the clash-free part of the profile:<br />
Proposition 25 Let P = 〈AF 1 , . . ., AF n 〉 be a profile of AFs. For each i, we<br />
have:<br />
• A CFP(P) = A expC (AF i ,P) ,<br />
• R CFP(P) ⊆ R expC (AF i ,P) ,<br />
• N CFP(P) ⊆ N expC (AF i ,P) .<br />
Proof : Consi<strong>de</strong>r AF i , <strong>de</strong>noted by 〈A i , R i , N i 〉, and the set conf(P) =<br />
( ⋃ i R i ) ∩ ( ⋃ i N i ). Each exp C (AF i , P) is <strong>de</strong>noted by 〈A ′ i , R′ i , I′ i , N ′ i 〉.<br />
• By <strong>de</strong>finition, the set of argum<strong>en</strong>ts is the same for CFP(AF 1 , . . .,AF n ) and<br />
for each exp C (AF i , P), ∀AF i : it is equal to ⋃ i A i .<br />
• Consi<strong>de</strong>r a, b ∈ ⋃ i A i such that (a, b) ∈ R CFP(P) = ( ⋃ i R i ) \ ( ⋃ i N i ) ; so,<br />
we have (a, b) ∉ conf(P) and (a, b) ∉ N i . So, (a, b) ∈ R ′ i = R i ∪ ((( ⋃ i R i ) \<br />
conf(P)) \ N i ).<br />
• Consi<strong>de</strong>r a, b ∈ ⋃ i A i such that (a, b) ∈ N CFP(P) = ( ⋃ i N i ) \ ( ⋃ i R i ) ; so, we<br />
have (a, b) ∉ conf(P) and (a, b) ∉ ⋃ i R i . So, (a, b) ∉ I ′ i , and (a, b) ∉ R′ i . So,<br />
(a, b) ∈ N ′ i .<br />
✷<br />
Now, concordance betwe<strong>en</strong> AFs can be <strong>de</strong>fined as follows:<br />
Definition 26 (Concordance) Let AF 1 = 〈A 1 , R 1 〉, AF 2 = 〈A 2 , R 2 〉 be two<br />
AFs. AF 1 , AF 2 are said to be concordant if and only if ∀(a, b) ∈ (A 1 ∩ A 2 ) ×<br />
(A 1 ∩ A 2 ), (a, b) ∈ R 1 if and only if (a, b) ∈ R 2 . Otherwise they are said to be<br />
discordant.<br />
Let P = 〈AF 1 , . . .,AF n 〉 be a profile of AFs. P is said to be concordant if<br />
and only if all its AFs are pairwise concordant. Otherwise it is said to be<br />
discordant.<br />
Of course, concordance is related to the set conf(P) repres<strong>en</strong>ting clashs betwe<strong>en</strong><br />
attack and non-attack relations in the differ<strong>en</strong>t AFs of the profile:<br />
Proposition 27 Let P = 〈AF 1 , . . .,AF n 〉 be a profile of argum<strong>en</strong>tation systems.<br />
P is concordant if and only if conf(P) = ⋃ i R i ∩ ⋃ i N i is empty.<br />
Proof :<br />
P is concordant ⇔<br />
∀AF i , AF j ∈ P, ∄a, b ∈ A i ∩ A j such that (a, b) ∈ (R i \ R j ) ∪ (R j \ R i ) ⇔<br />
∀AF i , AF j ∈ P, ∄a, b ∈ A i ∩ A j such that (a, b) ∈ R i and (a, b) ∈ N j ⇔<br />
⋃<br />
i R i ∩ ⋃ i N i = ∅.<br />
✷<br />
22 194
Wh<strong>en</strong> a profile of AFs is concordant, its clash-free part is the union of its<br />
elem<strong>en</strong>ts, and the converse also holds:<br />
Proposition 28 Let P = 〈AF 1 , . . .,AF n 〉 be a profile of AFs. P is concordant<br />
if and only if CFP(P) = ⋃ i AF i .<br />
Proof : CFP(P) is <strong>de</strong>noted by 〈A CFP , R CFP , I CFP , N CFP 〉.<br />
The proof for “P concordant ⇒ CFP(P) = ⋃ i AF i ” is ma<strong>de</strong> using a reductio<br />
ad absurdum. We suppose that CFP(P) ≠ ⋃ i AF i and we have the following<br />
possibilities:<br />
• ∃(a, b) ∈ R CFP and (a, b) ∉ ⋃ i R i ; this case is impossible because, by <strong>de</strong>finition,<br />
(a, b) ∈ ( ⋃ i R i ) \ ( ⋃ i N i );<br />
• ∃(a, b) ∈ N CFP and (a, b) ∉ ⋃ i N i ; this case is impossible because, by <strong>de</strong>finition,<br />
(a, b) ∈ ( ⋃ i N i ) \ ( ⋃ i R i );<br />
• ∃(a, b) ∉ R CFP and (a, b) ∈ ⋃ i R i ; so, by <strong>de</strong>finition, (a, b) ∈ ( ⋃ i N i ); so,<br />
∃AF k , AF j such that (a, b) ∈ R k and (a, b) ∈ N j ; so, ∃AF k , AF j such that<br />
(a, b) ∈ A k ∩ A j and (a, b) ∈ R k \ R j ; so, contradiction with the hypothesis<br />
P concordant;<br />
• ∃(a, b) ∉ N CFP and (a, b) ∈ ⋃ i N i ; so, by <strong>de</strong>finition, (a, b) ∈ ( ⋃ i R i ); so,<br />
∃AF k , AF j such that (a, b) ∈ R k and (a, b) ∈ N j ; so, ∃AF k , AF j such that<br />
(a, b) ∈ A k ∩ A j and (a, b) ∈ R k \ R j ; so, contradiction with the hypothesis<br />
P concordant.<br />
For each possibility, we obtain a contradiction. So, if P is concordant, th<strong>en</strong><br />
CFP(P) = ⋃ i AF i .<br />
The proof for “P concordant ⇐ CFP(P) = ⋃ i AF i ” is also ma<strong>de</strong> using a reductio<br />
ad absurdum. If P is discordant th<strong>en</strong> ∃AF i , AF j such that ∃(a, b) ∈ A i ∩A j<br />
and (a, b) ∈ (R i \ R j ) ∪ (R j \ R i ). So, a, b ∈ ⋃ k A k , (a, b) ∈ ⋃ k R k and (a, b) ∈<br />
⋃<br />
⋃ k N k ; so, (a, b) appears in the attack relation and in the non-attack relation of<br />
i AF i . However, by <strong>de</strong>finition, (a, b) cannot appear in the same time in R CFP<br />
and in N CFP . So, contradiction with the hypothesis CFP(AF 1 , . . .,AF n ) =<br />
⋃<br />
i AF i .<br />
✷<br />
Proposition 29 Let P = 〈AF 1 , . . ., AF n 〉 be a profile of AFs. P is concordant<br />
if and only if exp C (P) = 〈exp C (AF 1 , P), . . .,exp C (AF n , P)〉 is reduced to<br />
〈 ⋃ i AF i , . . ., ⋃ i AF i 〉 (i.e., each of the n elem<strong>en</strong>ts of the vector is ⋃ i AF i ).<br />
Proof : Consi<strong>de</strong>r a concordant profile of AFs P. ∀AF i = 〈A i , R i , N i 〉, let us<br />
consi<strong>de</strong>r exp C (AF i , P) = 〈A ′ i, R i, ′ I i, ′ N i〉. ′ ∀a, b ∈ ⋃ i A i , there are several cases:<br />
• if (a, b) ∈ R i th<strong>en</strong> (a, b) ∈ R ′ i ;<br />
• if (a, b) ∉ R i and (a, b) ∈ A i × A i th<strong>en</strong> (a, b) ∈ N i , so (a, b) ∈ N ′ i; with P<br />
concordant, we also know that ∄AF j ∈ P such that (a, b) ∈ R j ;<br />
• if (a, b) ∉ R i and (a, b) ∉ A i × A i th<strong>en</strong> there are two cases:<br />
· either ∃AF j ∈ P such that (a, b) ∈ R j : because P is concordant, (a, b) ∈<br />
23 195
R ′ i ;<br />
· or ∄AF j ∈ P such that (a, b) ∈ R j : so, (a, b) ∈ N ′ i .<br />
In all the cases, if (a, b) is an attack interaction for one of the AF i , (a, b) is also<br />
an attack interaction for the cons<strong>en</strong>sual PAFs. So, all the cons<strong>en</strong>sual PAFs<br />
are equal to ⋃ i AF i .<br />
For the second part of the proof, consi<strong>de</strong>rexp C<br />
(P) = 〈 ⋃ i AF i , , . . ., ⋃ i AF i 〉. We<br />
suppose that P is discordant. So, ∃AF i , AF j ∈ P such that ∃a, b ∈ A i ∩A j and<br />
(a, b) ∈ (R i \R j ) ∪(R j \R i ). If we suppose that (a, b) ∈ R i , th<strong>en</strong> exp C (AF j , P)<br />
cannot contain the attack (a, b); so, exp C<br />
(AF j , P) ≠ ⋃ i AF i : contradiction.<br />
And the same problem appears wh<strong>en</strong> we suppose that (a, b) ∈ R j . So, P is<br />
concordant.<br />
✷<br />
Note that ⋃ i AF i may appear into exp C (P), ev<strong>en</strong> if P is discordant. This is<br />
illustrated by the following example:<br />
Example 30 Consi<strong>de</strong>r the profile P = 〈AF 1 , AF 2 , AF 3 〉 consisting of the following<br />
three AFs:<br />
• AF 1 = 〈{a, b, c}, {(a, b), (a, c)}〉,<br />
• AF 2 = 〈{a, b, c}, {(a, c)}〉,<br />
• AF 3 = 〈{a, d}, {(a, d)}〉.<br />
The profile P = 〈AF 1 , AF 2 , AF 3 〉 is discordant and exp C (P) = 〈PAF 1 , PAF 2 ,<br />
PAF 3 〉 is such that:<br />
• PAF 1 = 〈{a, b, c, d}, {(a, b), (a, c), (a, d)}, ∅〉 (= ⋃ i AF i ),<br />
• PAF 2 = 〈{a, b, c, d}, {(a, c), (a, d)}, ∅〉,<br />
• PAF 3 = 〈{a, b, c, d}, {(a, c), (a, d)}, {(a, b)}〉.<br />
PAF 1<br />
b<br />
PAF 3<br />
b<br />
PAF 2<br />
b<br />
a<br />
c<br />
a<br />
c<br />
a<br />
c<br />
d<br />
d<br />
d<br />
The following proposition states that wh<strong>en</strong>ever the pres<strong>en</strong>ce of an attack (a, b)<br />
does not clash with a profile of AFs, such an attack is pres<strong>en</strong>t in all the<br />
corresponding PAFs obtained by cons<strong>en</strong>sual expansion if and only if it is<br />
pres<strong>en</strong>t in one of the input AFs.<br />
Proposition 31 Let P = 〈AF 1 , . . .,AF n 〉 be a profile of AFs. Let (a, b) be a<br />
pair of argum<strong>en</strong>ts such that a, b ∈ ⋃ i A i and ∄AF i , AF j ∈ P such that (a, b) ∈<br />
(R i \ R j ) ∪ (R j \ R i ).<br />
24 196
∃AF l ∈ P such that (a, b) ∈ R l if and only if ∀AF k ∈ P, (a, b) ∈ R ′ k with R′ k<br />
<strong>de</strong>noting the attack relation of the PAF exp C<br />
(AF k , P).<br />
Proof : Consi<strong>de</strong>r AF k ∈ P. Since ∃AF l ∈ P such that (a, b) ∈ R l and<br />
∄AF i , AF j ∈ P such that (a, b) ∈ (R i \R j )∪(R j \R i ), (a, b) ∉ N k ; so, (a, b) ∈ R k.<br />
′<br />
The second part of the proof is obvious with a reductio ad absurdum: if we<br />
suppose that ∄AF l ∈ P such that (a, b) ∈ R l th<strong>en</strong> we obtain ∀AF k ∈ P,<br />
(a, b) ∈ N k ′ which is a contradiction with ∀AF k ∈ P, (a, b) ∈ R k ′ . ✷<br />
A notion of compatibility of a profile of PAFs over the same set of argum<strong>en</strong>ts<br />
can also be <strong>de</strong>fined:<br />
Definition 32 (Compatibility) Let P = 〈PAF 1 , . . .,PAF n 〉 be a profile of<br />
PAFs over a set of argum<strong>en</strong>ts A. PAF 1 , . . .,PAF n are said to be compatible<br />
if and only if they have at least one common completion. Otherwise they are<br />
said to be incompatible.<br />
Let P = 〈AF 1 , ..., AF n 〉 be a profile of AFs. Let exp be an expansion function.<br />
AF 1 , ..., AF n are said to be compatible giv<strong>en</strong> exp if and only if exp(AF i , P),<br />
∀i = 1 . . .n, are said to be compatible. Otherwise they are said to be incompatible.<br />
There is a clear link betwe<strong>en</strong> concordance and compatibility in the case of the<br />
cons<strong>en</strong>sual expansion applied to a profile of AFs:<br />
Proposition 33 Let P = 〈AF 1 , . . .,AF n 〉 be a profile of AFs. P is concordant<br />
if and only if exp C (AF 1 , P), ..., exp C (AF n , P) are compatible.<br />
Proof : The first part of the proof is obvious: if P is concordant th<strong>en</strong> the profile<br />
exp C<br />
(P) = 〈exp C<br />
(AF 1 , P), ..., exp C<br />
(AF n , P)〉 is reduced to 〈 ⋃ i AF i , . . .,<br />
⋃<br />
i AF i 〉 (see Proposition 29); so, exp C (AF 1 , P), ..., exp C (AF n , P) are equal<br />
and have a common completion.<br />
The second part of the proof uses a reductio ad absurdum: if we suppose that<br />
P is discordant th<strong>en</strong> ∃AF i , AF j such that ∃(a, b) ∈ R i ∩ N j ; so, (a, b) ∈ R i<br />
′<br />
with R i ′ <strong>de</strong>noting the attack relation of exp C (AF i , P) and (a, b) ∈ N j ′ with N j<br />
′<br />
<strong>de</strong>noting the non-attack relation of exp C (AF j , P); so, all the completions of<br />
exp C<br />
(AF i , P) must contain the attack (a, b) and no completion ofexp C<br />
(AF j , P)<br />
can contain the attack (a, b); so, AF i and AF j do not have a common completion<br />
which is in contradiction with the hypothesis of compatibility. ✷<br />
Example 34 Consi<strong>de</strong>r the following argum<strong>en</strong>tation systems AF 1 , AF 2 and<br />
AF 3 .<br />
The completions of their respective cons<strong>en</strong>sual expansions PAF 1 , PAF 2 and<br />
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PAF 3 are:<br />
AF 3<br />
AF 1 AF 2<br />
c a<br />
c a b<br />
a b<br />
Completions of AF 1<br />
c a b<br />
c a b<br />
Completions of AF 3<br />
c a b<br />
Completions of AF 2<br />
c a b<br />
c<br />
a<br />
b<br />
AF 1 and AF 2 are discordant and incompatible giv<strong>en</strong> exp C<br />
. AF 3 and AF 1 are<br />
concordant and compatible giv<strong>en</strong> exp C .<br />
6.2 Properties of merging operators<br />
Let us now give some properties of merging operators, focusing on those based<br />
on the edit distance:<br />
Proposition 35 Let P = 〈AF 1 , . . .,AF n 〉 be a profile of AFs. Assume that<br />
the expansion function used for each ag<strong>en</strong>t is the cons<strong>en</strong>sual one. If P is<br />
concordant th<strong>en</strong> ∆ ⊗ <strong>de</strong> (P) = {⋃ i AF i }.<br />
Proof : If P is concordant, th<strong>en</strong> by Proposition 29, we have exp C (〈AF 1 , . . .,<br />
AF n 〉) = 〈 ⋃ i AF i , . . ., ⋃ i AF i 〉. It remains to show that ∆ ⊗ <strong>de</strong> (〈 ⋃ i AF i , . . ., ⋃ i AF i 〉)<br />
= { ⋃ i AF i }, which is obvious since <strong>de</strong>, as a distance, satisfies the minimality<br />
requirem<strong>en</strong>t ( ⋃ i AF i is the unique PAF at edit distance 0 from itself). ✷<br />
Now we show an expected property: that the clash-free part of any profile P<br />
is inclu<strong>de</strong>d in each AF from the merging of P wh<strong>en</strong> the edit distance is used.<br />
Proposition 36 Let P = 〈AF 1 , . . .,AF n 〉 be a profile of argum<strong>en</strong>tation systems.<br />
Assume that the expansion function used for each ag<strong>en</strong>t is the cons<strong>en</strong>sual<br />
one. For any aggregation function ⊗, we have that : ∀AF = 〈A, R, N〉 ∈<br />
∆ ⊗ <strong>de</strong> (〈AF 1, . . ., AF n 〉):<br />
• A CFP(P) ⊆ A,<br />
• R CFP(P) ⊆ R,<br />
• N CFP(P) ⊆ N.<br />
Proof : Let CFP(P) = 〈A CFP , R CFP , N CFP 〉 (the ignorance relation does<br />
not appear here because argum<strong>en</strong>tation systems (and not partial ones) are<br />
consi<strong>de</strong>red).<br />
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• A CFP = ⋃ i A i ⊆ A = ⋃ i A i .<br />
• By Proposition 25, we know that CFP(P) is pointwise inclu<strong>de</strong>d in each<br />
exp C (AF i , P). Let first consi<strong>de</strong>r the case (a, b) ∈ R CFP , we have (a, b) ∈<br />
R expC (AF i ,P) , ∀AF i.<br />
Consi<strong>de</strong>r AF = 〈A, R〉 ∈ ∆ ⊗ <strong>de</strong> (〈AF 1, . . .,AF n 〉). Suppose that (a, b) ∉ R and<br />
consi<strong>de</strong>r AF ′ = 〈A ′ = A, R ′ = R ∪ {(a, b)}〉.<br />
∀AF i , <strong>de</strong>(AF ′ ,exp C (AF i , P)) = <strong>de</strong>(AF,exp C (AF i , P)) − 1, since (a, b) ∈<br />
R ′ ∩ R expC (AF and ∉ R ∩ R i ,P) exp C<br />
(AF i<br />
; so, since ⊗ respects mono-<br />
,P)<br />
tonicity, we have ⊗ n i=1 <strong>de</strong>(AF′ ,exp C<br />
(P)) < ⊗ n i=1 <strong>de</strong>(AF,exp C<br />
(P)) and we obtain<br />
a contradiction with AF ∈ ∆ ⊗ <strong>de</strong> (〈AF 1, . . ., AF n 〉); so, (a, b) ∈ R. H<strong>en</strong>ce<br />
R CFP(P) ⊆ R.<br />
• In the same way, we can prove that if (a, b) ∈ N CFP th<strong>en</strong> (a, b) ∈ N. So<br />
N CFP(P) ⊆ N.<br />
✷<br />
As a direct corollary of Propositions 23 and 36, we get that:<br />
Corollary 37 Let P = 〈AF 1 , . . ., AF n 〉 be a profile of argum<strong>en</strong>tation systems.<br />
Assume that the expansion function used for each ag<strong>en</strong>t is the cons<strong>en</strong>sual<br />
one. For any aggregation function ⊗, we have that: ∀AF = 〈A, R, N〉 ∈<br />
∆ ⊗ <strong>de</strong> (〈AF 1, . . ., AF n 〉):<br />
• A CP(P) ⊆ A,<br />
• R CP(P) ⊆ R,<br />
• N CP(P) ⊆ N.<br />
Wh<strong>en</strong> sum is used as the aggregation function and all AFs are over the same<br />
set of argum<strong>en</strong>ts, the merging of a profile can be characterized in a concise<br />
way, thanks to the notion of majority graph. Intuitively the majority graph<br />
of a profile of AFs over the same set of argum<strong>en</strong>ts is the PAF obtained by<br />
applying the strict majority rule to <strong>de</strong>ci<strong>de</strong> whether a attacks b or not, for<br />
every or<strong>de</strong>red pair (a, b) of argum<strong>en</strong>ts. Wh<strong>en</strong>ever there is no strict majority,<br />
an ignorance edge is g<strong>en</strong>erated.<br />
Definition 38 (Majority PAF) Let P = 〈AF 1 , ..., AF n 〉 be a profile of<br />
AFs over the same set A of argum<strong>en</strong>ts. The majority PAF MP(P) of P is<br />
the triple 〈R, N, I〉 such that ∀a, b ∈ A: 4<br />
• (a, b) ∈ R if and only if #({i ∈ 1 . . .n | (a, b) ∈ R i }) > #({i ∈ 1 . . .n | (a, b) ∈<br />
N i });<br />
• (a, b) ∈ N if and only if #({i ∈ 1 . . .n | (a, b) ∈ N i }) > #({i ∈ 1 . . .n | (a, b) ∈<br />
R i });<br />
4 For any set S, #(S) <strong>de</strong>notes the cardinality of S.<br />
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• (a, b) ∈ I otherwise.<br />
The next proposition states that, as expected, the majority PAF of a profile<br />
of AFs over the same set of argum<strong>en</strong>ts is a PAF:<br />
Proposition 39 Let P = 〈AF 1 , ..., AF n 〉 be a profile of AFs over the same<br />
set A of argum<strong>en</strong>ts. The majority PAF MP(P) of P is a PAF.<br />
Proof : Obvious since by construction, R and I are disjoint sets and N is<br />
the complem<strong>en</strong>t of R ∪ I into A × A.<br />
✷<br />
Example 40 Consi<strong>de</strong>r AF 1 = 〈{a, b, c}, {(a, b), (b, c), (a, c)}〉, AF 2 = 〈{a, b, c},<br />
{(a, b), (b, a), (a, c)}〉.<br />
AF 1<br />
b<br />
AF 2<br />
b<br />
MP(〈AF 1 , AF 2 〉)<br />
b<br />
a<br />
a<br />
a<br />
c<br />
c<br />
c<br />
We have MP(〈AF 1 , AF 2 〉) = 〈{a, b, c}, {(a, b), (a, c)}, {(b, c), (b, a)}, {(a, a),<br />
(b, b), (c, c), (c, a), (c, b)}〉.<br />
Proposition 41 Let P = 〈AF 1 , ..., AF n 〉 be a profile of AFs over the same<br />
set A of argum<strong>en</strong>ts. ∆ Σ <strong>de</strong>(P) = C(MP(P)).<br />
Proof : The key is that the edit distance betwe<strong>en</strong> an AF <strong>de</strong>noted by AF<br />
and a profile of AFs over A wh<strong>en</strong> Σ is the aggregation operator is the sum<br />
over the AF i of the profile of the sum over every or<strong>de</strong>red pair of argum<strong>en</strong>ts<br />
over A of the edit distances betwe<strong>en</strong> AF and AF i (this is a consequ<strong>en</strong>ce of the<br />
associativity of the sum).<br />
Let AF be an AF over A which minimizes ∑ n<br />
i=1 <strong>de</strong>(AF, AF i ). Let a, b ∈ A. If<br />
#({i ∈ 1 . . .n | (a, b) ∈ R i } > #({i ∈ 1 . . .n | (a, b) ∈ N i }, th<strong>en</strong> (a, b) must<br />
be in the attack relation of AF; otherwise, the AF AF ′ over A which coinci<strong><strong>de</strong>s</strong><br />
with AF except that (a, b) is in the attack relation of AF ′ would be such that<br />
∑ ni=1<br />
<strong>de</strong>(AF ′ , AF i ) < ∑ n<br />
i=1 <strong>de</strong>(AF, AF i ). Similarly, if #({i ∈ 1 . . .n | (a, b) ∈<br />
N i } > #({i ∈ 1 . . .n | (a, b) ∈ R i }, th<strong>en</strong> (a, b) must not be in the attack<br />
relation of AF.<br />
In the remaining case, i.e., wh<strong>en</strong> #({i ∈ 1 . . .n | (a, b) ∈ R i } = #({i ∈<br />
1 . . .n | (a, b) ∈ N i }, let AF ′ be the AF over A which coinci<strong><strong>de</strong>s</strong> with AF except<br />
that (a, b) is in the attack relation of AF ′ if and only if (a, b) is not in the<br />
attack relation of AF. Th<strong>en</strong> ∑ n<br />
i=1 <strong>de</strong>(AF ′ , AF i ) = ∑ n<br />
i=1 <strong>de</strong>(AF, AF i ).<br />
28 200
This shows that every AF over A which minimizes ∑ n<br />
i=1 <strong>de</strong>(AF, AF i ) is a completion<br />
of the majority PAF MP(P).<br />
Conversely, since every completion AF ′ of MP(P) is such that ∑ n<br />
i=1 <strong>de</strong>(AF ′ , AF i ) =<br />
∑ ni=1<br />
<strong>de</strong>(AF, AF i ) where AF minimizes ∑ n<br />
i=1 <strong>de</strong>(AF, AF i ), the conclusion follows.<br />
✷<br />
Let us illustrate the previous proposition on Example 7:<br />
Example 7 (continued) The cons<strong>en</strong>sual expansions of AF 1 , AF 2 and AF 3<br />
are respectively:<br />
b<br />
c<br />
e<br />
b<br />
c<br />
e<br />
b<br />
c<br />
e<br />
a<br />
d<br />
f<br />
a<br />
d<br />
f<br />
a<br />
d<br />
f<br />
So, the majority PAF of 〈AF 1 , AF 2 , AF 3 〉 is:<br />
b<br />
c<br />
e<br />
a<br />
d<br />
f<br />
Using the edit distance and sum as the aggregation function, this PAF also<br />
repres<strong>en</strong>ts the result of the merging in the s<strong>en</strong>se that the latter is the set of all<br />
completions of this PAF.<br />
Computing the majority PAF of a profile of AFs over the same set of argum<strong>en</strong>ts<br />
amounts to voting on the attack relations associated to each AF. As<br />
explained in Section 3, this can prove more suited to our goal than the approach<br />
which consists in voting directly on the acceptable sets of argum<strong>en</strong>ts<br />
for each ag<strong>en</strong>t. The previous proposition shows that such a simple voting approach<br />
corresponds to a specific merging operator in our framework (but many<br />
other operators, especially arbitration ones, can also be used).<br />
7 Acceptability for Merged AFs<br />
Starting from a profile of AFs (over possibly differ<strong>en</strong>t sets of argum<strong>en</strong>ts), a<br />
merging operator <strong>en</strong>ables the computation of a set of AFs (this time, over the<br />
same set of argum<strong>en</strong>ts) which are the best candidates to repres<strong>en</strong>t the AFs of<br />
the group (a kind of “cons<strong>en</strong>sus”).<br />
There is an important epistemic differ<strong>en</strong>ce betwe<strong>en</strong> those two sets of AFs, the<br />
first one reflects differ<strong>en</strong>t points of view giv<strong>en</strong> by differ<strong>en</strong>t ag<strong>en</strong>ts (and it can<br />
29 201
e the case that two distinct ag<strong>en</strong>ts give the same AF), while the second set<br />
expresses some uncertainty on the merging due to the pres<strong>en</strong>ce of conflicts.<br />
Let us recall that the main goal of this paper is to characterize the sets of<br />
argum<strong>en</strong>ts acceptable by the whole group of ag<strong>en</strong>ts. In or<strong>de</strong>r to achieve it, it<br />
remains to <strong>de</strong>fine some mechanisms for exploiting the resulting set of AFs.<br />
This calls for a notion of joint acceptability.<br />
Definition 42 (Joint acceptability) A joint acceptability relation for a profile<br />
〈AF 1 , . . .,AF n 〉 of AFs, <strong>de</strong>noted by Acc 〈AF 1,...,AF n〉<br />
, is a total function from<br />
2 ⋃ i A i<br />
to {true,false} which associates each subset E of ⋃ i A i with true if E<br />
is a jointly acceptable set for 〈AF 1 , . . ., AF n 〉 and with false otherwise.<br />
For instance, a joint acceptability relation for a profile 〈AF 1 , . . .,AF n 〉 can<br />
be <strong>de</strong>fined by the acceptability relations Acc AFi (based themselves on some<br />
semantics and some selection principles), which can coinci<strong>de</strong> for every AF i (but<br />
this is not mandatory) and a voting method V : {true,false} n ↦→ {true,false}:<br />
Acc 〈AF 1,...,AF n〉 (E) = V (Acc AF 1<br />
(E), . . ., Acc AFn (E)).<br />
Here are some instances of Definition 42 based on voting methods:<br />
Definition 43 (Acceptabilities for profiles of AFs) Let P = 〈AF 1 , . . .,<br />
AF n 〉 be a profile of n AFs over the same set of argum<strong>en</strong>ts A. Let Acc AFi be<br />
the (local) acceptability relation associated with AF i . If n = 1, th<strong>en</strong> we <strong>de</strong>fine<br />
Acc 〈AF 1〉 = Acc AF 1<br />
. Otherwise, for any subset S of A, we say that:<br />
• S is skeptically jointly acceptable for P if and only if S is inclu<strong>de</strong>d in at<br />
least one acceptable set for each AF i :<br />
∀AF i ∈ P, ∃E i such that Acc AFi (E i ) is true and S ⊆ E i .<br />
• S is credulously jointly acceptable for P if and only if S is inclu<strong>de</strong>d in at<br />
least one acceptable set for at least one AF i :<br />
∃AF i ∈ P, ∃E i such that Acc AFi (E i ) is true and S ⊆ E i .<br />
• S is jointly acceptable by majority for P if and only if S is inclu<strong>de</strong>d in at<br />
least one acceptable set for at least a weak majority of AF i :<br />
#({AF i | ∃E i such that Acc AFi (E i ) is true and S ⊆ E i }) ≥ n 2 .<br />
Obviously <strong>en</strong>ough, wh<strong>en</strong> none of the local acceptabilities Acc AFi is trivial (i.e.,<br />
equival<strong>en</strong>t to the constant function false) for the profile un<strong>de</strong>r consi<strong>de</strong>ration,<br />
we have that any set of argum<strong>en</strong>ts which is skeptically jointly acceptable is<br />
also jointly acceptable by majority, and that any set of argum<strong>en</strong>ts which is<br />
jointly acceptable by majority is also credulously jointly acceptable.<br />
Note that skeptical (resp. credulous) joint acceptability does not require that<br />
the skeptical (resp. credulous) infer<strong>en</strong>ce principle is at work for <strong>de</strong>fining local<br />
30 202
acceptabilities Acc AFi , which remain unconstrained.<br />
Focusing on the preferred semantics together with credulous local acceptabilities,<br />
let us re-consi<strong>de</strong>r some previous examples:<br />
Example 20 (continued) Using the edit distance and ⊗ = Leximax (or<br />
Max) as the aggregation function, we get two AFs AF ′ 1 and AF′ 2 in the merging.<br />
If the local acceptability relations are based on credulous infer<strong>en</strong>ce from preferred<br />
ext<strong>en</strong>sions, we have:<br />
• Acc AF<br />
′ (E) = true if and only if E ⊆ {c, d} or E ⊆ {b, e};<br />
1<br />
• Acc AF<br />
′ (E) = true if and only if E ⊆ {a, c, e}.<br />
2<br />
{c} and {e} are skeptically jointly acceptable and {b, e},{c, d} and {a, c, e}<br />
(and their subsets) are credulously (and by majority) jointly acceptable for the<br />
merging.<br />
Using this method, the argum<strong>en</strong>t a can still be <strong>de</strong>rived credulously, contrariwise<br />
to what happ<strong>en</strong>s wh<strong>en</strong> the union of the two AFs AF 1 and AF 2 is consi<strong>de</strong>red.<br />
Example 7 (continued) Using the edit distance and the sum as the aggregation<br />
function, we get one AF in the merging, <strong>de</strong>noted AF:<br />
b<br />
c<br />
e<br />
a<br />
d<br />
f<br />
AF has two preferred ext<strong>en</strong>sions : {a, c, e} and {b, d, e}. So, Acc AF (E) = true<br />
if and only if E ⊆ {a, c, e} or E ⊆ {b, d, e}. The three joint acceptability<br />
relations coinci<strong>de</strong> here (as there is only one AF in the result). The sets {a, c, e}<br />
and {b, d, e} (and their subsets) are credulously, skeptically and by majority,<br />
jointly acceptable for the merging, which is a more s<strong>en</strong>sible result that the one<br />
obtained using a voting method on the <strong>de</strong>rived argum<strong>en</strong>ts of the initial AFs<br />
(as explained in Section 3).<br />
Example 14 (continued) Using the edit distance and the sum as the aggregation<br />
function, we get two AFs in the merging:<br />
b<br />
c<br />
b<br />
c<br />
a<br />
d<br />
a<br />
d<br />
The preferred ext<strong>en</strong>sions for these 2 AFs coinci<strong>de</strong> (they are {a, c} and {b, d}).<br />
As the preferred ext<strong>en</strong>sions for the 2 AFs are the same ones, the three relations<br />
31 203
of joint acceptability coinci<strong>de</strong> here. Thus, the sets {a, c} and {b, d} (and their<br />
subsets) are skeptically, credulously and by majority jointly acceptable for the<br />
merging.<br />
It is interesting to compare the joint acceptability relation for the input profile<br />
P = 〈AF 1 , . . .,AF n 〉 with the joint acceptability relation for the merging<br />
∆ ⊗ d (P). Unsurprisingly, both predicates are not logically connected (i.e., none<br />
of them implies the other one), ev<strong>en</strong> in the case wh<strong>en</strong> the two joint acceptability<br />
relations are based on the same notion of local acceptability (for instance,<br />
consi<strong>de</strong>ring a set of argum<strong>en</strong>ts E as acceptable for an AF wh<strong>en</strong> it is inclu<strong>de</strong>d<br />
in at least one of its preferred ext<strong>en</strong>sions) and the same voting method (for<br />
instance, the simple majority rule).<br />
Thus, it can be the case that new jointly acceptable sets are obtained after<br />
merging while they were not jointly acceptable at start:<br />
Proposition 44 Let P = 〈AF 1 , . . .,AF n 〉 be a profile of AFs over the same<br />
set of argum<strong>en</strong>ts A. The set of all jointly acceptable sets for the profile P is<br />
not necessarily equal to the set of all jointly acceptable sets for the merging of<br />
P.<br />
A counter-example is giv<strong>en</strong> by Example 14.<br />
Wh<strong>en</strong> each local acceptability relation corresponds exactly to the collective<br />
acceptability proposed by Dung (for a giv<strong>en</strong> semantics and ∀AF i , Acc AFi (E) =<br />
true if and only if E is an ext<strong>en</strong>sion of AF i for this semantics), the following<br />
remarks can be done:<br />
• If a set of argum<strong>en</strong>ts is inclu<strong>de</strong>d in one of the acceptable sets for an ag<strong>en</strong>t,<br />
it is not necessarily inclu<strong>de</strong>d into one of the acceptable sets of any AF from<br />
the merging (and it also holds for singletons). The converse is also true.<br />
• More surprisingly, ev<strong>en</strong> if a set of argum<strong>en</strong>ts is inclu<strong>de</strong>d into each acceptable<br />
set for an ag<strong>en</strong>t, it is not guaranteed to be inclu<strong>de</strong>d into an acceptable set<br />
of an AF from the merging. Conversely, if a set of argum<strong>en</strong>ts is inclu<strong>de</strong>d<br />
into every acceptable set of the AFs from the merging, it is not guaranteed<br />
to be inclu<strong>de</strong>d into an acceptable set for one of the ag<strong>en</strong>ts. Intuitively, this<br />
can be explained by the fact that if an argum<strong>en</strong>t is accepted by all ag<strong>en</strong>ts<br />
for bad reasons (for instance, because they lack information about attacks<br />
on it), it can be rejected by the group after the merging. More formally,<br />
this is due to the fact that nothing <strong>en</strong>sures that one of the initial AFs will<br />
belong to the result of the merging and also to the fact that acceptability is<br />
nonmonotonic (in the s<strong>en</strong>se that adding a single attack (a, b) in an AF may<br />
drastically change its ext<strong>en</strong>sions).<br />
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8 Conclusion and Perspectives<br />
We have pres<strong>en</strong>ted a framework for <strong>de</strong>riving s<strong>en</strong>sible information from a collection<br />
of argum<strong>en</strong>tation systems <strong>à</strong> la Dung. Our approach consists in merging<br />
such systems. The proposed framework is g<strong>en</strong>eral <strong>en</strong>ough to allow for the<br />
repres<strong>en</strong>tation of many differ<strong>en</strong>t sc<strong>en</strong>arios. It is not assumed that all ag<strong>en</strong>ts<br />
must share the same sets of argum<strong>en</strong>ts. No assumption is ma<strong>de</strong> concerning<br />
the meaning of the attack relations, so that such relations may differ not only<br />
because ag<strong>en</strong>ts have differ<strong>en</strong>t points of view on the way argum<strong>en</strong>ts interact<br />
but more g<strong>en</strong>erally may disagree on what an interaction is. Each ag<strong>en</strong>t may be<br />
associated to a specific expansion function, which <strong>en</strong>ables for <strong>en</strong>coding many<br />
attitu<strong><strong>de</strong>s</strong> wh<strong>en</strong> facing a new argum<strong>en</strong>t. Many differ<strong>en</strong>t distances betwe<strong>en</strong> PAFs<br />
and many differ<strong>en</strong>t aggregation functions can be used to <strong>de</strong>fine argum<strong>en</strong>tation<br />
systems which best repres<strong>en</strong>t the whole group.<br />
By means of example, we have shown that our merging-based approach leads<br />
to results which are much more expected than those furnished by a direct vote<br />
on the (sets of) argum<strong>en</strong>ts acceptable by each ag<strong>en</strong>t. We have also shown that<br />
union cannot be tak<strong>en</strong> as a valuable merging operator in the g<strong>en</strong>eral case. We<br />
have investigated formally some properties of the merging operators which we<br />
point out. Among other results, we have shown that merging operators based<br />
on the edit distance preserve all the information on which all the ag<strong>en</strong>ts participating<br />
in the merging process agree, and more g<strong>en</strong>erally, all the information<br />
on which the ag<strong>en</strong>ts participating in the merging process do not disagree. We<br />
have also shown that the merging operator based on the edit distance and the<br />
sum as aggregation function is closely related to the merging approach which<br />
consists in voting on the attack relations wh<strong>en</strong> the input profile gathers argum<strong>en</strong>tation<br />
systems over the same set of argum<strong>en</strong>ts. Finally, we have prov<strong>en</strong><br />
that in the g<strong>en</strong>eral case, the <strong>de</strong>rivable sets of argum<strong>en</strong>ts wh<strong>en</strong> joint acceptability<br />
concerns the input profile may drastically differ from the the <strong>de</strong>rivable<br />
sets of argum<strong>en</strong>ts wh<strong>en</strong> joint acceptability concerns the profile obtained after<br />
the merging step.<br />
We plan to refine our framework in several directions:<br />
Merging PAFs. Our framework can be ext<strong>en</strong><strong>de</strong>d to PAFs merging (instead of<br />
AFs). This <strong>en</strong>ables us to take into account ag<strong>en</strong>ts with incomplete belief states<br />
regarding the attack relation betwe<strong>en</strong> argum<strong>en</strong>ts. Expansions of PAFs can be<br />
<strong>de</strong>fined in a very similar way to expansions of AFs (what mainly changes is<br />
the way ignorance is handled). As PAFs are more expressive than AFs, an<br />
interesting issue for further research is to <strong>de</strong>fine acceptability for PAFs.<br />
Attacks str<strong>en</strong>gths. Assume that each attack believed by Ag<strong>en</strong>t i is associated<br />
to a numerical value reflecting the str<strong>en</strong>gth of the attack according to the<br />
33 205
ag<strong>en</strong>t, i.e., the <strong>de</strong>gree to which Ag<strong>en</strong>t i believes that a attacks b. It is easy to<br />
take into account those values by modifying slightly the <strong>de</strong>finition of the edit<br />
distance over an or<strong>de</strong>red pair of argum<strong>en</strong>ts (for instance, viewing such values<br />
as weights once normalized within [0, 1]). Another possibility regarding attack<br />
str<strong>en</strong>gths is, from unweighted attack relations, to g<strong>en</strong>erate a weighted one,<br />
repres<strong>en</strong>ting differ<strong>en</strong>t <strong>de</strong>grees of accordance in the group. For instance, each<br />
attack (a, b) in the majority PAF of a profile 〈AF 1 , . . .,AF n 〉 can be labelled<br />
by the ratio #({i∈1...n|(a,b)∈R i})<br />
and similarly for the non-attack relation (this<br />
n<br />
leads to consi<strong>de</strong>r both the attack and the non-attack relations of the majority<br />
PAF as fuzzy relations). Corresponding acceptability relations remain to be<br />
<strong>de</strong>fined. This is another perspective of this work.<br />
Merging audi<strong>en</strong>ces. In (7), an ext<strong>en</strong>sion of the notion of AF, called valued<br />
AF — VAF for short —, has be<strong>en</strong> proposed in or<strong>de</strong>r to take advantage of<br />
values repres<strong>en</strong>ting the ag<strong>en</strong>t’s prefer<strong>en</strong>ces in the context of a giv<strong>en</strong> audi<strong>en</strong>ce.<br />
A further perspective of our work concerns the merging of such VAFs.<br />
Acknowledgm<strong>en</strong>ts<br />
The authors are grateful to the anonymous referees for their helpful comm<strong>en</strong>ts.<br />
This work has be<strong>en</strong> partly supported by the Région Nord/Pas-<strong>de</strong>-Calais, the<br />
IRCICA consortium and by the European Community FEDER Program.<br />
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35 207
BELIEF BASE MERGING AS A GAME<br />
Sébasti<strong>en</strong> Konieczny.<br />
Journal of Applied Non-Classical Logics. 14(3).<br />
pages 275-294.<br />
2004.<br />
209
Belief base merging as a game<br />
Sébasti<strong>en</strong> Konieczny<br />
CRIL - CNRS<br />
Université d’Artois<br />
62300 L<strong>en</strong>s (France)<br />
konieczny@cril.univ-artois.fr<br />
ABSTRACT. We propose in this paper a new family of belief merging operators, that is based on<br />
a game betwe<strong>en</strong> sources : until a coher<strong>en</strong>t set of sources is reached, at each round a contest<br />
is organized to find out the weakest sources, th<strong>en</strong> those sources has to conce<strong>de</strong> (weak<strong>en</strong> their<br />
point of view). This i<strong>de</strong>a leads to numerous new interesting operators (<strong>de</strong>p<strong>en</strong>ding of the exact<br />
meaning of “weakest” and “conce<strong>de</strong>”, that gives the two parameters for this family) and op<strong>en</strong>s<br />
new perspectives for belief merging. Some existing operators are also recovered as particular<br />
cases. Those operators can be se<strong>en</strong> as a special case of Booth’s Belief Negotiation Mo<strong>de</strong>ls<br />
[BOO 02], but the achieved restriction forms a consist<strong>en</strong>t family of merging operators that<br />
worths to be studied on its own.<br />
KEYWORDS: belief merging, belief negotiation.<br />
1. Introduction<br />
The problem of (propositional) belief merging [REV 97, LIN 99, LIB 98, KON 99,<br />
KON 02a, KON 04] can be summarized by the following question: giv<strong>en</strong> a set of<br />
sources (propositional belief bases) that are (typically) mutually inconsist<strong>en</strong>t, how do<br />
we obtain a coher<strong>en</strong>t belief base reflecting the beliefs of the set?<br />
The i<strong>de</strong>a here is that some/each sources has to conce<strong>de</strong> on some points in or<strong>de</strong>r<br />
to solve the conflicts. If one has some notion of relative reliability betwe<strong>en</strong> sources,<br />
it is <strong>en</strong>ough and s<strong>en</strong>sible to force the less reliable ones to give up first. There is<br />
a variety of differ<strong>en</strong>t means to do that, which has provi<strong>de</strong>d a large literature, e.g.<br />
[CHO 93, CHO 95, CHO 98, BEN 98a, BEN 98b]. But oft<strong>en</strong> we do not have such<br />
information, and ev<strong>en</strong> if we get it, it remains the more fundam<strong>en</strong>tal problem of how<br />
to merge sources of equal reliability [KON 99, KON 02a].<br />
In this paper we will investigate the merging methods based on a notion of game<br />
betwe<strong>en</strong> the sources. The intuitive i<strong>de</strong>a is simple: wh<strong>en</strong> trying to impose its wish,<br />
each source will try to form some coalition with the closest (more compatible) other<br />
Journal of Applied Non-Classical Logics.Volume 14 – n 3/2004, pages 275 to 294<br />
211
276 JANCL – 14/2004. Uncertainty in multiple data sources<br />
sources. So the source that is the “furthest” from the other ones will certainly be the<br />
weakest one. And it will be that source that will have to conce<strong>de</strong> first. In this work,<br />
we will not focus on how the coalitions form, we only take this i<strong>de</strong>a to <strong><strong>de</strong>s</strong>ignate the<br />
weakest ones.<br />
So the merging is based on the following game: until a coher<strong>en</strong>t set of sources<br />
is reached, at each round a contest is organized to find out the weakest sources, th<strong>en</strong><br />
those sources have to conce<strong>de</strong> (weak<strong>en</strong> their point of view).<br />
We can state several intuitions and justifications for the use of such operators.<br />
We have already giv<strong>en</strong> the first one: coalition with near-min<strong>de</strong>d sources. In a group<br />
<strong>de</strong>cision process betwe<strong>en</strong> rational sources, it can be s<strong>en</strong>sible to expect the sources<br />
to look for near-min<strong>de</strong>d sources in or<strong>de</strong>r to find help to <strong>de</strong>f<strong>en</strong>d their view, so the<br />
“furthest” source is the more likely to have to conce<strong>de</strong> on its view.<br />
A second intuition is the one giv<strong>en</strong> by a social pressure on the sources. Wh<strong>en</strong><br />
confronting several points of view, usually people that have the more exotic views try<br />
to change their opinion in or<strong>de</strong>r to be accepted by the other members of the group, so<br />
opinions that are <strong>de</strong>f<strong>en</strong><strong>de</strong>d by the least number of sources are usually giv<strong>en</strong> up more<br />
easily in the negotiation process.<br />
A last intuition that gives the main rationale for that kind of operator is Condorcet’s<br />
Jury theorem. This theorem states that if all the members of a jury are reliable (in the<br />
s<strong>en</strong>se that they have greater than ¡£¢¥¤ chance to find the truth), th<strong>en</strong> list<strong>en</strong>ing to the<br />
majority is the more rational choice.<br />
After stating some useful <strong>de</strong>finitions and notations in Section 2, we will <strong>de</strong>fine the<br />
new family of operators we propose in Section 3. The <strong>de</strong>finition will use a notion<br />
of weak<strong>en</strong>ing and choice functions. We will explore these notions in Section 4. We<br />
will give some examples of specific operators in Section 5 in or<strong>de</strong>r to illustrate their<br />
behaviour. We will look at the logical properties of those operators in Section 6. In<br />
Section 7 we will look at the links betwe<strong>en</strong> this work and related works (especially<br />
Booth’s proposal [BOO 01, BOO 02]). We conclu<strong>de</strong> in Section 8 with some op<strong>en</strong><br />
issues and perspectives of this work.<br />
2. Definitions<br />
of propositional<br />
We consi<strong>de</strong>r a propositional ¦ language over a finite § alphabet<br />
symbols. An interpretation is a function § from ¨<br />
¢©<br />
to . The set of all the interpretations<br />
is <strong>de</strong>noted . An interpretation is a mo<strong>de</strong>l of a formula , noted ,<br />
if and only if it makes it true in the usual classical truth functional way. Let be a<br />
formula, <strong>de</strong>notes the set of mo<strong>de</strong>ls of , ¨!#"$%##<br />
i.e. .<br />
Conversely, & let be a set of '(*)+,&- interpretations, <strong>de</strong>notes the formula (up to<br />
logical equival<strong>en</strong>ce) whose set of mo<strong>de</strong>ls & is .<br />
212
d<br />
Belief base merging as a game 277<br />
A belief base is a consist<strong>en</strong>t propositional formula (or, equival<strong>en</strong>tly, a finite<br />
consist<strong>en</strong>t set of propositional formulae consi<strong>de</strong>red conjunctively). Let us note . the<br />
set of all belief bases.<br />
/<br />
©1010102©<br />
43 Let 5 be belief bases (not necessarily differ<strong>en</strong>t). We call belief profile<br />
the 6 multi-set consisting of 5 those belief 6 #78/<br />
©1010202©<br />
43( bases: (i.e. two sources<br />
can have the same belief base). We 6 note the conjunction of the belief bases 6 of ,<br />
6#9/;:=
t4x6CG? 6 –<br />
– 6y a#z If , {|"}t4x6] th<strong>en</strong> ~y a#z s.t.<br />
|‡ˆ„T –<br />
– |aP„T If , |a#z th<strong>en</strong><br />
<br />
278 JANCL – 14/2004. Uncertainty in multiple data sources<br />
Booth’s work, one can use the whole history of profiles to make the choice. We think<br />
that those hypothesis are more realistic (and necessary) on a belief merging point<br />
of view, whereas Booth’s framework allows to mo<strong>de</strong>l more g<strong>en</strong>eralized negotiation<br />
schemes, where one can <strong>de</strong>ci<strong>de</strong> for example that each source has to weak<strong>en</strong> one after<br />
the other (see Section 7 for a <strong>de</strong>eper comparison of the two approaches).<br />
DEFINITION 1. — A choice function is a function tuH_vJw_ such that:<br />
– If 6a€6 , th<strong>en</strong> t4x6]Yat4x67<br />
The choice function aims to find which are the sources that must weak<strong>en</strong> at a giv<strong>en</strong><br />
round (see <strong>de</strong>finition 3). As the weak<strong>en</strong>ing function aims to weak<strong>en</strong> the belief base,<br />
and as there is no weaker base than a tautological one, the second condition states that<br />
at least one non-tautological base must be selected. So it states that at each round at<br />
least one base will be weak<strong>en</strong>. This condition is necessary to <strong>en</strong>sures to always reach<br />
a result with Belief Game Mo<strong>de</strong>l. Note that a consequ<strong>en</strong>ce of this condition is that<br />
we t4x6]‚y ƒ have as soon as the profile contains at least one non-tautological base.<br />
Last condition is an irrelevance of syntax condition. It states that the selection of the<br />
bases to weak<strong>en</strong> does not <strong>de</strong>p<strong>en</strong>d on the particular form of the bases, but only on their<br />
informational cont<strong>en</strong>t. Note that we also have an additional property: anonymity, that<br />
means that the result does not <strong>de</strong>p<strong>en</strong>d on the “name” of the source, but only on its<br />
point of view. This is due to the fact that we work with multi-sets, that are equival<strong>en</strong>t<br />
by permutation. If one works with another repres<strong>en</strong>tation (or<strong>de</strong>red lists of sources for<br />
example), this anonymity property can be giv<strong>en</strong> by the last condition, provi<strong>de</strong>d that<br />
the equival<strong>en</strong>ce betwe<strong>en</strong> two belief profiles is rightly <strong>de</strong>fined (as in Section 2).<br />
DEFINITION 2. — A weak<strong>en</strong>ing function is a function „vH…¦J†¦ such that:<br />
– If |a , th<strong>en</strong> „T7YaP„T <br />
The weak<strong>en</strong>ing function aims to give the new beliefs of a source that have be<strong>en</strong><br />
chos<strong>en</strong> to be weak<strong>en</strong>ed. The two first conditions <strong>en</strong>sure that the base will be replaced<br />
by a strictly weaker one (unless the base is already a tautological one). The last condition<br />
is an irrelevance of syntax requirem<strong>en</strong>t : the result of the weak<strong>en</strong>ing must only<br />
<strong>de</strong>p<strong>en</strong>d on the information conveyed by the base, not on its syntactical form.<br />
We ext<strong>en</strong>d the weak<strong>en</strong>ing functions on belief profiles as follows: let 6 be a subset<br />
of 6 ,<br />
„pNOUx6] ‰<br />
„T(> ‰<br />
[ NOU<br />
[ N4Š!NOU<br />
This means that we only weak<strong>en</strong> the belief bases of 6 that are in 6T , and the other<br />
ones do not change.<br />
214
¤<br />
©<br />
¢<br />
¢<br />
©<br />
©<br />
¢<br />
Belief base merging as a game 279<br />
DEFINITION 3. — A Belief Game Mo<strong>de</strong>l is a ‹ŒŽ t pair<br />
function „ and is a weak<strong>en</strong>ing function.<br />
The solution to a belief 6 profile for a Belief Game ‹ Mo<strong>de</strong>l<br />
, is the belief profile 6T’ , <strong>de</strong>fined as:<br />
‹#x6]<br />
6T“”#6 –<br />
6–•˜—/€„Oš…› N8œž x6Ÿ•*<br />
–<br />
6T’ – is the 6–• first that is consist<strong>en</strong>t<br />
„ where t is a choice<br />
‘ t<br />
„ , noted<br />
So the solution to a belief profile is the result of a game on the beliefs of the<br />
sources. At each round there is a contest to find out the weakest bases (the losers), and<br />
the losers have to conce<strong>de</strong> on their belief by weak<strong>en</strong>ing them.<br />
It may prove reasonable that each source has its own weak<strong>en</strong>ing function, that <strong>de</strong>notes<br />
differ<strong>en</strong>t conceding politics. After all, the point is that the source has to weak<strong>en</strong><br />
its beliefs, not how she does so. So we can figure out a g<strong>en</strong>eralization of the belief<br />
game mo<strong>de</strong>l, where there is no one weak<strong>en</strong>ing function, but one for each source 2 . But<br />
this is not the point in this paper, so we will suppose that there is a unique weak<strong>en</strong>ing<br />
function for all the sources.<br />
In some cases, the result of the merging has to obey some constraints (physical<br />
constraints, norms, etc...). We will assume that these integrity constraints are <strong>en</strong>co<strong>de</strong>d<br />
as a propositional formula (a belief base), and we will note this base . Th<strong>en</strong> we<br />
introduce the following notion:<br />
as:<br />
mt<br />
DEFINITION 4. — The solution to a belief profile 6 for a Belief Game Mo<strong>de</strong>l ‹¡<br />
’ <strong>de</strong>fined<br />
„ un<strong>de</strong>r the integrity constraints , noted ‹£¢(x6] , is the belief profile 6<br />
6T“”#6 –<br />
6–•˜—/€„Oš…› N œ x6Ÿ•*<br />
–<br />
6 ’ – is the 6–• first that is consist<strong>en</strong>t with<br />
Oft<strong>en</strong> in the following in this paper we will call result of the merging operator (Belief<br />
Game Mo<strong>de</strong>l), the belief 6 ’ :( base . This abuse of notation is not problematic,<br />
since this belief base <strong>de</strong>notes the cons<strong>en</strong>sus point obtained by the belief 6 profile<br />
solution of the Belief Game Mo<strong>de</strong>l process.<br />
Note that the <strong>de</strong>finition of the Belief Game Mo<strong>de</strong>l and of the weak<strong>en</strong>ing and choice<br />
functions <strong>en</strong>sures that each belief profile 6 has a solution as soon as the constraints<br />
are consist<strong>en</strong>t.<br />
¢<br />
’<br />
. Technically, it forces to drop out the weak<strong>en</strong>ing function from the belief game mo<strong>de</strong>l ¥ and<br />
to put it in the input, i.e. the input would be a list of sources ¦ that are couples compound of a<br />
belief base and a weak<strong>en</strong>ing function: §,Ÿ©«ª7¬8©«<br />
215
¢<br />
©<br />
¢<br />
¢<br />
<br />
280 JANCL – 14/2004. Uncertainty in multiple data sources<br />
THEOREM 5. — Let 6 be a belief profile, and be a belief base. If 6 is non-empty<br />
and is consist<strong>en</strong>t, 6 th<strong>en</strong><br />
of rounds.<br />
To prove that 6<br />
¢<br />
:O is consist<strong>en</strong>t and 6<br />
’<br />
’ is reached for a finite number<br />
that, at a giv<strong>en</strong> round, 6–• either is consist<strong>en</strong>t with , 6 ’ ®6Ÿ• so , and the process<br />
<strong>en</strong>d, so we are done. 6Ÿ• Or is not consist<strong>en</strong>t with , in this case there is an other<br />
round. From the <strong>de</strong>fintion of the weak<strong>en</strong>ing of a profile, the resulting 6Ÿ•˜—/ profile is<br />
logically strictly weaker 6–• than , that means that each base 6–•˜—/ of is either logically<br />
equival<strong>en</strong>t to the corresponding base 6–• in , or logically strictly weaker than this base<br />
(see <strong>de</strong>finition 2). Now just note that the logically weakest profile, is the one where<br />
each base is equival<strong>en</strong>t z to , and that this profile is achievable from every giv<strong>en</strong> belief<br />
profile by successive applications of the weak<strong>en</strong>ing function. Since we work in a finite<br />
propositional logic setting, this can be done by a finite number of rounds. Finally,<br />
note that this profile is consist<strong>en</strong>t with every consist<strong>en</strong>t integrity constraint . So if<br />
the process <strong><strong>de</strong>s</strong>cribed in <strong>de</strong>finition 4 has not stopped before, it is guaranteed to give a<br />
result with this belief profile.<br />
’ is reached for a finite number of rounds, it is suffici<strong>en</strong>t to note<br />
4. Weak<strong>en</strong>ing and choice functions<br />
In or<strong>de</strong>r to <strong>de</strong>fine a particular Belief Game Mo<strong>de</strong>l, we have to choose a choice<br />
function and a weak<strong>en</strong>ing function. We will give in this section some natural choices<br />
for these functions and see what are the resulting BGM operators.<br />
4.1. Weak<strong>en</strong>ing functions<br />
Let us first turn out to weak<strong>en</strong>ing functions. Can we find a “natural” one ? In fact<br />
it is a difficult task, since the exact choice of a weak<strong>en</strong>ing function <strong>de</strong>p<strong>en</strong>ds on the<br />
expected behaviour for the Belief Game Mo<strong>de</strong>l and <strong>de</strong>p<strong>en</strong>ds also on the exist<strong>en</strong>ce of<br />
some “prefer<strong>en</strong>tial” information. But if we have no such additional information, we<br />
have at least two natural candidates : drastic weak<strong>en</strong>ing and dilation.<br />
DEFINITION 6. — Let be a belief base. The drastic weak<strong>en</strong>ing function forget all<br />
the information about one source, i.e. : „p¯=z .<br />
This weak<strong>en</strong>ing function simply forget all the information in !<br />
After this rough function, let us see a more fine grained one. Let us first recall<br />
what is the Hamming’s distance betwe<strong>en</strong> interpretations (also called Dalal’s distance<br />
[DAL 88]) since we will use it several times in this paper.<br />
DEFINITION 7. — The Hamming distance betwe<strong>en</strong> interpretations is the number of<br />
propositional symbols on which the two interpretations differ. Let and 8 be two<br />
interpretations, th<strong>en</strong><br />
°g,<br />
Yv@AB¨^‚"V§Ž…=^y ± 7^<br />
<br />
<br />
216
À<br />
– <br />
©<br />
©<br />
¢<br />
iff $:$»½¼¿¾<br />
©<br />
Belief base merging as a game 281<br />
Th<strong>en</strong> the dilation weak<strong>en</strong>ing function is <strong>de</strong>fined as :<br />
DEFINITION 8. — Let be a belief base. The dilation weak<strong>en</strong>ing function is <strong>de</strong>fined<br />
as :<br />
<br />
*„O²£B¨!~"V³{<br />
9r°‚m<br />
Y´<br />
This weak<strong>en</strong>ing function takes as mo<strong>de</strong>ls of the weak<strong>en</strong>ed base „O² , all the<br />
mo<strong>de</strong>ls that are at an Hamming distance less or equal to 1 from the mo<strong>de</strong>ls of , i.e.<br />
all the mo<strong>de</strong>ls that are in the base and all the mo<strong>de</strong>ls that are achievable by flipping<br />
the truth value of one propositional symbol in a mo<strong>de</strong>l of . This is close to the<br />
dilation operator used in morpho-logics [BLO 00, BLO 04].<br />
4.2. Choice functions<br />
Let us now turn to choice functions. The aim of this function is to <strong>de</strong>termine the<br />
“losers”, that are the sources that have to conce<strong>de</strong> by weak<strong>en</strong>ing their beliefs at a giv<strong>en</strong><br />
round.<br />
One of the simplest choice functions is i<strong>de</strong>ntity (<strong>de</strong>notedtµ•m ). It is not the expected<br />
behaviour for this function, but it can prove the rationality of our operators if, ev<strong>en</strong> in<br />
this case, we obtain a s<strong>en</strong>sible merging.<br />
We will focus on two families of choice functions. The first one is mo<strong>de</strong>l-based,<br />
the second one is formula-based. We think that most of the s<strong>en</strong>sible choice functions<br />
belong to one of those families.<br />
4.2.1. Mo<strong>de</strong>l-based choice functions<br />
We will focus here on some mo<strong>de</strong>lizations of what can be called “social pressure”,<br />
and can be viewed as a majority principle. Namely, at each round it is the “furthest”<br />
sources from the group that will conce<strong>de</strong>. The exact choice of the meaning of “furthest”<br />
will fix the chos<strong>en</strong> operator for this family. Technically we will use a distance<br />
betwe<strong>en</strong> belief bases and an aggregation function to evaluate the distance of a belief<br />
base with respect to the others.<br />
We will start from the <strong>de</strong>finition of the distance betwe<strong>en</strong> two belief bases.<br />
DEFINITION 9. — A (pseudo)distance 3 betwe<strong>en</strong> two belief bases is a function ·H<br />
¦ ¸£¦J†¹mº such that:<br />
– <br />
»Y<br />
»Y9» © <br />
. Remark that we miss an important property of distances: we have only Álm¨8ªÂ¨pÃÄnÅorÆ if ¨±o<br />
¨ Ã , but not the only if part. Remark also that we do not require the triangular inequality.<br />
217
– Çi7<br />
– °g<br />
©<br />
©<br />
¢<br />
<br />
¢<br />
‰<br />
Ñ<br />
©<br />
©<br />
Ó<br />
©<br />
¢<br />
282 JANCL – 14/2004. Uncertainty in multiple data sources<br />
Two examples of such distances are :<br />
$:$»½¼¿¾ if<br />
otherwise<br />
»7Y<br />
‰(Ð<br />
<br />
DEFINITION 10. — An aggregation function is a total function ' associating a<br />
nonnegative integer to every finite tuple of nonnegative integers and verifying (non<strong>de</strong>creasingness),<br />
(minimality) and (i<strong>de</strong>ntity).<br />
– ÑÒ´Ó if , 'pmÑ/<br />
©1010102©<br />
th<strong>en</strong><br />
Ñ43(G´'pmÑ/<br />
©1010102©<br />
©1010102©<br />
©1010102©<br />
. Ñ43(<br />
(non-<strong>de</strong>creasingness)<br />
'pmÑ/<br />
©1010202©<br />
Ñ43(Y<br />
01010<br />
±Ñ43$<br />
– . (minimality)<br />
– for every nonnegative Ñ integer 'pmÑY±Ñ , . (i<strong>de</strong>ntity)<br />
if and only if Ñ/–<br />
We say that an aggregation function is symmetric if it also satisfies :<br />
– For any Ô permutation 'pmÑ/<br />
©1010102©<br />
Ñ43(Y9'pmÑ4Õ› /Ö , Ñ4Õ› 3… Ð×<br />
©1010201©<br />
DEFINITION 11. — A mo<strong>de</strong>l-based choice function t <br />
is <strong>de</strong>fined as :<br />
(symmetry)<br />
Y<br />
ÈÊÉÌË<br />
Í¥ÎÏ<br />
Í U ÎÏ<br />
U °gm<br />
Ð×<br />
434B is maximal <br />
where Ø is an aggregation function, and is a distance betwe<strong>en</strong> belief bases.<br />
We say that the mo<strong>de</strong>l-based choice function is symmetric if the aggregation function<br />
is symmetric.<br />
We will focus on some specific aggregation functions in this paper, but we can use<br />
differ<strong>en</strong>t aggregation functions here. In particular we will only focus on symmetrical<br />
aggregation functions in this paper (to fit with choice function requirem<strong>en</strong>ts) but note<br />
that the <strong>de</strong>finition allows non-symmetrical functions. This allows to <strong>de</strong>fine operators<br />
that are not anonymous, i.e. where each base has not the same importance. So one<br />
can use priorities (a weight or a pre-or<strong>de</strong>r on the sources) for <strong>de</strong>noting differ<strong>en</strong>t level<br />
of reliability, differ<strong>en</strong>t hierarchical importance, etc.<br />
We will use in the following as examples of aggregation functions, two typical<br />
ones, the sum (noted Ù ) and the maximum (noted ^1Ñ ).<br />
©1010102©<br />
t <br />
x6]¨4•Y"±6Ø(74•<br />
/2<br />
74•<br />
4.2.2. Formula-based choice functions<br />
Not all interesting choice functions are captured in the <strong>de</strong>finition giv<strong>en</strong> in the previous<br />
section. In particular, a lot of interesting choice functions can be <strong>de</strong>fined by using<br />
maximal consist<strong>en</strong>t subsets. Note, however that, conversely to usual formula-based<br />
merging operators [BAR 92, KON 00], we use multi-sets instead of simple sets.<br />
DEFINITION 12. — MAXCONSx6C Let be the set of the maxcons 6 of , i.e. the<br />
maximal (with respect to multi-set inclusion) consist<strong>en</strong>t subsets 6 of . Formally,<br />
is the set of all multi-sets Ú such that:<br />
MAXCONSx6C<br />
– ÚÛ? 6 and<br />
218
– ØÝŸÞ / <br />
©<br />
©<br />
©<br />
©<br />
©<br />
©<br />
©<br />
<br />
<br />
<br />
Belief base merging as a game 283<br />
– Ú¡Ü-Ú~p? 6<br />
if Ú-ž<br />
0<br />
th<strong>en</strong><br />
DEFINITION 13. — A formula-based choice tÝŸÞ function is a function of the set of<br />
the maxcons 6 of and the belief base, i.e. :<br />
MAXCONSx6]x is minimal <br />
t ÝTÞ x6]¨p•G"6Ø»4•<br />
Examples of the use of maxcons are numerous, let us see two of them.<br />
DEFINITION 14. —<br />
– ØÝŸÞ ` <br />
<br />
The first function computes the number of maxcons the belief base belongs to. The<br />
second function computes the size of the biggest maxcons the belief base belongs to.<br />
MAXCONSx6]xYvÈ£<strong>à</strong>á4B¨â@AÚKGÚß" MAXCONSB6C and |"ÒÚ<br />
We will note t ÝŸÞ /<br />
(respectively t ÝŸÞ `<br />
) the formula-based choice function that use<br />
Ø;ÝŸÞ /<br />
(resp. Ø;ÝŸÞ `<br />
).<br />
MAXCONSx6]xYv@ˆ¨ÚßÚß" MAXCONSx6] and |"ÒÚ<br />
5. Instantiating the BGM framework<br />
In this section we will try to illustrate how interesting the <strong>de</strong>fined Belief Game<br />
Mo<strong>de</strong>l framework is by giving several examples. We will first see some of the simplest<br />
operators that we can <strong>de</strong>fine with this framework. Th<strong>en</strong> we will illustrate the behaviour<br />
of more complex operators on a typical merging example.<br />
5.1. Some simple examples<br />
Let us first see what operators are obtained with the simplest weak<strong>en</strong>ing and choice<br />
functions (that means that we will either choose the weak<strong>en</strong>ing function to be the<br />
drastic one, or the choice function to be i<strong>de</strong>ntity).<br />
– t½•,<br />
„p¯= : In this case the belief base result of the BGM on 6 un<strong>de</strong>r the constraint<br />
is the conjunction of all the bases of the profile with the integrity constraints<br />
( 69:£ ) if this conjunction is consist<strong>en</strong>t, and otherwise. This operator is called<br />
the basic merging operator [KON 99].<br />
t½•, –<br />
dilation. This gives the well known mo<strong>de</strong>l-based merging ã …ä<br />
operator<br />
: In this case, at each step of the game, each source weak<strong>en</strong>s using ÐåGæ*ç<br />
„O²<br />
<strong>de</strong>fined<br />
in [REV 93, REV 97, KON 02a].<br />
– t è<br />
if it is unique and consist<strong>en</strong>t with the constraints , and it is simply otherwise. This<br />
operator is a new one. It is interesting since it can be viewed as a g<strong>en</strong>eralized conjunction<br />
: it gives the conjunction of all the bases and the constraints if it is consist<strong>en</strong>t,<br />
Ðé<br />
„p¯= : Here, the result is the cardinality-maximal consist<strong>en</strong>t subset of 6<br />
219
õ<br />
ë<br />
©<br />
<br />
ë<br />
©<br />
<br />
<br />
©<br />
<br />
<br />
©<br />
©<br />
©<br />
<br />
©<br />
©<br />
<br />
©<br />
<br />
<br />
©<br />
<br />
©<br />
284 JANCL – 14/2004. Uncertainty in multiple data sources<br />
but if it is not, it tries to find the result by doing the least number of repairs (forget<br />
one belief base) of the belief profile. If there is no ambiguity on the correction (i.e. a<br />
unique cardinality-maxcons), th<strong>en</strong> it accepts it as the result.<br />
t è „p¯ê – : This operator gives as result the conjunction of all the formulas<br />
that belong to all maxcons (also called free formulas in [BEN 97, BEN 99]) and the<br />
integrity constraints if it is consist<strong>en</strong>t, and otherwise.<br />
ÐåYæxç<br />
t ÝŸÞ / ©<br />
„p¯= – : This operator gives the conjunction of the formulas that belong to<br />
the maximum number of maxcons and the integrity constraints if consist<strong>en</strong>t, and<br />
otherwise.<br />
tÝŸÞ ` ©<br />
„p¯= – : In this case, the belief base result of the merging is the conjunction<br />
of the belief bases that belong to the biggest maxcons for cardinality and the integrity<br />
constraints if consist<strong>en</strong>t, and otherwise.<br />
These operators are not logically in<strong>de</strong>p<strong>en</strong><strong>de</strong>nt, some of them are logically stronger<br />
than others, as stated in the following proposition.<br />
THEOREM 15. — In figure 1 an arrow betwe<strong>en</strong> an I operator and an operator<br />
) means that I operator is logically stronger 4 (or less cautious) than<br />
operator . Results obtained by transitivity are not repres<strong>en</strong>ted.<br />
ë<br />
(Iwì4J<br />
Ðé<br />
tÝŸÞ ` ©<br />
t è<br />
„p¯=<br />
„p¯ê<br />
tµ•,<br />
tµ•,<br />
„p¯ê<br />
©1í<br />
ÐåGæ*ç<br />
tÝŸÞ / ©<br />
t è<br />
„p¯=<br />
„p¯ê<br />
Figure 1. Cautiousness<br />
5.2. An example<br />
We will see on an example [REV 97], what is the behaviour or some BGM opera-<br />
Ð×2î Ð×ïð«ñ<br />
tors, t …ä „O² namely t …ä „O² the tÝTÞ / ©<br />
„O² operators mtÝŸÞ ` ©<br />
„O² , , and .<br />
Here is the example : There 6 ¨/<br />
are three sources with the<br />
Ú€;»`;ó<br />
,<br />
following belief bases Ú€/2ó<br />
¢M©©B¢ ¢M©B¢M©<br />
¨£<br />
+z so .<br />
¨µ<br />
ÚP7(ò½]ô¨µ<br />
©©<br />
,<br />
. There are no constraints on the result,<br />
»`<br />
»ò<br />
©B¢M©B¢<br />
¢M©B¢M©<br />
…©B¢M©<br />
. An h operator is logically stronger than an ö operator iff for all s profile hbl7sŸn8÷Cö‚l7s–n , ,<br />
hbl7sŸn where <strong>de</strong>notes the belief base result of the h BGM on the s profile .<br />
220
Belief base merging as a game 285<br />
Ð×î<br />
©<br />
„O² – : 6 As is not consist<strong>en</strong>t, let us do the first /<br />
©<br />
»`;£<br />
ä<br />
round. é<br />
t é<br />
©<br />
»ò;ß<br />
<br />
, »`<br />
©<br />
»ò;ß ø , . Ø<br />
¢<br />
/1Û<br />
<br />
N »`;ß ø So ,<br />
é<br />
Ð×î<br />
/<br />
N<br />
. That gives t …ä x6CrR¨»ò<br />
<br />
. So »ò is replaced 5 by „O²£»ò;r<br />
»ò;róù N Ø<br />
Ø ,<br />
<br />
©<br />
<br />
©©B¢<br />
<br />
©<br />
<br />
©B¢M©<br />
<br />
©<br />
<br />
¢M©©<br />
<br />
<br />
. We have not yet reached a consist<strong>en</strong>t<br />
, so let us do a further round. Let us first compute the new dis-<br />
é<br />
6 ©<br />
'»!)…+B¨£<br />
©g©<br />
(`;ú<br />
¢<br />
tances.<br />
/<br />
©<br />
(ò;ú<br />
¢<br />
, 7»`<br />
©<br />
(ò½K<br />
<br />
, . Ø N /2ú<br />
¢<br />
So é<br />
<br />
,<br />
é /<br />
Ð×2î<br />
<br />
, . That t …ä © <br />
gives . »` So is re-<br />
Ø<br />
, <br />
N »`;¿ N 7(ò½¿<br />
x6]¿Û¨»` »ò<br />
Ø<br />
placed „O²£7»`;9'(*)vB¨µ<br />
¢M©©B¢<br />
<br />
©<br />
<br />
¢©B¢M©<br />
<br />
©<br />
<br />
…©……©B¢<br />
<br />
©<br />
<br />
¢M©B¢©x¢<br />
<br />
©<br />
<br />
¢M©©<br />
<br />
©<br />
<br />
©B¢M©<br />
<br />
<br />
by<br />
»ò and is replaced „O²»ò;Pû'(*)+¨µ<br />
©…©…<br />
<br />
©<br />
<br />
©©B¢<br />
<br />
©<br />
<br />
©B¢M©<br />
<br />
©<br />
<br />
¢M©©<br />
<br />
©<br />
<br />
¢M©<br />
by<br />
<br />
©<br />
<br />
¢M©B¢M©<br />
<br />
<br />
…©B¢ © ©B¢©x¢<br />
Ð î<br />
äYþ ÿ ¡£¢ Y¨µ<br />
¢M©B¢M©<br />
<br />
©<br />
<br />
©B¢M©<br />
<br />
<br />
Ð× ïðÂñ šý Ú€x6]ü is<br />
. We have reached a consist<strong>en</strong>t belief profile, so the result<br />
<br />
.<br />
t<br />
©<br />
– 6 : As is not consist<strong>en</strong>t, let us do the /<br />
©<br />
»`;Y<br />
¢<br />
first round. åGæ*ç<br />
ä<br />
åGæ*ç<br />
„O² åYæxç<br />
<br />
,<br />
7»`<br />
©<br />
(ò½V®ø , . Ø<br />
©<br />
/2}<br />
<br />
So Ø N »`½VWø , Ø N »ò;V<br />
;/ Ð× N ïð«ñ<br />
»ò;V<br />
,<br />
t …ä x6]w ¨»`<br />
©<br />
»ò<br />
<br />
. That »` gives . So „O²£»`;w<br />
ø ¢M©©B¢<br />
is<br />
©<br />
replaced<br />
¢M©B¢M©<br />
by<br />
©<br />
<br />
©©B¢<br />
<br />
©<br />
<br />
¢M©B¢M©B¢<br />
<br />
©<br />
<br />
¢M©©<br />
<br />
©<br />
<br />
…©x¢©<br />
<br />
<br />
, and (ò is replaced<br />
'»!)…+B¨£<br />
„O²»ò;i<br />
…©……© ©<br />
<br />
©…©x¢<br />
<br />
©<br />
<br />
©B¢M©<br />
<br />
©<br />
<br />
¢M©©<br />
<br />
<br />
by ïð«ñ '(*)vB¨µ<br />
Ð <br />
consist<strong>en</strong>t, so Ú€x6 ü š…ý äYþ ÿ ¡¤¢2Y#¨µ<br />
©B¢M©<br />
<br />
<br />
the result is<br />
. The obtained profile is<br />
<br />
.<br />
tÝŸÞ / ©<br />
„O² – 6 : is not consist<strong>en</strong>t, MAXCONSx6]9 ¨£¨/<br />
©<br />
»`<br />
©<br />
¨»ò<br />
£<br />
and<br />
Ø ÝŸÞ /<br />
N /2 Ø ÝTÞ /<br />
N 7(`½ Ø ÝŸÞ /<br />
N »ò; <br />
<br />
So , t ÝŸÞ / x6] <br />
and<br />
. So we weak<strong>en</strong> the three bases, which gives respectively „O²£/1 <br />
6 ©B¢M©B¢<br />
<br />
©<br />
<br />
¢M©B¢M©<br />
<br />
©<br />
<br />
©B¢M©<br />
<br />
©<br />
<br />
¢M©B¢M©»¢<br />
<br />
©<br />
<br />
©©B¢<br />
<br />
©<br />
<br />
¢M©©<br />
<br />
©<br />
<br />
©©<br />
<br />
<br />
, „O²£»`;}<br />
¢M©©B¢ © ¢M©B¢M© © ©©B¢ © ¢M©B¢M©B¢ © ¢M©© © ©B¢M© <br />
'»!)…+B¨£<br />
<br />
'»!)…+B¨£ ©© ©<br />
<br />
©©B¢<br />
<br />
©<br />
<br />
©B¢M©<br />
<br />
©<br />
<br />
¢M©©<br />
<br />
<br />
'»!)…+T¨£<br />
Ð <br />
the resulting ÚPB6 ü š¦¥¨§£© ¡ ¢ Y¨µ<br />
©B¢M©<br />
<br />
©<br />
<br />
base is<br />
, and „O²»ò; <br />
<br />
. This belief profile is consist<strong>en</strong>t, and<br />
©©B¢ ¢M©© <br />
<br />
.<br />
<br />
tÝŸÞ ` ©<br />
„O² – 6 : is not consist<strong>en</strong>t, and we MAXCONSx6] <br />
have<br />
©<br />
»`<br />
©<br />
¨»ò<br />
£<br />
. So ØÝTÞ `<br />
N 78/1 Ø;ÝŸÞ `<br />
N »`; ø and Ø;ÝŸÞ `<br />
N »ò; <br />
¨¨/<br />
, tÝŸÞ ` x6C ¨»ò<br />
<br />
and . »ò So is replaced „O²»ò; <br />
by<br />
<br />
consist<strong>en</strong>t, so we need one more MAXCONSx6] <br />
. The belief profile is still not<br />
round. Now we have<br />
©©<br />
<br />
©<br />
<br />
©©B¢<br />
<br />
©<br />
<br />
©B¢M©<br />
<br />
©<br />
<br />
¢M©©<br />
<br />
<br />
'»!)…+B¨£<br />
©<br />
»`<br />
©<br />
¨/<br />
©<br />
»ò<br />
£<br />
. So Ø ÝŸÞ `<br />
N /2 Ø ÝŸÞ `<br />
N »`; Ø ÝTÞ `<br />
N »ò; ø ,<br />
¨¨/<br />
tÝŸÞ ` x6] 6 and . So we weak<strong>en</strong> the three bases, which gives respectively<br />
„O²/2 '»!)…+B¨£<br />
©B¢M©B¢<br />
<br />
©<br />
<br />
¢M©B¢M©<br />
<br />
©<br />
<br />
©B¢M©<br />
<br />
©<br />
<br />
¢M©B¢M©B¢<br />
<br />
©<br />
<br />
©©B¢<br />
<br />
©<br />
<br />
¢©<br />
…© © ©…©… <br />
¢M©©B¢ © ¢M©B¢M© © ©©B¢ © ¢M©B¢M©B¢ © ¢M©© ©<br />
, „O²£»`; '(*)+¨µ <br />
©B¢M© <br />
, and „O²£»ò;ó '(*)vˆ¨µ<br />
…©……©<br />
<br />
©<br />
<br />
©…©x¢<br />
<br />
©<br />
<br />
©B¢M©<br />
<br />
©<br />
<br />
¢M©©<br />
<br />
<br />
<br />
Ð<br />
<br />
belief profile is consist<strong>en</strong>t, and the resulting ÚPx6 ü š¥§ ¡ ¢ <br />
base is<br />
¢M©B¢M©<br />
<br />
©<br />
<br />
©B¢M©<br />
<br />
©<br />
<br />
©©B¢<br />
<br />
©<br />
<br />
¢M©©<br />
<br />
<br />
¨£<br />
.<br />
As we can note, on this example the four operators give differ<strong>en</strong>t (non trivial)<br />
results. As all these operators take dilation as weak<strong>en</strong>ing functions, we sometimes<br />
have the interpretation as mo<strong>de</strong>l of the base result of the merging, whereas<br />
©©B¢<br />
<br />
. In or<strong>de</strong>r to avoid unnecessary notations, we do not use subscripts to <strong>de</strong>note the differ<strong>en</strong>t<br />
weak<strong>en</strong>ing steps of the bases, we simply replace the belief bases by their weak<strong>en</strong>ed counterparts.<br />
Hopefully, it can not lead to confusions.<br />
221<br />
©<br />
<br />
.<br />
.<br />
The
(IC5) 㣢»x6]/2(:uãÊ¢»x6T`;GãÊ¢(x6]/Y>r6`;<br />
©<br />
<br />
(IC6) If 㣢»x6]/2M:}ãÊ¢(B6T`½ is consist<strong>en</strong>t, th<strong>en</strong> 㣢»x6]/p>V6T`½YãÊ¢(x6]/2M:}ãÊ¢»x6T`½<br />
(IC7) 㣢 © x6](:” pì㣢 © ¢ x6]<br />
©<br />
<br />
286 JANCL – 14/2004. Uncertainty in multiple data sources<br />
it is a mo<strong>de</strong>l of none of the initial belief bases. This means that, conversely to usual<br />
formula-based merging operators [BAR 92, KON 00, KON 04], the result of the BGM<br />
does not (always) imply the disjunction of the belief bases of the profile.<br />
6. Logical properties<br />
Some work in belief merging aims at finding sets of axiomatic properties operators<br />
may exhibit in or<strong>de</strong>r to <strong>en</strong>sure the expected behaviour [REV 93, REV 97, LIB 98,<br />
KON 98, KON 99, KON 02b]. We focus here on the characterization of Integrity<br />
Constraints (IC) merging operators [KON 99, KON 02a].<br />
DEFINITION 16 (IC MERGING OPERATORS). — ã is an IC merging operator if and<br />
only if it satisfies the following properties:<br />
(IC0) 㣢»x6]Y±<br />
(IC1) If is consist<strong>en</strong>t, th<strong>en</strong> 㣢»x6] is consist<strong>en</strong>t<br />
(IC2) 6 If is consist<strong>en</strong>t with , ãÊ¢(B6CGa 6¿:i<br />
th<strong>en</strong><br />
(IC3) 6]/–a 6` If T/Ÿa p` and , ãÊ¢ © B6C/2Ga#㣢 x6`;<br />
th<strong>en</strong><br />
»`<br />
(IC4) /g If »`r and , 㣢»B¨/<br />
th<strong>en</strong><br />
is consist<strong>en</strong>t<br />
(:·(`<br />
G:ˆ/ is consist<strong>en</strong>t if and only if<br />
ãÊ¢»B¨/<br />
»`<br />
(IC8) If ãÊ¢ © x6](:” p` is consist<strong>en</strong>t, th<strong>en</strong> 㣢 © ¢ x6]Y㣢 © B6C<br />
The intuitive meaning of the properties is the following: (IC0) <strong>en</strong>sures that the<br />
result of merging satisfies the integrity constraints. (IC1) states that, if the integrity<br />
constraints are consist<strong>en</strong>t, th<strong>en</strong> the result of merging will be consist<strong>en</strong>t. (IC2) states<br />
that if possible, the result of merging is simply the conjunction of the belief bases with<br />
the integrity constraints. (IC3) is the principle of irrelevance of syntax : the result of<br />
merging has to <strong>de</strong>p<strong>en</strong>d only on the expressed opinions and not on their syntactical<br />
pres<strong>en</strong>tation. (IC4) is a fairness postulate meaning that the result of merging of two<br />
belief bases should not give prefer<strong>en</strong>ce to one of them (if it is consist<strong>en</strong>t with one of<br />
both, it has to be consist<strong>en</strong>t with the other one.) It is a symmetry condition, that aims<br />
to rule out operators that can give priority to one of the bases. (IC5) expresses the<br />
following i<strong>de</strong>a: if belief profiles are viewed as expressing the beliefs of the members<br />
of a group, th<strong>en</strong> if 6C/ (corresponding to a first group) compromises on a set of alternatives<br />
which I belongs to, and 6` (corresponding to a second group) compromises<br />
on another set of alternatives which contains I too, th<strong>en</strong> I has to be in the chos<strong>en</strong><br />
222
6<br />
©<br />
¢<br />
¢<br />
Belief base merging as a game 287<br />
alternatives if we join the two groups. (IC5) and (IC6) together state that if one could<br />
find two subgroups which agree on at least one alternative, th<strong>en</strong> the result of the global<br />
merging will be exactly those alternatives the two groups agree on. (IC7) and (IC8)<br />
state that the notion of clos<strong>en</strong>ess is well-behaved, i.e. that an alternative that is preferred<br />
among the possible alternatives ( T/ ), will remain preferred if one restricts the<br />
possible choices ( T/M:Ÿ p` ). For more explanations on those properties see [KON 02a].<br />
So, let us see now what are the properties of BGM operators.<br />
THEOREM 17. — BGM operators 6 satisfy properties (IC0), (IC1), (IC2), (IC3),<br />
(IC7), (IC8). They do not necessarily satisfy properties (IC4), (IC5), (IC6).<br />
PROOF. —<br />
(IC0) is satisfied by <strong>de</strong>finition (see <strong>de</strong>finition 4), since the result of the BGM process<br />
is the 6 ’ :” base , so it implies .<br />
(IC1) is satisfied by <strong>de</strong>finition (see theorem 5).<br />
(IC2) by <strong>de</strong>finition of the BGM (see <strong>de</strong>finition 4), 6 if is consist<strong>en</strong>t with ,<br />
th<strong>en</strong> the halting condition is satisfied before any weak<strong>en</strong>ing round, 6 ’ c6 so , and<br />
¢<br />
:” va 6¿:” ’<br />
(IC3) is a straighforward consequ<strong>en</strong>ce of the third condition of the weak<strong>en</strong>ing<br />
functions, and of the last condition of the choice functions.<br />
(IC7) and (IC8) are satisfied. First note that if ãÊ¢ © B6C:= p` is not consist<strong>en</strong>t, th<strong>en</strong><br />
© x6C :p p`”㣢 © ¢ x6] , so (IC7) is trivially satisfied. Now, in the case ãÊ¢ © x6] :<br />
ãÊ¢<br />
consist<strong>en</strong>t, let us prove (IC7) and (IC8), that is 㣢 © ¢ x6]Êaó㣢 © x6]:V p` .<br />
p`<br />
㣢 © x6]£ 6<br />
¢ ©<br />
’ :V T/ As , and by <strong>de</strong>finition 6<br />
¢ ©<br />
’ 4, is the first 6Ÿ• profile such that<br />
T/ is consist<strong>en</strong>t. As we also know that, by hypothesis, 6–•:· T/–:· p` is<br />
6–•:V<br />
6Ÿ• consist<strong>en</strong>t, is also the first profile such 6–•T:$ T/b:u p` that<br />
is consist<strong>en</strong>t. By<br />
<strong>de</strong>finition it means 6 that<br />
x6<br />
¢ © get<br />
© ¢ ¢<br />
K6–• . So we get that 6 ’ K6<br />
¢ © ¢ <br />
¢ © ¢ <br />
’<br />
:4 T/:p p` , which is exactly 㣢 © x6]â:p p`”㣢 © ¢ x6] .<br />
’<br />
¢ ©<br />
’ . From that we<br />
’ :4 T/2â:4 p`‚#6<br />
So, as stated in the previous proposition, BGM operators do not fit all properties<br />
of IC merging operators. On the other hand, we know for example that the ÐåYæxç<br />
operator<br />
…ä<br />
satisfies also (IC4), (IC5) [KON 02a]. So the question is to<br />
mt½•, „O²$‘ã<br />
know if we can <strong>en</strong>sure more logical properties by making some restrictions on the<br />
weak<strong>en</strong>ing and/or the choice functions.<br />
A first remark is that (IC4) cannot be proved to hold for any BGM operator, but it<br />
is satisfied for all the particular operators we have <strong>de</strong>fined in this paper.<br />
THEOREM 18. — If the weak<strong>en</strong>ing function is dilation or drastic weak<strong>en</strong>ing, and if<br />
the choice function is a symmetric mo<strong>de</strong>l-based choice function or the formula-based<br />
choice function t ÝŸÞ /<br />
or t ÝŸÞ `<br />
, th<strong>en</strong> the BGM operator satisfies (IC4).<br />
■<br />
. Defined from any choice function and any weak<strong>en</strong>ing function.<br />
223
©<br />
<br />
©<br />
<br />
©<br />
¢<br />
×<br />
¢<br />
©<br />
©<br />
¢<br />
©<br />
¢<br />
©<br />
©<br />
©<br />
©<br />
¢<br />
288 JANCL – 14/2004. Uncertainty in multiple data sources<br />
PROOF. — First note that for all those operators if the two bases are consist<strong>en</strong>t with<br />
the constraints /b:v(`£:u (i.e.<br />
), th<strong>en</strong> the result of the merging is (from (IC2))<br />
, so (IC4) is trivially satisfied. The interesting part of<br />
=c/:A»`C:Ê<br />
(IC4) is /Y:$»ề:” wh<strong>en</strong> is not consist<strong>en</strong>t.<br />
ãÊ¢»B¨/<br />
»`<br />
If the choice function is a symmetric mo<strong>de</strong>l-based choice function, th<strong>en</strong> simply<br />
note that the distance used to <strong>de</strong>fine the mo<strong>de</strong>l-based functions are symmetric by<br />
<strong>de</strong>finition, V‘7 so . Let note also that by <strong>de</strong>finition of the distance<br />
. So, now, using the symmetry condition of the aggregation<br />
function, it is easy to show ػ7<br />
x that , that<br />
K» © »7<br />
;<br />
; B}óØ»7 7<br />
<br />
means that » and are always both maximal, and that they always both have to be<br />
weak<strong>en</strong>ed. If we use drastic weak<strong>en</strong>ing as the weak<strong>en</strong>ing function, we have that<br />
» and are replaced z by z and . So the result of the merging is consist<strong>en</strong>t with and<br />
. So (IC4) is satisfied. If the weak<strong>en</strong>ing function is dilation, th<strong>en</strong> wh<strong>en</strong> we » weak<strong>en</strong><br />
» (resp. ) for 5¨ the time, we get as the result the 3<br />
base » (resp. ) that is the<br />
<br />
formula (up to logical equival<strong>en</strong>ce) whose mo<strong>de</strong>ls are the ones that are at a 3 (Dalal)<br />
5 »<br />
6Ÿ•”®¨ • ©<br />
» • 6Ÿ•G:Ò …°g»<br />
<br />
©<br />
”<br />
°g • ©<br />
»7ê<br />
°‚<br />
© • 8Y °‚» • ©<br />
(7Y 6–•M:£»<br />
distance less or equal to to mo<strong>de</strong>ls of (resp. ). So, let note the final profile of<br />
the BGM , if is consist<strong>en</strong>t, it means that ,<br />
but by symmetry it implies that , so (recall also that, by <strong>de</strong>finition of<br />
dilation and ) that is consist<strong>en</strong>t. So (IC4) is<br />
satisfied.<br />
If the choice function tÝTÞ /<br />
is tÝŸÞ `<br />
or , let us note that the maximal consist<strong>en</strong>t sets<br />
¨» and , so the and » are both minimal, and that they always both have<br />
<br />
¨ are<br />
to be weak<strong>en</strong>ed. If we use drastic weak<strong>en</strong>ing as the weak<strong>en</strong>ing function, we have that<br />
» and . So (IC4) is satisfied. If the weak<strong>en</strong>ing function is dilation, we use the same<br />
symmetry argum<strong>en</strong>t as for symmetric mo<strong>de</strong>l-based choice functions.<br />
■<br />
and » are replaced by z and z . So the result of the merging is consist<strong>en</strong>t with <br />
The property (IC5) can also be recovered for some BGM operators, but (IC6)<br />
seems hardly recoverable. Those two properties ((IC5) and (IC6)) are important for<br />
classical merging operators, so we can won<strong>de</strong>r if the BGM operators missing those<br />
properties can still be called “merging” operators. One answer to this is that the BGM<br />
operators aim at focusing on the interaction betwe<strong>en</strong> the beliefs of the sources, so it<br />
seems natural to loose property (IC6). In<strong>de</strong>ed, whereas classical merging operators<br />
aim at giving the result of the merging process in an i<strong>de</strong>al framework, BGM operators<br />
seem more a<strong>de</strong>quately reflect the behaviour of a real multi-source merging process.<br />
Another important logical link to be un<strong>de</strong>rlined is the relationship betwe<strong>en</strong> BGM<br />
operators and AGM belief revision operators [ALC 85, GÄR 88, KAT 91, GÄR 92].<br />
Belief revision aims to make the minimum change in a belief base in or<strong>de</strong>r to take into<br />
account new information that is more reliable than the curr<strong>en</strong>t belief base (and that<br />
usually contradicts the curr<strong>en</strong>t belief base). Technically those operators can be <strong><strong>de</strong>s</strong>cribed<br />
as follows : until the belief base is consist<strong>en</strong>t with the new item of information<br />
224
©<br />
<br />
©<br />
©<br />
<br />
<br />
Ð<br />
/<br />
©<br />
<br />
Ð<br />
H<br />
<br />
<br />
&<br />
Belief base merging as a game 289<br />
(se<strong>en</strong> as an integrity constraint) th<strong>en</strong> weak<strong>en</strong> the belief base 7 . Stated this way, we can<br />
immediately see the parallel with BGM operators since they are <strong><strong>de</strong>s</strong>cribed as follows :<br />
until the belief profile is consist<strong>en</strong>t with the constraint th<strong>en</strong> weak<strong>en</strong> some belief bases.<br />
The following result shows more formally that, as explained above, BGM operators<br />
can be se<strong>en</strong> as a direct g<strong>en</strong>eralization of AGM belief revision operators.<br />
THEOREM 19. — ‹%W t „ Let be a BGM operator. Let and be two belief<br />
bases. The operator <strong>de</strong>fined £ +‹}¢»B¨ as is an AGM belief revision operator<br />
(i.e. it satisfies properties (R1-R6) of [KAT 91]).<br />
PROOF. — It is easy to see that if we restrict belief 6 profiles to ¨ singletons , th<strong>en</strong><br />
postulates (IC0), (IC1), (IC2), (IC3), (IC7), (IC8) directly translate to (R1-R6).<br />
After that the proof follows from theorem 17.<br />
■<br />
In particular, we have that each BGM using the dilation weak<strong>en</strong>ing function is a<br />
g<strong>en</strong>eralization of Dalal’s revision operator [DAL 88].<br />
Finally let us see another cardinality restriction on the belief profile.<br />
„O² be a BGM operator <strong>de</strong>fined from a symmet-<br />
Ð×<br />
THEOREM 20. ‹ Ž t <br />
— Let<br />
ric mo<strong>de</strong>l-based choice function and dilation weak<strong>en</strong>ing / function. (` Let , and<br />
be three belief bases, th<strong>en</strong> the ‹}¢»B¨/<br />
ÐåYæxç<br />
operator<br />
[KON 02a]. <br />
operator ã ä<br />
is the mo<strong>de</strong>l-based merging<br />
Note that the previous result holds only wh<strong>en</strong> we merge two belief bases.<br />
»`<br />
¢ B¨/<br />
»`<br />
7. Comparison betwe<strong>en</strong> BGM and BNM<br />
In this section we will mainly compare our proposal with Booth’s Belief Negotiation<br />
Mo<strong>de</strong>l (BNM) [BOO 02]. Let us first briefly recall Booth’s proposal.<br />
Belief profiles in this framework are no longer multi-sets but vectors of belief<br />
6 bases, noted . Let us _ note the set of belief profiles, and let us note Ù the set of<br />
all sequ<strong>en</strong>ces (vectors) of belief profiles, and <br />
<br />
one elem<strong>en</strong>t of this set. Wh<strong>en</strong> is a<br />
<br />
Ô<br />
vector, we will note <br />
<br />
the th elem<strong>en</strong>t of the vector and <br />
&±Ý the last elem<strong>en</strong>t of the<br />
& • <br />
vector.<br />
So a sequ<strong>en</strong>ce of belief profiles Ô is of the form <br />
<br />
being a vector of belief bases, i.e.<br />
6Ÿ•<br />
notation <br />
Ô ò<br />
stands for <br />
©1010102© <br />
6Ÿ3 6]/<br />
, with each<br />
©1010102©<br />
Ô#†¨<br />
3 œ . And, for example, the<br />
4•<br />
6Ÿ•c4•<br />
Ý stands for <br />
6Ÿ3 .<br />
Ô<br />
Th<strong>en</strong> a BNM negotiation (choice) function is <strong>de</strong>fined as :<br />
DEFINITION 21. — A BNM negotiation function is a t<br />
BNM<br />
function<br />
that:<br />
<br />
ÙKJ<br />
<br />
such _<br />
6Tò , and the notation <br />
. It is the intuitive meaning behind Katsuno and M<strong>en</strong><strong>de</strong>lzon repres<strong>en</strong>tation theorem in terms<br />
of faithful assignm<strong>en</strong>ts [KAT 91].<br />
225
¢<br />
t<br />
BNM<br />
<br />
–<br />
<br />
<br />
<br />
–<br />
– ' ’ <br />
<br />
<br />
<br />
H<br />
<br />
<br />
<br />
<br />
<br />
<br />
<br />
6T“<br />
<br />
6M<br />
©1010102©<br />
<br />
<br />
<br />
H<br />
290 JANCL – 14/2004. Uncertainty in multiple data sources<br />
– If 4•Y"}t<br />
BNM<br />
ÔÅG?<br />
t<br />
BNM<br />
–<br />
Ô Ý<br />
<br />
ÔÅ<br />
<br />
, 4•êy a#z th<strong>en</strong><br />
And a BNM weak<strong>en</strong>ing function is <strong>de</strong>fined as :<br />
DEFINITION 22. — A BNM weak<strong>en</strong>ing function is a „<br />
BNM<br />
function<br />
that:<br />
<br />
ÙKJ<br />
<br />
such _<br />
ƒ ÔÅy<br />
– If <br />
Ô Ý‚ • ‡A„<br />
BNM<br />
<br />
Ô Ý‚ • a#z<br />
<br />
Finally the solution to a BNM is <strong>de</strong>fined as :<br />
DEFINITION 23. — The solution to a belief profile 6 for a Belief Negotiation Mo<strong>de</strong>l<br />
<br />
BNM<br />
#mt<br />
BNM©<br />
„<br />
BNM<br />
un<strong>de</strong>r the integrity constraints , noted ‹<br />
BNM<br />
¢ B6C , is giv<strong>en</strong> by the<br />
‹<br />
' ’<br />
function<br />
Ù <strong>de</strong>fined as:<br />
ÔÅ •<br />
Ý‚ • aP„<br />
BNM Ôp •<br />
, th<strong>en</strong> <br />
Ô<br />
with <br />
6“‚<br />
6 , is the smallest integer such that<br />
Ô"! <strong>de</strong>notes <br />
<br />
6T“<br />
<br />
6#! ):<br />
©1020101©<br />
<br />
is consist<strong>en</strong>t, and for each<br />
6b:”<br />
_vJ<br />
6CY<br />
Ôr#<br />
´<br />
we have ( <br />
<br />
—/2 • 6!<br />
„<br />
BNM<br />
Ô"!… • <br />
if<br />
• "}t<br />
BNM 6!<br />
•<br />
otherwise<br />
6#!<br />
Ô"!<br />
<br />
Finally, the belief base result of the BNM is<br />
<br />
. 6]:i<br />
We change some notation, in or<strong>de</strong>r to show the clos<strong>en</strong>ess with our pres<strong>en</strong>t work.<br />
For the original pres<strong>en</strong>tation and <strong>de</strong>tailed explanations on the <strong>de</strong>finitions see [BOO 02,<br />
BOO 01].<br />
The main differ<strong>en</strong>ces betwe<strong>en</strong> BNM and BGM are :<br />
– BNM’s <strong>de</strong>finition of belief profile as vectors allows us to speak about sources<br />
separately. So wh<strong>en</strong> there are two i<strong>de</strong>ntical belief bases in the belief profile, it is possible<br />
to weak<strong>en</strong> only one of these bases. This is not possible in the BGM framework.<br />
– The BNM negotiation function takes as input the whole negotiation history from<br />
the initial belief profile. So it is possible to implem<strong>en</strong>t a negotiation process such that<br />
each source weak<strong>en</strong>s after the previous one (for example, source , th<strong>en</strong> source ø , 01010 ),<br />
or such that we prev<strong>en</strong>t a source to weak<strong>en</strong> two times successively. The BGM choice<br />
functions are more Markovian, taking only into account the curr<strong>en</strong>t belief profile.<br />
– Similarly, the BNM weak<strong>en</strong>ing function also takes as input the whole negotiation<br />
history. It allows us to weak<strong>en</strong> differ<strong>en</strong>tly two i<strong>de</strong>ntical belief bases obtained at<br />
differ<strong>en</strong>t rounds or to weak<strong>en</strong> differ<strong>en</strong>tly two i<strong>de</strong>ntical belief bases of the same belief<br />
profile.<br />
– According to the previous items i<strong>de</strong>as, the irrelevance of syntax condition of the<br />
BGM weak<strong>en</strong>ing function, and the anonymity condition of the BGM choice function<br />
226
Belief base merging as a game 291<br />
are not required in the BNM framework.<br />
The main differ<strong>en</strong>ce betwe<strong>en</strong> Booth’s proposal and our is that Booth’s takes the<br />
sources as candidates to weak<strong>en</strong>ing, whereas we restrict ourselves to “points of view”<br />
(logical cont<strong>en</strong>t of the bases). That means that in Booth’s if one source has to weak<strong>en</strong>,<br />
it can be the case that another source with exactly the same beliefs do not have to<br />
weak<strong>en</strong> too. Our proposal adds more anonymity by saying that only beliefs <strong>de</strong>ci<strong>de</strong><br />
who has to weak<strong>en</strong>, not the i<strong>de</strong>ntity of one source. Similarly, the choice functions are<br />
more “Markovian” in our framework than in Booth’s one. We think that those hypothesis<br />
are more realistic (and necessary) on a belief merging point of view. Whereas<br />
Booth’s framework allows us to mo<strong>de</strong>l more g<strong>en</strong>eralized negotiation frameworks,<br />
where one can <strong>de</strong>ci<strong>de</strong> for example that each source has to weak<strong>en</strong> one after the other.<br />
So, <strong><strong>de</strong>s</strong>pite the clos<strong>en</strong>ess of the mo<strong>de</strong>ls, and the objective fact that our proposal is a<br />
particular case of Booth’s one (i.e. each of our operators can be <strong>de</strong>fined in Booth’s<br />
framework), the int<strong>en</strong><strong>de</strong>d applications of those two frameworks are quite differ<strong>en</strong>t.<br />
And the particular properties achieved by adding those restrictions shows that this<br />
framework forms a consist<strong>en</strong>t family of merging operators. It explains why it is worth<br />
focusing on the mo<strong>de</strong>l we have <strong>de</strong>fined.<br />
A last differ<strong>en</strong>ce is that, in this paper, we are interested on the result of the process<br />
(as a belief base), whereas BNM framework aims at studying the resulting profile, in<br />
connection with a notion of “social contraction”. See [BOO 02] for a study of logical<br />
properties for social contraction.<br />
An additional contribution of this work is to give examples of purely propositional<br />
logic BNM operators. In [BOO 02], Booth propose two examples of BNM,<br />
both working on ordinal conditional functions (OCF) [SPO 88], but none on a propositional<br />
belief base. So this work can be se<strong>en</strong> as an investigation of what kind of<br />
operators this <strong>de</strong>finition can give on propositional belief bases (through adding additional<br />
requirem<strong>en</strong>ts).<br />
8. Conclusion<br />
We have proposed in this paper a new family of belief merging operators, that we<br />
call Belief Game Mo<strong>de</strong>l (BGM) operators. The hypothesis for those operators is that<br />
all the sources are a priori reliable, or that we know that some sources are less reliable<br />
than the others, but without knowing which ones. This hypothesis leads to choosing<br />
a majority approach, justified by Condorcet’s Jury Theorem. The i<strong>de</strong>a behind the<br />
Belief Game Mo<strong>de</strong>l is simple : Until a coher<strong>en</strong>t set of sources is reached, at each<br />
round a contest is organized to find out the weakest sources, th<strong>en</strong> those sources have<br />
to conce<strong>de</strong> (weak<strong>en</strong> their point of view). This i<strong>de</strong>a leads to numerous new interesting<br />
operators and op<strong>en</strong>s new perspectives for belief merging. Some existing operators are<br />
also recovered as particular cases.<br />
Not surprisingly, the operators <strong>de</strong>fined do not satisfy all logical properties proposed<br />
for IC merging operators. The reason is that those logical properties aim at<br />
227
292 JANCL – 14/2004. Uncertainty in multiple data sources<br />
giving constraints on the result of the merging in an i<strong>de</strong>al framework, whereas BGM<br />
operators aim at <strong><strong>de</strong>s</strong>cribing more accurately what can happ<strong>en</strong> in a real multi-source<br />
<strong>en</strong>vironm<strong>en</strong>t. So usual IC merging operators can be se<strong>en</strong> as a normative approach<br />
to merging. They show the way to a purely logical result. Conversely, BGM operators<br />
adopt a <strong><strong>de</strong>s</strong>criptive approach to merging, taking into account the interaction<br />
betwe<strong>en</strong> the sources. They try to simulate more a<strong>de</strong>quately what can happ<strong>en</strong> in a<br />
group-<strong>de</strong>cision process. So they are maybe more realistic.<br />
This paper mainly aims to introduce BGM operators, but it provi<strong><strong>de</strong>s</strong> several op<strong>en</strong><br />
questions that are left for further research.<br />
The first one is about the <strong>de</strong>finition of BGM operators and the computation of<br />
the result. We give an iterative <strong>de</strong>finition of BGM operators, that leads to an iterative<br />
computation of the result. The question is to know if we can find a non-iterative equival<strong>en</strong>t<br />
<strong>de</strong>finition. We know that some simple operators can be <strong>de</strong>fined non-iteratively.<br />
But the question is to know if all operators or a non-trivial subclass of them are also<br />
<strong>de</strong>finable non-iteratively.<br />
Another op<strong>en</strong> question is about the logical characterization of this family. In this<br />
paper we study the logical properties of this family with respect to the g<strong>en</strong>eral <strong>de</strong>finition<br />
of IC merging operators. The question is to know if we can find a set of logical<br />
properties that characterizes BGM operators.<br />
Finally, we have rec<strong>en</strong>tly studied the strategy-proofness of the usual propositional<br />
merging operators, showing that most of them are not strategy-proof [EVE 04]. And<br />
we have exhibited several restrictions on which strategy-proofness can be achieved.<br />
So an interesting question is to compare the strategy-proofness of BGM operators<br />
with the one of the classical merging operators.<br />
Acknowledgem<strong>en</strong>ts<br />
The author wish to thank Richard Booth for hepful discussions and for its comm<strong>en</strong>ts<br />
on a previous version of this paper, the anonymous referees for their interesting<br />
comm<strong>en</strong>ts and Anthony Hunter for its help. The author has be<strong>en</strong> supported by the IUT<br />
<strong>de</strong> L<strong>en</strong>s, the Université d’Artois, the Région Nord/Pas-<strong>de</strong>-Calais un<strong>de</strong>r the TACT-TIC<br />
project, and by the European Community FEDER Program.<br />
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[CHO 95] CHOLVY L., “Automated reasoning with merged contradictory information whose<br />
reliability <strong>de</strong>p<strong>en</strong>ds on topics”, Proceedings of the Third European Confer<strong>en</strong>ce on Symbolic<br />
and Quantitative Approaches to Reasoning and Uncertainty (ECSQARU’95), 1995.<br />
[CHO 98] CHOLVY L., “Reasoning about data provi<strong>de</strong>d by fe<strong>de</strong>rated <strong>de</strong>ductive databases”,<br />
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[DAL 88] DALAL M., “Investigations into a theory of knowledge base revision: preliminary<br />
report”, Proceedings of the Sev<strong>en</strong>th American National Confer<strong>en</strong>ce on Artificial Intellig<strong>en</strong>ce<br />
(AAAI’88), 1988, p. 475-479.<br />
[EVE 04] EVERAERE P., KONIECZNY S., MARQUIS P., “On merging strategy-proofness”,<br />
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(KR’04), 2004, p. 357-358.<br />
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[KAT 91] KATSUNO H., MENDELZON A. O., “Propositional knowledge base revision and<br />
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Repres<strong>en</strong>tation and Reasoning (KR’00), 2000, p. 135-144.<br />
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[KON 02b] KONIECZNY S., PINO PÉREZ R., “On the Frontier betwe<strong>en</strong> Arbitration and Majority”,<br />
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IEEE Transactions on Knowledge and Data Engineering, vol. 10, num. 1, 1998, p. 76-90.<br />
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Worlds: From the Frame Problem to Knowledge Managem<strong>en</strong>t, Kluwer, 1999.<br />
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vol. 2, p. 105-134, Kluwer, 1988.<br />
230
CONFLUENCE OPERATORS<br />
Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez.<br />
Elev<strong>en</strong>th European Confer<strong>en</strong>ce on Logics in Artificial Intellig<strong>en</strong>ce<br />
(JELIA’08).<br />
pages 272-284.<br />
2008.
Conflu<strong>en</strong>ce Operators<br />
Sébasti<strong>en</strong> Konieczny 1 and Ramón Pino Pérez 2<br />
1 CRIL - CNRS<br />
Faculté <strong><strong>de</strong>s</strong> Sci<strong>en</strong>ces, Université d’Artois, L<strong>en</strong>s, France<br />
e-mail: konieczny@cril.fr<br />
2 Departam<strong>en</strong>to <strong>de</strong> Matemáticas<br />
Facultad <strong>de</strong> ci<strong>en</strong>cias, Universidad <strong>de</strong> Los An<strong><strong>de</strong>s</strong>, Mérida, V<strong>en</strong>ezuela<br />
e-mail: pino@ula.ve<br />
Abstract. In the logic based framework of knowledge repres<strong>en</strong>tation and reasoning<br />
many operators have be<strong>en</strong> <strong>de</strong>fined in or<strong>de</strong>r to capture differ<strong>en</strong>t kinds of<br />
change: revision, update, merging and many others. There are close links betwe<strong>en</strong><br />
revision, update, and merging. Merging operators can be consi<strong>de</strong>red as ext<strong>en</strong>sions<br />
of revision operators to multiple belief bases. And update operators can be consi<strong>de</strong>red<br />
as pointwise revision, looking at each mo<strong>de</strong>l of the base, instead of taking<br />
the base as a whole. Thus, a natural question is the following one: Are there natural<br />
operators that are pointwise merging, just as update are pointwise revision?<br />
The goal of this work is to give a positive answer to this question. In or<strong>de</strong>r to do<br />
that, we introduce a new class of operators: the conflu<strong>en</strong>ce operators. These new<br />
operators can be useful in mo<strong>de</strong>lling negotiation processes.<br />
1 Introduction<br />
Belief change theory has produced a lot of differ<strong>en</strong>t operators that mo<strong>de</strong>ls the differ<strong>en</strong>t<br />
ways the beliefs of one (or some) ag<strong>en</strong>t(s) evolve over time. Among these operators,<br />
one can quote revision [1, 5, 10, 6], update [9, 8], merging [19, 14], abduction [16], extrapolation<br />
[4], etc.<br />
In this paper we will focus on revision, update and merging. Let us first briefly<br />
<strong><strong>de</strong>s</strong>cribe these operators informally:<br />
Revision Belief revision is the process of accomodating a new piece of evi<strong>de</strong>nce that<br />
is more reliable than the curr<strong>en</strong>t beliefs of the ag<strong>en</strong>t. In belief revision the world is<br />
static, it is the beliefs of the ag<strong>en</strong>ts that evolve.<br />
Update In belief update the new piece of evi<strong>de</strong>nce <strong>de</strong>notes a change in the world. The<br />
world is dynamic, and these (observed) changes modify the beliefs of the ag<strong>en</strong>t.<br />
Merging Belief merging is the process of <strong>de</strong>fining the beliefs of a group of ag<strong>en</strong>ts.<br />
So the question is: Giv<strong>en</strong> a set of ag<strong>en</strong>ts that have their own beliefs, what can be<br />
consi<strong>de</strong>red as the beliefs of the group?<br />
Apart from these intuitive differ<strong>en</strong>ces betwe<strong>en</strong> these operators, there are also close<br />
links betwe<strong>en</strong> them. This is particularly clear wh<strong>en</strong> looking at the technical <strong>de</strong>finitions.<br />
There are close relationship betwe<strong>en</strong> revision [1, 5, 10] and KM update operators [9].<br />
The first ones looking at the beliefs of the ag<strong>en</strong>ts globally, the second ones looking at<br />
them locally (this s<strong>en</strong>t<strong>en</strong>ce will be ma<strong>de</strong> formally clear later in the paper) 3 . There is<br />
3 See [8, 4, 15] for more discussions on update and its links with revision.<br />
233
also a close connection betwe<strong>en</strong> revision and merging operators. In fact revision operators<br />
can be se<strong>en</strong> as particular cases of merging operators. From these two facts a<br />
very natural question arises: What is the family of operators that are a g<strong>en</strong>eralization<br />
of update operators in the same way merging operators g<strong>en</strong>eralize revision operators?<br />
Or, equival<strong>en</strong>tly, what are the operators that can be consi<strong>de</strong>red as pointwise merging,<br />
just as KM update operators can be consi<strong>de</strong>red as pointwise belief revision. This can<br />
be outlined in the figure below. The aim of this paper is to introduce and study the operators<br />
corresponding to the question mark. We will call these new operators conflu<strong>en</strong>ce<br />
operators.<br />
Revision<br />
Update<br />
Merging ?<br />
Fig. 1. Revision - Update - Merging - Conflu<strong>en</strong>ce<br />
these new operators are more cautious than merging operators. This suggest that<br />
they can be used to <strong>de</strong>fine negotiation operators (see [2, 20, 18, 17, 12]), or as a first step<br />
of a negotiation process, in or<strong>de</strong>r to find all the possible negotiation results.<br />
In or<strong>de</strong>r to illustrate the need for these new operators and also the differ<strong>en</strong>ce of<br />
behaviour betwe<strong>en</strong> merging and conflu<strong>en</strong>ce we pres<strong>en</strong>t the following small example.<br />
Example 1. Mary and Peter are planning to buy a car. Mary does not like a German car<br />
nor an exp<strong>en</strong>sive car. She likes small cars. Peter hesitates betwe<strong>en</strong> a German, exp<strong>en</strong>sive<br />
but small car or a car which is not German, nor exp<strong>en</strong>sive and is a big car. Taking<br />
three propositional variables German car, Exp<strong>en</strong>sive car and Small car in<br />
this or<strong>de</strong>r, Mary’s <strong><strong>de</strong>s</strong>ires are repres<strong>en</strong>ted by mod(A) = {001} and Peter’s <strong><strong>de</strong>s</strong>ires by<br />
mod(B) = {111, 000}. Most of the merging operators 4 give as solution (in semantical<br />
terms) the set {001, 000}. That is the same solution obtained wh<strong>en</strong> we suppose<br />
that Peter’s <strong><strong>de</strong>s</strong>ires are only a car which is not German nor exp<strong>en</strong>sive but a big car<br />
(mod(B ′ ) = {000}). The conflu<strong>en</strong>ce operators will take into account the disjunctive<br />
nature of Peter’s <strong><strong>de</strong>s</strong>ires in a better manner and they will incorporate also the interpretations<br />
that are a tra<strong>de</strong>-off betwe<strong>en</strong> 001 and 111. For instance, the worlds 011 and 101<br />
will be also in the solution if one use the conflu<strong>en</strong>ce operator ✸ dH,Gmax (<strong>de</strong>fined in<br />
Section 7).<br />
This kind of operators is particularly a<strong>de</strong>quate wh<strong>en</strong> the base <strong><strong>de</strong>s</strong>cribes a situation<br />
that is not perfectly known, or that can evolve in the future. For instance Peter’s <strong><strong>de</strong>s</strong>ires<br />
can either be imperfectly known (he wants one of the two situations but we do not<br />
know which one), or can evolve in the future (he will choose later betwe<strong>en</strong> the two<br />
situations). In these situations the solutions proposed by conflu<strong>en</strong>ce operators will be<br />
4 Such as △ d H ,Σ and △ d H ,Gmax [14].<br />
234
more a<strong>de</strong>quate than the one proposed by merging operators. The solutions proposed by<br />
the conflu<strong>en</strong>ce operators can be se<strong>en</strong> as all possible agreem<strong>en</strong>ts in a negotiation process.<br />
In the next section we will give the required <strong>de</strong>finitions and notations. In Section<br />
3 we will recall the postulates and repres<strong>en</strong>tation theorems for revision, update, and<br />
merging, and state the links betwe<strong>en</strong> these operators. In Section 4 we <strong>de</strong>fine conflu<strong>en</strong>ce<br />
operators. We provi<strong>de</strong> a repres<strong>en</strong>tation theorem for these operators in Section 5. In<br />
Section 6 we study the links betwe<strong>en</strong> conflu<strong>en</strong>ce operators and update and merging. In<br />
Section 7 we give examples of conflu<strong>en</strong>ce operators. And we conclu<strong>de</strong> in Section 8.<br />
2 Preliminaries<br />
We consi<strong>de</strong>r a propositional language L <strong>de</strong>fined from a finite set of propositional variables<br />
P and the standard connectives, including ⊤ and ⊥.<br />
An interpretation ω is a total function from P to {0, 1}. The set of all interpretations<br />
is <strong>de</strong>noted by W. An interpretation ω is a mo<strong>de</strong>l of a formula φ ∈ L if and only if it<br />
makes it true in the usual truth functional way. mod(ϕ) <strong>de</strong>notes the set of mo<strong>de</strong>ls of<br />
the formula ϕ, i.e., mod(ϕ) = {ω ∈ W | ω |= ϕ}. Wh<strong>en</strong> M is a set of mo<strong>de</strong>ls we<br />
<strong>de</strong>note by ϕ M a formula such that mod(ϕ M ) = M.<br />
A base K is a finite set of propositional formulae. In or<strong>de</strong>r to simplify the notations,<br />
in this work we will i<strong>de</strong>ntify the base K with the formula ϕ which is the conjunction of<br />
the formulae of K 5 .<br />
A profile Ψ is a non-empty multi-set (bag) of bases Ψ = {ϕ 1 , . . . , ϕ n } (h<strong>en</strong>ce<br />
differ<strong>en</strong>t ag<strong>en</strong>ts are allowed to exhibit i<strong>de</strong>ntical bases), and repres<strong>en</strong>ts a group of n<br />
ag<strong>en</strong>ts.<br />
We <strong>de</strong>note by ∧ Ψ the conjunction of bases of Ψ = {ϕ 1 , . . . , ϕ n }, i.e., ∧ Ψ =<br />
ϕ 1 ∧ . . . ∧ ϕ n . A profile Ψ is said to be consist<strong>en</strong>t if and only if ∧ Ψ is consist<strong>en</strong>t. The<br />
multi-set union is <strong>de</strong>noted by ⊔.<br />
A formula ϕ is complete if it has only one mo<strong>de</strong>l. A profile Ψ is complete if all the<br />
bases of Ψ are complete formulae.<br />
If ≤ <strong>de</strong>notes a pre-or<strong>de</strong>r on W (i.e., a reflexive and transitive relation), th<strong>en</strong> <<br />
<strong>de</strong>notes the associated strict or<strong>de</strong>r <strong>de</strong>fined by ω < ω ′ if and only if ω ≤ ω ′ and ω ′ ≰ ω,<br />
and ≃ <strong>de</strong>notes the associated equival<strong>en</strong>ce relation <strong>de</strong>fined by ω ≃ ω ′ if and only if<br />
ω ≤ ω ′ and ω ′ ≤ ω. A pre-or<strong>de</strong>r is total if ∀ω, ω ′ ∈ W, ω ≤ ω ′ or ω ′ ≤ ω. A preor<strong>de</strong>r<br />
that is not total is called partial. Let ≤ be a pre-or<strong>de</strong>r on A, and B ⊆ A, th<strong>en</strong><br />
min(B, ≤) = {b ∈ B | ∄a ∈ B a < b}.<br />
3 Revision, Update and Merging<br />
Let us now recall in this section some background on revision, update and merging, and<br />
their repres<strong>en</strong>tation theorems in terms of pre-or<strong>de</strong>rs on interpretations. This will allow<br />
us to give the relationships betwe<strong>en</strong> these operators.<br />
5 Some approaches are s<strong>en</strong>sitive to syntactical repres<strong>en</strong>tation. In that case it is important to<br />
distinguish betwe<strong>en</strong> K and the conjonction of its formulae (see e.g. [13]). But operators of<br />
this work are all syntax in<strong>de</strong>p<strong>en</strong>dant.<br />
235
3.1 Revision<br />
Definition 1 (Katsuno-M<strong>en</strong><strong>de</strong>lzon [10]). An operator ◦ is an AGM belief revision operator<br />
if it satisfies the following properties:<br />
(R1) ϕ ◦ µ ⊢ µ<br />
(R2) If ϕ ∧ µ ⊥ th<strong>en</strong> ϕ ◦ µ ≡ ϕ ∧ µ<br />
(R3) If µ ⊥ th<strong>en</strong> ϕ ◦ µ ⊥<br />
(R4) If ϕ 1 ≡ ϕ 2 and µ 1 ≡ µ 2 th<strong>en</strong> ϕ 1 ◦ µ 1 ≡ ϕ 2 ◦ µ 2<br />
(R5) (ϕ ◦ µ) ∧ φ ⊢ ϕ ◦ (µ ∧ φ)<br />
(R6) If (ϕ ◦ µ) ∧ φ ⊥ th<strong>en</strong> ϕ ◦ (µ ∧ φ) ⊢ (ϕ ◦ µ) ∧ φ<br />
Wh<strong>en</strong> one works with a finite propositional language the previous postulates, proposed<br />
by Katsuno and M<strong>en</strong><strong>de</strong>lzon, are equival<strong>en</strong>t to AGM ones [1, 5]. In [10] Katsuno<br />
and M<strong>en</strong><strong>de</strong>lzon give also a repres<strong>en</strong>tation theorem for revision operators, showing that<br />
each revision operator corresponds to a faithful assignm<strong>en</strong>t, that associates to each base<br />
a plausibility preor<strong>de</strong>r on interpretations (this i<strong>de</strong>a can be traced back to Grove systems<br />
of spheres [7] ).<br />
Definition 2. A faithful assignm<strong>en</strong>t is a function mapping each base ϕ to a pre-or<strong>de</strong>r<br />
≤ ϕ over interpretations such that:<br />
1. If ω |= ϕ and ω ′ |= ϕ, th<strong>en</strong> ω ≃ ϕ ω ′<br />
2. If ω |= ϕ and ω ′ ̸|= ϕ, th<strong>en</strong> ω < ϕ ω ′<br />
3. If ϕ ≡ ϕ ′ , th<strong>en</strong> ≤ ϕ =≤ ϕ ′<br />
Theorem 1 (Katsuno-M<strong>en</strong><strong>de</strong>lzon [10]). An operator ◦ is a revision operator (ie. it<br />
satisfies (R1)-(R6)) if and only if there exists a faithful assignm<strong>en</strong>t that maps each base<br />
ϕ to a total pre-or<strong>de</strong>r ≤ ϕ such that<br />
mod(ϕ ◦ µ) = min(mod(µ), ≤ ϕ ).<br />
This repres<strong>en</strong>tation theorem is important because it provi<strong><strong>de</strong>s</strong> a way to easily <strong>de</strong>fine<br />
revision operators by <strong>de</strong>fining faithful assignm<strong>en</strong>ts. But also because their are similar<br />
such theorems for update and merging (we will also show a similar result for conflu<strong>en</strong>ce),<br />
and that these repres<strong>en</strong>tations in term of assignm<strong>en</strong>ts allow to more easily find<br />
links betwe<strong>en</strong> these operators.<br />
3.2 Update<br />
Definition 3 (Katsuno-M<strong>en</strong><strong>de</strong>lzon [9, 11]). An operator ⋄ is a (partial) update operator<br />
if it satisfies the properties (U1)-(U8). It is a total update operator if it satisfies the<br />
properties (U1)-(U5), (U8), (U9).<br />
(U1) ϕ ⋄ µ ⊢ µ<br />
(U2) If ϕ ⊢ µ, th<strong>en</strong> ϕ ⋄ µ ≡ ϕ<br />
(U3) If ϕ ⊥ and µ ⊥ th<strong>en</strong> ϕ ⋄ µ ⊥<br />
(U4) If ϕ 1 ≡ ϕ 2 and µ 1 ≡ µ 2 th<strong>en</strong> ϕ 1 ⋄ µ 1 ≡ ϕ 2 ⋄ µ 2<br />
236
(U5) (ϕ ⋄ µ) ∧ φ ⊢ ϕ ⋄ (µ ∧ φ)<br />
(U6) If ϕ ⋄ µ 1 ⊢ µ 2 and ϕ ⋄ µ 2 ⊢ µ 1 , th<strong>en</strong> ϕ ⋄ µ 1 ≡ ϕ ⋄ µ 2<br />
(U7) If ϕ is a complete formula, th<strong>en</strong> (ϕ ⋄ µ 1 ) ∧ (ϕ ⋄ µ 2 ) ⊢ ϕ ⋄ (µ 1 ∨ µ 2 )<br />
(U8) (ϕ 1 ∨ ϕ 2 ) ⋄ µ ≡ (ϕ 1 ⋄ µ) ∨ (ϕ 2 ⋄ µ)<br />
(U9) If ϕ is a complete formula and (ϕ ⋄ µ) ∧ φ ⊥, th<strong>en</strong> ϕ ⋄ (µ ∧ φ) ⊢ (ϕ ⋄ µ) ∧ φ<br />
As for revision, there is a repres<strong>en</strong>tation theorem in terms of faithful assignm<strong>en</strong>t.<br />
Definition 4. A faithful assignm<strong>en</strong>t is a function mapping each interpretation ω to a<br />
pre-or<strong>de</strong>r ≤ ω over interpretations such that if ω ≠ ω ′ , th<strong>en</strong> ω < ω ω ′ .<br />
One can easily check that this faithful assignm<strong>en</strong>t on interpretations is just a special<br />
case of the faithful assignm<strong>en</strong>t on bases <strong>de</strong>fined in the previous section on the complete<br />
base corresponding to the interpretation.<br />
Katsuno and M<strong>en</strong><strong>de</strong>lzon give two repres<strong>en</strong>tation theorems for update operators. The<br />
first repres<strong>en</strong>tation theorem corresponds to partial pre-or<strong>de</strong>rs.<br />
Theorem 2 (Katsuno-M<strong>en</strong><strong>de</strong>lzon [9, 11]). An update operator ⋄ satisfies (U1)-(U8)<br />
if and only if there exists a faithful assignm<strong>en</strong>t that maps each interpretation ω to a<br />
partial pre-or<strong>de</strong>r ≤ ω such that<br />
mod(ϕ ⋄ µ) = ⋃<br />
min(mod(µ), ≤ ϕ{ω} )<br />
ω|=ϕ<br />
And the second one corresponds to total pre-or<strong>de</strong>rs.<br />
Theorem 3 (Katsuno-M<strong>en</strong><strong>de</strong>lzon [9, 11]). An update operator ⋄ satisfies (U1)-(U5),<br />
(U8) and (U9) if and only if there exists a faithful assignm<strong>en</strong>t that maps each interpretation<br />
ω to a total pre-or<strong>de</strong>r ≤ ω such that<br />
mod(ϕ ⋄ µ) = ⋃<br />
min(mod(µ), ≤ ϕ{ω} )<br />
ω|=ϕ<br />
3.3 Merging<br />
Definition 5 (Konieczny-Pino Pérez [14]). An operator △ mapping a pair Ψ, µ (profile,<br />
formula) into a formula <strong>de</strong>noted △ µ (Ψ) is an IC merging operator if it satisfies<br />
the following properties:<br />
(IC0) △ µ (Ψ) ⊢ µ<br />
(IC1) If µ is consist<strong>en</strong>t, th<strong>en</strong> △ µ (Ψ) is consist<strong>en</strong>t<br />
(IC2) If ∧ Ψ is consist<strong>en</strong>t with µ, th<strong>en</strong> △ µ (Ψ) ≡ ∧ Ψ ∧ µ<br />
(IC3) If Ψ 1 ≡ Ψ 2 and µ 1 ≡ µ 2 , th<strong>en</strong> △ µ1 (Ψ 1 ) ≡ △ µ2 (Ψ 2 )<br />
(IC4) If ϕ 1 ⊢ µ and ϕ 2 ⊢ µ, th<strong>en</strong> △ µ ({ϕ 1 , ϕ 2 }) ∧ ϕ 1 is consist<strong>en</strong>t if and only if<br />
△ µ ({ϕ 1 , ϕ 2 }) ∧ ϕ 2 is consist<strong>en</strong>t<br />
(IC5) △ µ (Ψ 1 ) ∧ △ µ (Ψ 2 ) ⊢ △ µ (Ψ 1 ⊔ Ψ 2 )<br />
(IC6) If △ µ (Ψ 1 ) ∧ △ µ (Ψ 2 ) is consist<strong>en</strong>t, th<strong>en</strong> △ µ (Ψ 1 ⊔ Ψ 2 ) ⊢ △ µ (Ψ 1 ) ∧ △ µ (Ψ 2 )<br />
(IC7) △ µ1 (Ψ) ∧ µ 2 ⊢ △ µ1∧µ 2<br />
(Ψ)<br />
237
(IC8) If △ µ1 (Ψ) ∧ µ 2 is consist<strong>en</strong>t, th<strong>en</strong> △ µ1∧µ 2<br />
(Ψ) ⊢ △ µ1 (Ψ)<br />
There is also a repres<strong>en</strong>tation theorem for merging operators in terms of pre-or<strong>de</strong>rs<br />
on interpretations [14].<br />
Definition 6. A syncretic assignm<strong>en</strong>t is a function mapping each profile Ψ to a total<br />
pre-or<strong>de</strong>r ≤ Ψ over interpretations such that:<br />
1. If ω |= Ψ and ω ′ |= Ψ, th<strong>en</strong> ω ≃ Ψ ω ′<br />
2. If ω |= Ψ and ω ′ ̸|= Ψ, th<strong>en</strong> ω < Ψ ω ′<br />
3. If Ψ 1 ≡ Ψ 2 , th<strong>en</strong> ≤ Ψ1 =≤ Ψ2<br />
4. ∀ω |= ϕ ∃ω ′ |= ϕ ′ ω ′ ≤ {ϕ}⊔{ϕ} ′ ω<br />
5. If ω ≤ Ψ1 ω ′ and ω ≤ Ψ2 ω ′ , th<strong>en</strong> ω ≤ Ψ1⊔Ψ 2<br />
ω ′<br />
6. If ω < Ψ1 ω ′ and ω ≤ Ψ2 ω ′ , th<strong>en</strong> ω < Ψ1⊔Ψ 2<br />
ω ′<br />
Theorem 4 (Konieczny-Pino Pérez [14]). An operator △ is an IC merging operator<br />
if and only if there exists a syncretic assignm<strong>en</strong>t that maps each profile Ψ to a total<br />
pre-or<strong>de</strong>r ≤ Ψ such that<br />
mod(△ µ (Ψ)) = min(mod(µ), ≤ Ψ )<br />
3.4 Revision vs Update<br />
Intuitively revision operators bring a minimal change to the base by selecting the most<br />
plausible mo<strong>de</strong>ls among the mo<strong>de</strong>ls of the new information. Whereas update operators<br />
bring a minimal change to each possible world (mo<strong>de</strong>l) of the base in or<strong>de</strong>r to take into<br />
account the change <strong><strong>de</strong>s</strong>cribed by the new infomation whatever the possible world. So, if<br />
we look closely to the two repres<strong>en</strong>tation theorems (propositions 1, 2 and 3), we easily<br />
find the following result:<br />
Theorem 5. If ◦ is a revision operator (i.e. it satisfies (R1)-(R6)), th<strong>en</strong> the operator ⋄<br />
<strong>de</strong>fined by:<br />
ϕ ⋄ µ = ∨<br />
ϕ {ω} ◦ µ<br />
ω|=ϕ<br />
is an update operator that satisfies (U1)-(U9).<br />
Moreover, for each update operator ⋄, there exists a revision operator ◦ such that<br />
the previous equation holds.<br />
As explained above this proposition states that update can be viewed as a kind of<br />
pointwise revision.<br />
3.5 Revision vs Merging<br />
Intuitively revision operators select in a formula (the new evi<strong>de</strong>nce) the closest information<br />
to a ground information (the old base). And, i<strong>de</strong>ntically, IC merging operators<br />
select in a formula (the integrity constraints) the closest information to a ground information<br />
(a profile of bases).<br />
So following this i<strong>de</strong>a it is easy to make a correspon<strong>de</strong>nce betwe<strong>en</strong> IC merging<br />
operators and belief revision operators [14]:<br />
238
Theorem 6 (Konieczny-Pino Pérez [14]). If △ is an IC merging operator (it satisfies<br />
(IC0-IC8)), th<strong>en</strong> the operator ◦, <strong>de</strong>fined as ϕ◦µ = △ µ (ϕ), is an AGM revision operator<br />
(it satisfies (R1-R6)).<br />
See [14] for more links betwe<strong>en</strong> belief revision and merging.<br />
4 Conflu<strong>en</strong>ce operators<br />
So now that we have ma<strong>de</strong> clear the connections sketched in figure 1 betwe<strong>en</strong> revision,<br />
update and merging, let us turn now to the <strong>de</strong>finition of conflu<strong>en</strong>ce operators, that aim<br />
to be a pointwise merging, similarly as update is a pointwise revision, as explained in<br />
Section 3.4. Let us first <strong>de</strong>fine p-consist<strong>en</strong>cy for profiles.<br />
Definition 7. A profile Ψ = {ϕ 1 , . . . ϕ n } is p-consist<strong>en</strong>t if all its bases are consist<strong>en</strong>t,<br />
i.e ∀ϕ i ∈ Ψ, ϕ i is consist<strong>en</strong>t.<br />
Note that p-consist<strong>en</strong>cy is much weaker than consist<strong>en</strong>cy, the former just asks that<br />
all the bases of the profile are consist<strong>en</strong>t, while the later asks that the conjunction of all<br />
the bases is consist<strong>en</strong>t.<br />
Definition 8. An operator ✸ is a conflu<strong>en</strong>ce operator if it satisfies the following properties:<br />
(UC0) ✸ µ (Ψ) ⊢ µ<br />
(UC1) If µ is consist<strong>en</strong>t and Ψ is p-consist<strong>en</strong>t, th<strong>en</strong> ✸ µ (Ψ) is consist<strong>en</strong>t<br />
(UC2) If Ψ is complete, Ψ is consist<strong>en</strong>t and ∧ Ψ ⊢ µ, th<strong>en</strong> ✸ µ (Ψ) ≡ ∧ Ψ<br />
(UC3) If Ψ 1 ≡ Ψ 2 and µ 1 ≡ µ 2 , th<strong>en</strong> ✸ µ1 (Ψ 1 ) ≡ ✸ µ2 (Ψ 2 )<br />
(UC4) If ϕ 1 and ϕ 2 are complete formulae and ϕ 1 ⊢ µ, ϕ 2 ⊢ µ,<br />
th<strong>en</strong> ✸ µ ({ϕ 1 , ϕ 2 }) ∧ ϕ 1 is consist<strong>en</strong>t if and only ✸ µ ({ϕ 1 , ϕ 2 }) ∧ ϕ 2 is consist<strong>en</strong>t<br />
(UC5) ✸ µ (Ψ 1 ) ∧ ✸ µ (Ψ 2 ) ⊢ ✸ µ (Ψ 1 ⊔ Ψ 2 )<br />
(UC6) If Ψ 1 and Ψ 2 are complete profiles and ✸ µ (Ψ 1 ) ∧ ✸ µ (Ψ 2 ) is consist<strong>en</strong>t,<br />
th<strong>en</strong> ✸ µ (Ψ 1 ⊔ Ψ 2 ) ⊢ ✸ µ (Ψ 1 ) ∧ ✸ µ (Ψ 2 )<br />
(UC7) ✸ µ1 (Ψ) ∧ µ 2 ⊢ ✸ µ1∧µ 2<br />
(Ψ)<br />
(UC8) If Ψ is a complete profile and if ✸ µ1 (Ψ) ∧ µ 2 is consist<strong>en</strong>t<br />
th<strong>en</strong> ✸ µ1∧µ 2<br />
(Ψ) ⊢ ✸ µ1 (Ψ) ∧ µ 2<br />
(UC9) ✸ µ (Ψ ⊔ {ϕ ∨ ϕ ′ }) ≡ ✸ µ (Ψ ⊔ {ϕ}) ∨ ✸ µ (Ψ ⊔ {ϕ ′ })<br />
Some of the (UC) postulates are exactly the same as (IC) ones, just like some (U)<br />
postulates for update are exactly the same as (R) ones for revision.<br />
In fact, (UC0), (UC3), (UC5) and (UC7) are exactly the same as the corresponding<br />
(IC) postulates. So the specificity of conflu<strong>en</strong>ce operators lies in postulates (UC1),<br />
(UC2), (UC6), (UC8) and (UC9). (UC2), (UC4), (UC6) and (UC8) are close to the<br />
corresponding (IC) postulates, but hold for complete profiles only. The pres<strong>en</strong>t formulation<br />
of (UC2) is quite similar to formulation of (U2) for update. Note that in the<br />
case of a complete profile the hypothesis of (UC2) is equival<strong>en</strong>t to ask coher<strong>en</strong>ce with<br />
the constraints, i.e. the hypothesis of (IC2). Postulates (UC8) and (UC9) are the main<br />
differ<strong>en</strong>ce with merging postulates, and correspond also to the main differ<strong>en</strong>ce betwe<strong>en</strong><br />
239
evision and KM update operators. (UC9) is the most important postulate, that <strong>de</strong>fines<br />
conflu<strong>en</strong>ce operators as pointwise agregation, just like (U8) <strong>de</strong>fines update operators as<br />
pointwise revision. This will be expressed more formally in the next Section (Lemma<br />
1).<br />
5 Repres<strong>en</strong>tation theorem for conflu<strong>en</strong>ce operators<br />
In or<strong>de</strong>r to state the repres<strong>en</strong>tation theorem for conflu<strong>en</strong>ce operators, we first have to be<br />
able to “localize” the problem. For update this is done by looking to each mo<strong>de</strong>l of the<br />
base, instead of looking at the base (set of mo<strong>de</strong>ls) as a whole. So for “localizing” the<br />
aggregation process, we have to find what is the local view of a profile. That is what we<br />
call a state.<br />
Definition 9. A multi-set of interpretations will be called a state. We use the letter e,<br />
possibly with subscripts, for <strong>de</strong>noting states. If Ψ = {ϕ 1 , . . . , ϕ n } is a profile and<br />
e = {ω 1 , . . . , ω n } is a state such that ω i |= ϕ i for each i, we say that e is a state of the<br />
profile Ψ, or that the state e mo<strong>de</strong>ls the profile Ψ, that will be <strong>de</strong>noted by e |= Ψ. If e =<br />
{ω 1 , . . . , ω n } is a state, we <strong>de</strong>fine the profile Ψ e by putting Ψ e = {ϕ {ω1}, . . . , ϕ {ωn}}.<br />
State is an interesting notion. If we consi<strong>de</strong>r each base as the curr<strong>en</strong>t point of view<br />
(goals) of the corresponding ag<strong>en</strong>t (that can be possibly str<strong>en</strong>gth<strong>en</strong>ed in the future) th<strong>en</strong><br />
states are all possible negotiation starting points.<br />
States are the points of interest for conflu<strong>en</strong>ce operators (like interpretations are for<br />
update), as stated in the following Lemma:<br />
Lemma 1. If ✸ satisfies (UC3) and (UC9) th<strong>en</strong> ✸ satisfies the following<br />
✸ µ (Ψ) ≡ ∨<br />
✸ µ (Ψ e )<br />
e|=Ψ<br />
Defining profile <strong>en</strong>tailm<strong>en</strong>t by putting Ψ ⊢ Ψ ′ iff every state of Ψ is a state of Ψ ′ ,<br />
the previous Lemma has as a corollary the following:<br />
Corollary 1. If ✸ is a conflu<strong>en</strong>ce operator th<strong>en</strong> it is monotonic in the profiles, that<br />
means that if Ψ ⊢ Ψ ′ th<strong>en</strong> ✸ µ (Ψ) ⊢ ✸ µ (Ψ ′ )<br />
This monotony property, that is not true in the case of merging operators, shows one<br />
of the big differ<strong>en</strong>ces betwe<strong>en</strong> merging and conflu<strong>en</strong>ce operators. Remark that there is<br />
a corresponding monotony property for update.<br />
Like revision’s faithful assignm<strong>en</strong>ts that have to be “localized” to interpretations for<br />
update, merging’s syncretic assignm<strong>en</strong>ts have to be localized to states for conflu<strong>en</strong>ce.<br />
Definition 10. A distributed assignm<strong>en</strong>t is a function mapping each state e to a total<br />
pre-or<strong>de</strong>r ≤ e over interpretations such that:<br />
1. ω < {ω,...,ω} ω ′ if ω ′ ≠ ω<br />
2. ω ≃ {ω,ω ′ } ω ′<br />
3. If ω ≤ e1 ω ′ and ω ≤ e2 ω ′ , th<strong>en</strong> ω ≤ e1⊔e 2<br />
ω ′<br />
240
4. If ω < e1 ω ′ and ω ≤ e2 ω ′ , th<strong>en</strong> ω < e1⊔e 2<br />
ω ′<br />
Now we can state the main result of this paper, that is the repres<strong>en</strong>tation theorem<br />
for conflu<strong>en</strong>ce operators.<br />
Theorem 7. An operator ✸ is a conflu<strong>en</strong>ce operator if and only if there exists a distributed<br />
assignm<strong>en</strong>t that maps each state e to a total pre-or<strong>de</strong>r ≤ e such that<br />
mod(✸ µ (Ψ)) = ⋃<br />
min(mod(µ), ≤ e ) (1)<br />
e|=Ψ<br />
Unfortunately, we have to omit the proof for space reasons. Nevertheless, we indicate<br />
the most important i<strong>de</strong>as therein. As it is usual, the if condition is done by checking<br />
each property without any major difficulty. In or<strong>de</strong>r to verify the only if condition we<br />
have to <strong>de</strong>fine a distributed assigm<strong>en</strong>t. This is done in the following way: for each<br />
state e we <strong>de</strong>fine a total pre-or<strong>de</strong>r ≤ e by putting ∀ω, ω ′ ∈ W ω ≤ e ω ′ if and only if<br />
ω |= ✸ ϕ{ω,ω ′ }<br />
(Ψ e ). Th<strong>en</strong>, the main difficulties are to prove that this is in<strong>de</strong>ed a distributed<br />
assigm<strong>en</strong>t and that the equation (1) holds. In particular, Lema 1 is very helpful<br />
for proving this last equation.<br />
Note that this theorem is still true if we remove respectively the postulate (UC4)<br />
from the required postulates for conflu<strong>en</strong>ce operators and the condition 2 from distributed<br />
assignm<strong>en</strong>ts.<br />
6 Conflu<strong>en</strong>ce vs Update and Merging<br />
So now we are able to state the proposition that shows that update is a special case of<br />
conflu<strong>en</strong>ce, just as revision is a special case of merging.<br />
Theorem 8. If ✸ is a conflu<strong>en</strong>ce operator (i.e. it satisfies (UC0-UC9)), th<strong>en</strong> the operator<br />
⋄, <strong>de</strong>fined as ϕ ⋄ µ = ✸ µ (ϕ), is an update operator (i.e. it satisfies (U1-U9)).<br />
Concerning merging operators, one can see easily that the restriction of a syncretic<br />
assignm<strong>en</strong>t to a complete profile is a distributed assignm<strong>en</strong>t. From that we obtain the<br />
following result (the one corresponding to Theorem 5):<br />
Theorem 9. If △ is an IC merging operator (i.e. it satisfies (IC0-IC8)) th<strong>en</strong> the operator<br />
✸ <strong>de</strong>fined by<br />
✸ µ (Ψ) = ∨<br />
△ µ (Ψ e )<br />
e|=Ψ<br />
is a conflu<strong>en</strong>ce operator (i.e. it satisfies (UC0-UC9)).<br />
Moreover, for each conflu<strong>en</strong>ce operator ✸, there exists a merging operator △ such<br />
that the previous equation holds.<br />
It is interesting to note that this theorem shows that every merging operator can be<br />
used to <strong>de</strong>fine a conflu<strong>en</strong>ce operator, and explains why we can consi<strong>de</strong>r conflu<strong>en</strong>ce as a<br />
pointwise merging.<br />
241
Unlike Theorem 5, the second part of the previous theorem doesn’t follow straightforwardly<br />
from the repres<strong>en</strong>tation theorems. We need to build a syncretic assignm<strong>en</strong>t<br />
ext<strong>en</strong>ding the distributed assignm<strong>en</strong>t repres<strong>en</strong>ting the conflu<strong>en</strong>ce operator. In or<strong>de</strong>r to<br />
do that we can use the following construction: Each pre-or<strong>de</strong>r ≤ e <strong>de</strong>fines naturally a<br />
rank function r e on natural numbers. Th<strong>en</strong> we put<br />
∑<br />
ω ≤ Ψ ω ′ if and only if r e (ω) ≤ ∑ r e (ω ′ )<br />
e|=Ψ<br />
e|=Ψ<br />
As a corollary of the repres<strong>en</strong>tation theorem we obtain the following<br />
Corollary 2. If ✸ is a conflu<strong>en</strong>ce operator th<strong>en</strong> the following property holds:<br />
If ∧ Ψ ⊢ µ and Ψ is consist<strong>en</strong>t th<strong>en</strong> ∧ Ψ ∧ µ ⊢ ✸ µ (Ψ)<br />
But unlike merging operators, we don’t have g<strong>en</strong>erally ✸ µ (Ψ) ⊢ ∧ Ψ ∧ µ.<br />
Note that this “half of (IC2)” property is similar to the “half of (R2)” satisfied by<br />
update operators.<br />
This corollary is interesting since it un<strong>de</strong>rlines an important differ<strong>en</strong>ce betwe<strong>en</strong><br />
merging and conflu<strong>en</strong>ce operators. If all the bases agree (i.e. if their conjunction is<br />
consist<strong>en</strong>t), th<strong>en</strong> a merging operator gives as result exactly the conjunction, whereas a<br />
conflu<strong>en</strong>ce operator will give this conjunction plus additional results. This is useful if<br />
the bases do not repres<strong>en</strong>t interpretations that are consi<strong>de</strong>red equival<strong>en</strong>t by the ag<strong>en</strong>t,<br />
but uncertain information about the ag<strong>en</strong>t’s curr<strong>en</strong>t or future state of mind.<br />
7 Example<br />
In this section we will illustrate the behaviour of conflu<strong>en</strong>ce operators on an example.<br />
We can <strong>de</strong>fine conflu<strong>en</strong>ce operators very similarly to merging operators, by using a<br />
distance and an aggregation function.<br />
Definition 11. A pseudo-distance betwe<strong>en</strong> interpretations is a total function d : W×<br />
W ↦→ R + s.t. for any ω, ω ′ ∈ W: d(ω, ω ′ ) = d(ω ′ , ω), and d(ω, ω ′ ) = 0 if and only if<br />
ω = ω ′ .<br />
A wi<strong>de</strong>ly used distance betwe<strong>en</strong> interpretations is the Dalal distance [3], <strong>de</strong>noted<br />
d H , that is the Hamming distance betwe<strong>en</strong> interpretations (the number of propositional<br />
atoms on which the two interpretations differ).<br />
Definition 12. An aggregation function f is a total function associating a nonnegative<br />
real number to every finite tuple of nonnegative real numbers s.t. for any x 1 , . . . , x n , x,<br />
y ∈ R + :<br />
– if x ≤ y, th<strong>en</strong> f(x 1 , . . . , x, . . . , x n ) ≤ f(x 1 , . . . , y, . . . , x n ) (non-<strong>de</strong>creasingness)<br />
– f(x 1 , . . . , x n ) = 0 if and only if x 1 = . . . = x n = 0 (minimality)<br />
– f(x) = x (i<strong>de</strong>ntity)<br />
242
S<strong>en</strong>sible aggregation functions are for instance max, sum, or leximax (Gmax) 6<br />
[14].<br />
Definition 13 (distance-based conflu<strong>en</strong>ce operators). Let d be a pseudo-distance betwe<strong>en</strong><br />
interpretations and f be an aggregation function. The result ✸ d,f<br />
µ (Ψ) of the<br />
⋃<br />
conflu<strong>en</strong>ce of Ψ giv<strong>en</strong> the integrity constraints µ is <strong>de</strong>fined by: mod(✸ d,f<br />
µ (Ψ)) =<br />
e|=Ψ min(mod(µ), ≤ e), where the pre-or<strong>de</strong>r ≤ e on W induced by e is <strong>de</strong>fined by:<br />
– ω ≤ e ω ′ if and only if d(ω, e) ≤ d(ω ′ , e), where<br />
– d(ω, e) = f(d(ω, ω 1 ) . . . , d(ω, ω n )) with e = {ω 1 , . . . , ω n }.<br />
It is easy to check that by using usual aggregation functions we obtain conflu<strong>en</strong>ce<br />
operators.<br />
Proposition 1. Let d be any distance, ✸ d,Σ<br />
µ (Ψ) and ✸d,Gmax µ (Ψ) are conflu<strong>en</strong>ce operators<br />
(i.e. they satisfy (UC0)-(UC9)).<br />
Example 2. Let us consi<strong>de</strong>r a profile Ψ = {ϕ 1 , ϕ 2 , ϕ 3 , ϕ 4 } and an integrity constraint<br />
µ <strong>de</strong>fined on a propositional language built over four symbols, as follows: mod(µ) =<br />
W \ {0110, 1010, 1100, 1110}, mod(ϕ 1 ) = mod(ϕ 2 ) = {1111, 1110}, mod(ϕ 3 ) =<br />
{0000}, and mod(ϕ 4 ) = {1110, 0110}.<br />
W 1111 1110 0000 0110 e 1 e 2 e 3 e 4 e 5 e 6 ✸ d,Σ<br />
µ ✸ d,Gmax<br />
µ<br />
Σ Gmax Σ Gmax Σ Gmax Σ Gmax Σ Gmax Σ Gmax<br />
0000 4 3 0 2 11 4430 10 4420 10 4330 9 4320 9 3330 8 3320<br />
0001 3 4 1 3 11 4331 10 3331 12 4431 11 4331 13 4441 12 4431<br />
0010 3 2 1 1 9 3321 8 3311 8 3221 7 3211 7 2221 6 2211 × ×<br />
0011 2 3 2 2 9 3222 8 2222 10 3322 9 3222 11 3332 10 3322 ×<br />
0100 3 2 1 1 9 3321 8 3311 8 3221 7 3211 7 2221 6 2211 × ×<br />
0101 2 3 2 2 9 3222 8 2222 10 3322 9 3222 11 3332 10 3322 ×<br />
0110 2 1 2 0 7 2221 6 2220 6 2211 5 2210 5 2111 4 2110<br />
0111 1 2 3 1 7 3211 6 3111 8 3221 7 3211 9 3222 8 3221 × ×<br />
1000 3 2 1 3 9 3321 10 3331 8 3221 9 3321 7 2221 8 3221 × ×<br />
1001 2 3 2 4 9 3222 10 4222 10 3322 11 4322 11 3332 12 4332<br />
1010 2 1 2 2 7 2221 8 2222 6 2211 7 2221 5 2111 6 2211<br />
1011 1 2 3 3 7 3211 8 3311 8 3221 9 3321 9 3222 10 3322 ×<br />
1100 2 1 2 2 7 2221 8 2222 6 2211 7 2221 5 2111 6 2211<br />
1101 1 2 3 3 7 3211 8 3311 8 3221 9 3321 9 3222 10 3322 ×<br />
1110 1 0 3 1 5 3110 6 3111 4 3100 5 3110 3 3000 4 3100<br />
1111 0 1 4 2 5 4100 6 4200 6 4110 7 4210 7 4111 8 4211 ×<br />
Table 1.<br />
The computations are reported in Table 1. The shadowed lines correspond to the interpretations<br />
rejected by the integrity constraints. Thus the result has to be tak<strong>en</strong> among<br />
6 leximax (Gmax) is usually <strong>de</strong>fined using lexicographic sequ<strong>en</strong>ces, but it can be easily repres<strong>en</strong>ted<br />
by reals to fit the above <strong>de</strong>finition (see e.g. [13]).<br />
243
the interpretations that are not shadowed. The states that mo<strong>de</strong>l the profile are the following<br />
ones:<br />
e 1 = {1111, 1111, 0000, 1110}, e 2 = {1111, 1111, 0000, 0110},<br />
e 3 = {1111, 1110, 0000, 1110}, e 4 = {1110, 1111, 0000, 0110},<br />
e 5 = {1110, 1110, 0000, 1110}, e 6 = {1110, 1110, 0000, 0110}.<br />
For each state, the Table gives the distance betwe<strong>en</strong> the interpretation and this state<br />
for the Σ aggregation function, and for the Gmax one. So one can th<strong>en</strong> look at the best<br />
interpretations for each state.<br />
So for instance for ✸ d,Σ<br />
µ (Ψ), e 1 selects the interpretation 1111, e 2 selects 0111<br />
and 1111, etc. So, taking the union of the interpretations selected by each state, gives<br />
mod(✸ d,Σ<br />
µ (Ψ)) = {0010, 0100, 0111, 1000, 1111}.<br />
Similarly we obtain mod(✸ d,Gmax<br />
µ (Ψ)) = {0100, 0011, 0010, 0101, 0111, 1000,<br />
1011, 1101}.<br />
8 Conclusion<br />
We have proposed in this paper a new family of change operators. Conflu<strong>en</strong>ce operators<br />
are pointwise merging, just as update can be se<strong>en</strong> as a pointwise revision. We provi<strong>de</strong><br />
an axiomatic <strong>de</strong>finition of this family, a repres<strong>en</strong>tation theorem in terms of pre-or<strong>de</strong>rs<br />
on interpretations, and provi<strong>de</strong> examples of these operators.<br />
In this paper we <strong>de</strong>fine conflu<strong>en</strong>ce operators as g<strong>en</strong>eralization to multiple bases of<br />
total update operators (i.e. which semantical counterpart are total pre-or<strong>de</strong>rs). A perspective<br />
of this work is to try to ext<strong>en</strong>d the result to partial update operators.<br />
As Example 1 suggests, these operator can prove meaningful to aggregate the goals<br />
of a group of ag<strong>en</strong>ts. They seem to be less a<strong>de</strong>quate for aggregating beliefs, where<br />
the global minimization done by merging operators is more appropriate for finding the<br />
most plausible worlds. This distinction betwe<strong>en</strong> goal and belief aggregation is a very<br />
interesting perspective, since, as far as we know, no such axiomatic distinction as be<strong>en</strong><br />
ever discussed.<br />
Acknowledgem<strong>en</strong>ts<br />
The i<strong>de</strong>a of this paper comes from discussions in the 2005 and 2007 Dagstuhl seminars<br />
(#5321 and #7351) “Belief Change in Rational Ag<strong>en</strong>ts” . The authors would like to<br />
thank the Schloss Dagstuhl institution, and the participants of the seminars, especially<br />
Andreas Herzig for the initial question “If merging can be se<strong>en</strong> as a g<strong>en</strong>eralization of<br />
revision, what is the g<strong>en</strong>eralization of update ?”. Here is an answer !<br />
The authors would like to thank the reviewers of the paper for their helpful comm<strong>en</strong>ts.<br />
The second author was partially supported by a research grant of the Mairie <strong>de</strong> Paris<br />
and by the project CDCHT-ULA N ◦ C-1451-07-05-A. Part of this work was done wh<strong>en</strong><br />
the second author was a visiting professor at CRIL (CNRS UMR 8188) from September<br />
to December 2007 and a visiting researcher at TSI Departm<strong>en</strong>t of Telecom ParisTech<br />
(CNRS UMR 5141 LTCI) from January to April 2008. The second author thanks to<br />
CRIL and TSI Departm<strong>en</strong>t for the excell<strong>en</strong>t working conditions.<br />
244
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5. Sébasti<strong>en</strong> Konieczny, Jérôme Lang, Pierre Marquis. DA 2 merging operators. Artificial<br />
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6. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Propositional belief base merging or how<br />
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choice theory. European Journal of Operational Research. 160(3). 785-802. 2005.<br />
7. Olivier Gauwin, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Conciliation through iterated<br />
belief merging. Journal of Logic and Computation. 17(5). 909-937. 2007.<br />
8. Sylvie Coste-Marquis, Caroline Devred, Sébasti<strong>en</strong> Konieczny, Marie-Christine Lagasquie-Schiex,<br />
Pierre Marquis. On the merging of Dung’s argum<strong>en</strong>tation systems.<br />
Artificial Intellig<strong>en</strong>ce. 171. 740-753. 2007.<br />
9. Patricia Everaere, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. The strategy-proofness landscape<br />
of merging. Journal of Artificial Intellig<strong>en</strong>ce Research. 28. 49-105. 2007.<br />
10. Philippe Besnard, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Bipolarity in bilattice logics.<br />
International Journal of Intellig<strong>en</strong>t Systems. 23(9). 1046-1061. 2008.<br />
Edition <strong>de</strong> Livres<br />
11. Eric Grégoire, Sébasti<strong>en</strong> Konieczny Ed. Information Fusion. 7(1). Special issue on<br />
Logic-based approaches to information fusion. 2006.<br />
Chapitres <strong>de</strong> Livre<br />
12. Hassan Bezzazi, Stéphane Janot, Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Analysing<br />
rational properties of change operators based on forward chaining. Transaction<br />
and Change in Logic Databases. Springer. LNCS 1472. 317-339. 1998.<br />
13. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Sur la représ<strong>en</strong>tation <strong><strong>de</strong>s</strong> états épistémiques<br />
et la révision itérée. Révision <strong><strong>de</strong>s</strong> Croyances. Hermes. 181-202. 2002.<br />
14. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. De la révision <strong>à</strong> la fusion <strong>de</strong> croyances.<br />
Révision <strong><strong>de</strong>s</strong> Croyances. Hermes. 203-226. 2002.<br />
15. Anthony Hunter, Sébasti<strong>en</strong> Konieczny. Approaches to measuring inconsist<strong>en</strong>t information.<br />
Inconsist<strong>en</strong>cy tolerance. Springer. LNCS 3300. 189-234. 2005.<br />
257
Revues Nationales<br />
16. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Sur la modélisation <strong>de</strong> la concertation<br />
<strong>en</strong>tre ag<strong>en</strong>ts : fusion <strong>de</strong> bases <strong>de</strong> croyances. Revue Information-Interaction-Intellig<strong>en</strong>ce.<br />
Cepadues. Hors Série.91-113. 2002.<br />
Confér<strong>en</strong>ces Internationales<br />
17. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. On the logic of merging. Sixth International<br />
Confer<strong>en</strong>ce on Principles of Knowledge Repres<strong>en</strong>tation and Reasoning<br />
(KR’98). 488-498. 1998.<br />
18. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Merging with integrity constraints. Fifth<br />
European Confer<strong>en</strong>ce on Symbolic and Quantitative Approaches to Reasoning with<br />
Uncertainty (ECSQARU’99). 233-244. 1999.<br />
19. Salem B<strong>en</strong>ferhat, Sébasti<strong>en</strong> Konieczny, Odile Papini, Ramón Pino Pérez. Iterated<br />
revision by epistemic states : axioms, semantics and syntax. Fourte<strong>en</strong>th European<br />
Confer<strong>en</strong>ce on Artificial Intellig<strong>en</strong>ce (ECAI’00). 13-17. 2000.<br />
20. Sébasti<strong>en</strong> Konieczny. On the differ<strong>en</strong>ce betwe<strong>en</strong> merging knowledge bases and<br />
combining them. Sev<strong>en</strong>th International Confer<strong>en</strong>ce on Principles of Knowledge Repres<strong>en</strong>tation<br />
and Reasoning (KR’00). 135-144. 2000.<br />
21. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Some operators for iterated revision. Sixth<br />
European Confer<strong>en</strong>ce on Symbolic and Quantitative Approaches to Reasoning with<br />
Uncertainty (ECSQARU’01). 498-509. 2001.<br />
22. Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Three-valued logics for inconsist<strong>en</strong>cy handling.<br />
Eighth European Confer<strong>en</strong>ce on Logic in Artifical Intellig<strong>en</strong>ce (JELIA’02).<br />
332-344. 2002.<br />
23. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. On the frontier betwe<strong>en</strong> arbitration and<br />
majority. Eighth International Confer<strong>en</strong>ce on Principles of Knowledge Repres<strong>en</strong>tation<br />
and Reasoning (KR’02). 109-118. 2002.<br />
24. Sébasti<strong>en</strong> Konieczny, Jérôme Lang, Pierre Marquis. Distance-based merging : a g<strong>en</strong>eral<br />
framework and some complexity results. Eighth International Confer<strong>en</strong>ce on<br />
Principles of Knowledge Repres<strong>en</strong>tation and Reasoning (KR’02). 97-108. 2002.<br />
25. Andreas Herzig, Sébasti<strong>en</strong> Konieczny, Laur<strong>en</strong>t Perussel. On iterated revision in the<br />
AGM framework. Sev<strong>en</strong>th European Confer<strong>en</strong>ce on Symbolic and Quantitative Approaches<br />
to Reasoning with Uncertainty (ECSQARU’03). 477-488. 2003.<br />
26. Sébasti<strong>en</strong> Konieczny, Jérôme Lang, Pierre Marquis. Quantifying information and<br />
contradiction in propositional logic through test actions. Eighte<strong>en</strong>th International<br />
Joint Confer<strong>en</strong>ce on Artificial Intellig<strong>en</strong>ce (IJCAI’03). 106-111. 2003.<br />
27. Patricia Everaere, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. On merging strategy-proofness.<br />
Ninth International Confer<strong>en</strong>ce on Principles of Knowledge Repres<strong>en</strong>tation<br />
and Reasoning (KR’04). 357-368. 2004.<br />
28. Sébasti<strong>en</strong> Konieczny, Jérôme Lang, Pierre Marquis. Reasoning un<strong>de</strong>r inconsist<strong>en</strong>cy :<br />
the forgott<strong>en</strong> connective. Ninete<strong>en</strong>th International Joint Confer<strong>en</strong>ce on Artificial Intellig<strong>en</strong>ce<br />
(IJCAI’05). 484-489. 2005.<br />
258
29. Sylvie Coste-Marquis, Caroline Devred, Sébasti<strong>en</strong> Konieczny, Marie-Christine Lagasquie-Schiex,<br />
Pierre Marquis. Merging Argum<strong>en</strong>tation Systems. Tw<strong>en</strong>tieth American<br />
National Confer<strong>en</strong>ce on Artificial Intellig<strong>en</strong>ce (AAAI’05). 614-619. 2005.<br />
30. Patricia Everaere, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Quota and Gmin merging<br />
operators. Ninete<strong>en</strong>th International Joint Confer<strong>en</strong>ce on Artificial Intellig<strong>en</strong>ce (IJ-<br />
CAI’05). 424-429. 2005.<br />
31. Olivier Gauwin, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Conciliation and cons<strong>en</strong>sus<br />
in iterated belief merging. Eigth European Confer<strong>en</strong>ce on Symbolic and Quantitative<br />
Approaches to Reasoning with Uncertainty (ECSQARU’05). 514-526. 2005.<br />
32. Anthony Hunter, Sébasti<strong>en</strong> Konieczny. Shapley Inconsist<strong>en</strong>cy Values. T<strong>en</strong>th International<br />
Confer<strong>en</strong>ce on Principles of Knowledge Repres<strong>en</strong>tation and Reasoning<br />
(KR’06). 249-259. 2006.<br />
33. Ramzi B<strong>en</strong> Larbi, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Ext<strong>en</strong>ding classical planning<br />
to the multi-ag<strong>en</strong>t case : a game-theoretic approach. Ninth European Confer<strong>en</strong>ce<br />
on Symbolic and Quantitative Approaches to Reasoning with Uncertainty<br />
(ECSQARU’07). 731-742. 2007.<br />
34. Ramzi B<strong>en</strong> Larbi, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. A mo<strong>de</strong>l for multiple outcomes<br />
games. Tw<strong>en</strong>tieth IEEE International Confer<strong>en</strong>ce on Tools with Artificial<br />
Intellig<strong>en</strong>ce (ICTAI’08). 27-34. 2008.<br />
35. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Improvem<strong>en</strong>t operators. Elev<strong>en</strong>th International<br />
Confer<strong>en</strong>ce on Principles of Knowledge Repres<strong>en</strong>tation and Reasoning<br />
(KR’08). 177-186. 2008.<br />
36. Anthony Hunter, Sébasti<strong>en</strong> Konieczny. Measuring inconsist<strong>en</strong>cy through minimal<br />
inconsist<strong>en</strong>t sets. Elev<strong>en</strong>th International Confer<strong>en</strong>ce on Principles of Knowledge<br />
Repres<strong>en</strong>tation and Reasoning (KR’08). 358-366. 2008.<br />
37. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Conflu<strong>en</strong>ce operators. Elev<strong>en</strong>th European<br />
Confer<strong>en</strong>ce on Logics in Artificial Intellig<strong>en</strong>ce (JELIA’08). 272-284. 2008.<br />
38. Patricia Everaere, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Propositional merging operators<br />
based on set-theoretic clos<strong>en</strong>ess (short paper). Eighte<strong>en</strong>th European Confer<strong>en</strong>ce<br />
on Artificial Intellig<strong>en</strong>ce (ECAI’08). 737-738. 2008.<br />
39. Patricia Everaere, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Conflict-based merging<br />
operators. Elev<strong>en</strong>th International Confer<strong>en</strong>ce on Principles of Knowledge Repres<strong>en</strong>tation<br />
and Reasoning (KR’08). 348-357. 2008.<br />
40. Sébasti<strong>en</strong> Konieczny. Using transfinite ordinal conditional functions. T<strong>en</strong>th European<br />
Confer<strong>en</strong>ce on Symbolic and Quantitative Approaches to Reasoning with Uncertainty<br />
(ECSQARU’09). 396-407. 2009.<br />
Workshops Internationaux<br />
41. Hassan Bezzazi, Stéphane Janot, Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Forward<br />
chaining and change operators. Workshop on (Trans)Actions and Change in Logic<br />
Programming and Deductive Databases (DYNAMICS’97), in conjunction with<br />
International Logic Programming Symposium (ILPS’97). 135-146. 1997.<br />
259
42. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. On the differ<strong>en</strong>ce betwe<strong>en</strong> arbitration and<br />
majority merging. Fourth Workshop on Logic, Language, Information and Computation<br />
(WoLLIC’97). 1997.<br />
43. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. An analysis of merging from a logical<br />
point of view. Elev<strong>en</strong>th Latin American Symposium on Mathematical Logic (SLA-<br />
LM’98). 1998.<br />
44. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Dynamical revision operators with memory.<br />
Ninth International Workshop on Non-Monotonic Reasoning (NMR’02). 171-<br />
179. 2002.<br />
45. Didier Dubois, Sébasti<strong>en</strong> Konieczny, H<strong>en</strong>ri Pra<strong>de</strong>. Quasi-possibilistic logic and measures<br />
of information and conflict. First International Workshop on Knowkedge Repres<strong>en</strong>tation<br />
and Approximate reasoning (KR&AR’03). 2003.<br />
46. Sébasti<strong>en</strong> Konieczny, Jérôme Lang, Pierre Marquis. Measures of contradiction and<br />
information in propositional bases. Third World Congress on Paraconsist<strong>en</strong>cy (WC-<br />
P’03). 2003.<br />
47. Sébasti<strong>en</strong> Konieczny. Belief as a game on beliefs. Uncertainty, Incomplet<strong>en</strong>ess, Imprecision<br />
and Conflict in Multiple Data Sources (Affiliated workshop to ECSQA-<br />
RU’03). 2003.<br />
48. Sébasti<strong>en</strong> Konieczny. Propositional belief merging and belief negotiation mo<strong>de</strong>l.<br />
T<strong>en</strong>th International Workshop on Non-Monotonic Reasoning (NMR’04). 249-257.<br />
2004.<br />
49. Patricia Everaere, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. On the logic of merging :<br />
Quota and Gmin merging operators. Sev<strong>en</strong>th International Symposium on Logical<br />
Formalizations of Commons<strong>en</strong>se Reasoning (CommonS<strong>en</strong>se’05). 2005.<br />
50. Olivier Gauwin, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Iterated belief merging as<br />
conciliation operators. Sev<strong>en</strong>th International Symposium on Logical Formalizations<br />
of Commons<strong>en</strong>se Reasoning (CommonS<strong>en</strong>se’05). 2005.<br />
51. Sébasti<strong>en</strong> Konieczny. More games in belief game mo<strong>de</strong>l : a preliminary report.<br />
Formal Approaches to Multi-Ag<strong>en</strong>t Systems (FAMAS’06) - Affiliated workshop<br />
to ECAI’06. 2006.<br />
52. Patricia Everaere, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Propositional merging operators<br />
based on set-theoretic clos<strong>en</strong>ess. Twelfth International Workshop on Non-<br />
Monotonic Reasoning (NMR’08). 2008.<br />
53. Ramzi B<strong>en</strong> Larbi, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. A Characterization of an<br />
optimality criterion for <strong>de</strong>cision making un<strong>de</strong>r complete ignorance. Twelfth International<br />
Workshop on Non Monotonic Reasoning (NMR’08). 2008.<br />
54. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Conflu<strong>en</strong>ce operators. Second International<br />
Workshop on Computational Social Choice (COMSOC’08). 2008.<br />
Workshops Internationaux (sans comité <strong>de</strong> selection)<br />
55. Sébasti<strong>en</strong> Konieczny. Conciliation, negotiation and merging. Dagsthul Seminar #53-<br />
21 : Belief Change in Rational Ag<strong>en</strong>ts : Perspectives from Artificial Intellig<strong>en</strong>ce,<br />
Philosophy, and Economics. 2005.<br />
260
56. Sébasti<strong>en</strong> Konieczny. Inconsist<strong>en</strong>cy measures and their application to belief change.<br />
Dagsthul Seminar #7351 : Formal Mo<strong>de</strong>ls of Belief Change in Rational Ag<strong>en</strong>ts.<br />
2007.<br />
57. Ramzi B<strong>en</strong> Larbi, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Qualitative multiple outcomes<br />
games with cons<strong>en</strong>sus. Third World Congress of the Game Theory Society<br />
(GAME’08). 2008.<br />
58. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Improvem<strong>en</strong>t operators. Dagsthul Seminar<br />
#9351 : Information processing, rational belief change and social interaction.<br />
2009<br />
Confér<strong>en</strong>ces Nationales<br />
59. Sébasti<strong>en</strong> Konieczny. Vers un modèle <strong>de</strong> la coopération. 6eme colloque <strong>de</strong> l’Association<br />
pour la <strong>Recherche</strong> Cognitive (ARC’96). 227-231. 1996.<br />
60. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Fusion avec contraintes d’intégrité. Journées<br />
Nationales sur les Modèles <strong>de</strong> Raisonnem<strong>en</strong>t (JNMR’99). 1999.<br />
61. Salem B<strong>en</strong>ferhat, Sébasti<strong>en</strong> Konieczny, Odile Papini, Ramón Pino Pérez. Révision<br />
itérée basée sur la primauté forte <strong><strong>de</strong>s</strong> observations. Journées Nationales sur les Modèles<br />
<strong>de</strong> Raisonnem<strong>en</strong>t (JNMR’99). 1999.<br />
62. Sébasti<strong>en</strong> Konieczny. Sur la logique <strong>de</strong> la concertation <strong>en</strong>tre ag<strong>en</strong>ts. Journées Francophones<br />
sur les Modèles Formels <strong>de</strong> l’Interaction (MFI’01). 289-302. 2001.<br />
63. Sébasti<strong>en</strong> Konieczny, Jérôme Lang, Pierre Marquis. Fusion <strong>de</strong> bases <strong>de</strong> croyances<br />
<strong>à</strong> partir <strong>de</strong> distances : modèle général et complexité algorithmique. Congrès Francophone<br />
<strong>de</strong> Reconnaissance <strong><strong>de</strong>s</strong> Formes et d’Intellig<strong>en</strong>ce Artificielle (RFIA’02).<br />
695-704. 2002.<br />
64. Patricia Everaere, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. De la manipulabilité <strong><strong>de</strong>s</strong><br />
opérateurs <strong>de</strong> fusion <strong>de</strong> croyances. Journées Nationales sur les Modèles <strong>de</strong> Raisonnem<strong>en</strong>t<br />
(JNMR’03). 107-121. 2003.<br />
65. Patricia Everaere, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Opérateurs <strong>de</strong> fusion <strong>à</strong><br />
quota. Journées Nationales sur les Modèles <strong>de</strong> Raisonnem<strong>en</strong>t (JNMR’03). 91-105.<br />
2003.<br />
66. Sébasti<strong>en</strong> Konieczny, Jérôme Lang, Pierre Marquis. Raisonnem<strong>en</strong>t <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce<br />
: le connecteur oublié. Congrès Francophone <strong>de</strong> Reconnaissance <strong><strong>de</strong>s</strong><br />
Formes et d’Intellig<strong>en</strong>ce Artificielle (RFIA’04). 1091-1099. 2004.<br />
67. Didier Dubois, Sébasti<strong>en</strong> Konieczny, H<strong>en</strong>ri Pra<strong>de</strong>. Logique quasi-possibiliste et mesures<br />
<strong>de</strong> conflit. Congrès Francophone <strong>de</strong> Reconnaissance <strong><strong>de</strong>s</strong> Formes et d’Intellig<strong>en</strong>ce<br />
Artificielle (RFIA’04). 1081-1090. 2004.<br />
68. Sébasti<strong>en</strong> Konieczny. SBMG : conciliation et mesures <strong>de</strong> conflit. Journées Francophones<br />
sur les Modèles Formels <strong>de</strong> l’Interaction (MFI’07). 189-200. 2007.<br />
69. Ramzi B<strong>en</strong> Larbi, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Planification multi-ag<strong>en</strong>t et<br />
diagnostique stratégique. Journées Francophones sur les Modèles Formels <strong>de</strong> l’Interaction<br />
(MFI’07). 25-36. 2007.<br />
261
70. Ramzi B<strong>en</strong> Larbi, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Jeux qualitatifs <strong>à</strong> résultats<br />
multiples. Journées Francophones <strong><strong>de</strong>s</strong> Systèmes Multi-Ag<strong>en</strong>ts (JFSMA’08). 65-74.<br />
2008.<br />
71. Patricia Everaere, Sébasti<strong>en</strong> Konieczny, Pierre Marquis. Un point <strong>de</strong> vue épistémique<br />
sur la fusion <strong>de</strong> croyances : l’i<strong>de</strong>ntification du mon<strong>de</strong> réel. Journées Francophones<br />
sur les Modèles Formels <strong>de</strong> l’Interaction (MFI’09). 135-145. 2009.<br />
72. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Opérateurs <strong>de</strong> conflu<strong>en</strong>ce. Journées Francophones<br />
sur les Modèles Formels <strong>de</strong> l’Interaction (MFI’09). 193-202. 2009.<br />
73. Sébasti<strong>en</strong> Konieczny, Ramón Pino Pérez. Opérateurs d’amélioration. Congrès Francophone<br />
<strong>de</strong> Reconnaissance <strong><strong>de</strong>s</strong> Formes et d’Intellig<strong>en</strong>ce Artificielle (RFIA’10). 50-<br />
57. 2010.<br />
74. Sébasti<strong>en</strong> Konieczny. Sur les fonctions ordinales conditionnelles transfinies. Congrès<br />
Francophone <strong>de</strong> Reconnaissance <strong><strong>de</strong>s</strong> Formes et d’Intellig<strong>en</strong>ce Artificielle (RFIA’10).<br />
302-309. 2010.<br />
Confér<strong>en</strong>ces Invitées<br />
75. Sébasti<strong>en</strong> Konieczny. Around propositional base merging. 8th Augustus De Morgan<br />
Workshop : Belief revision, belief merging and social choice (ADMW’06). King’s<br />
College, London, UK. 8-10 Novembre 2006.<br />
76. Sébasti<strong>en</strong> Konieczny. Iterated belief revision and improvem<strong>en</strong>t operators. Prefer<strong>en</strong>ce<br />
Change Workshop, London School of Economics (LSE). London, UK. 28-30<br />
mai 2009.<br />
Théses et Mémoires<br />
77. Sébasti<strong>en</strong> Konieczny. Sur la logique du changem<strong>en</strong>t : révision et fusion <strong>de</strong> bases<br />
<strong>de</strong> connaissance. Thèse <strong>de</strong> Doctorat. Laboratoire d’Informatique Fondam<strong>en</strong>tale <strong>de</strong><br />
Lille. 1999.<br />
262
INDEX<br />
A<br />
adéquation sémantique . . . . . . . . . . . . . . 48<br />
assignem<strong>en</strong>t fidèle . . . . . . . . . . . . . . . . . . 39<br />
assignem<strong>en</strong>t syncrétique . . . . . . . . . . . . 49<br />
assignem<strong>en</strong>t syncrétique juste . . . . . . . .49<br />
assignem<strong>en</strong>t syncrétique majoritaire . . 49<br />
B<br />
base . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10<br />
base <strong>de</strong> croyances . . . . . . . . . . . . . . . . . . 10<br />
C<br />
C-systèmes . . . . . . . . . . . . . . . . . . . . . . . . 21<br />
cadre d’argum<strong>en</strong>tation partiel . . . . . . . . 67<br />
changem<strong>en</strong>t minimal . . . . . . . . . . . . . . . .32<br />
cohér<strong>en</strong>ce . . . . . . . . . . . . . . . . . . . . . . . . . .32<br />
coher<strong>en</strong>ce . . . . . . . . . . . . . . . . . . . . . . . . . .27<br />
combinaison . . . . . . . . . . . . . . . . . . . . 53, 64<br />
comm<strong>en</strong>surabilité . . . . . . . . . . . . . . . . . . 64<br />
comparaison interpersonnelle <strong><strong>de</strong>s</strong> utilités<br />
. . . . . . . . . . . . . . . . . . . . . . . . . . 64<br />
conciliation . . . . . . . . . . . . . . . . . . . . . . . . 69<br />
contraction . . . . . . . . . . . . . . . . . . . . . 32, 33<br />
contraction par intersection partielle . . 37<br />
contraction par intersection totale . . . . 37<br />
D<br />
<strong>de</strong>gré d’incohér<strong>en</strong>ce . . . . . . . . . . . . . . . . 26<br />
dilatation . . . . . . . . . . . . . . . . . . . . . . . . . . 65<br />
distance . . . . . . . . . . . . . . . . . . . . . . . . . . . 51<br />
E<br />
<strong>en</strong>racinem<strong>en</strong>t épistémique . . . . . . . . . . . 38<br />
<strong>en</strong>semble minimal incohér<strong>en</strong>t . . . . . . . . 26<br />
états épistémiques . . . . . . . . . . . . . . . . . . 42<br />
ex falso quodlibet sequitur . . . . . . . . . . .21<br />
expansion . . . . . . . . . . . . . . . . . . . . . . 32, 33<br />
explosion . . . . . . . . . . . . . . . . . . . . . . . . . . 21<br />
ext<strong>en</strong>sion cohér<strong>en</strong>te projetée . . . . . . . . . 57<br />
ext<strong>en</strong>sion cohér<strong>en</strong>te symétrique . . . . . . 57<br />
F<br />
fonction d’agrégation . . . . . . . . . . . . . . . 51<br />
fonction <strong>de</strong> sélection . . . . . . . . . . . . . . . . 37<br />
fonction <strong>de</strong> sélection relationnelle . . . . 37<br />
fonction <strong>de</strong> sélection relationnelle transitive<br />
. . . . . . . . . . . . . . . . . . . . . . 37<br />
fonctions ordinales conditionnelles . . . 64<br />
fusion <strong>à</strong> base <strong>de</strong> conflits . . . . . . . . . . . . . 56<br />
fusion <strong>à</strong> base <strong>de</strong> défauts . . . . . . . . . . . . . 57<br />
fusion <strong>à</strong> base <strong>de</strong> formules . . . . . . . . . . . .52<br />
fusion <strong>à</strong> base <strong>de</strong> modèles . . . . . . . . . . . . 51<br />
fusion <strong>à</strong> base <strong>de</strong> similarité . . . . . . . . . . . 58<br />
fusion <strong>à</strong> quota . . . . . . . . . . . . . . . . . . . . . . 55<br />
fusion comme un jeu <strong>en</strong>tre sources . . . 70<br />
fusion contrainte . . . . . . . . . . . . . . . . . . . 47<br />
fusion DA 2 . . . . . . . . . . . . . . . . . . . . . . . . 54<br />
fusion <strong>de</strong> bases pondérées . . . . . . . . . . . 64<br />
fusion <strong>de</strong> programmes logiques . . . . . . 65<br />
fusion <strong>de</strong> réseaux <strong>de</strong> contraintes . . . . . .66<br />
fusion <strong>de</strong> systèmes d’argum<strong>en</strong>tation . . 66<br />
fusion disjonctive . . . . . . . . . . . . . . . . . . .55<br />
fusion <strong>en</strong> logique du premier ordre . . . 65<br />
fusion itérée . . . . . . . . . . . . . . . . . . . . . . . 70<br />
fusion majoritaire . . . . . . . . . . . . . . . . . . .48<br />
fusion prioritaire . . . . . . . . . . . . . . . . . . . 63<br />
263
I<br />
i-r<strong>en</strong>ommage . . . . . . . . . . . . . . . . . . . . . . 57<br />
i<strong>de</strong>ntité <strong>de</strong> Harper . . . . . . . . . . . . . . . . . . 35<br />
i<strong>de</strong>ntité <strong>de</strong> Levi . . . . . . . . . . . . . . . . . . . . .35<br />
indice <strong>de</strong> satisfaction . . . . . . . . . . . . . . . .61<br />
indice drastique faible . . . . . . . . . . . . . . .61<br />
indice drastique fort . . . . . . . . . . . . . . . . 61<br />
indice probabliste . . . . . . . . . . . . . . . . . . .61<br />
infér<strong>en</strong>ce <strong>à</strong> base <strong>de</strong> cardinalité . . . . . . . 19<br />
infér<strong>en</strong>ce argum<strong>en</strong>tative . . . . . . . . . . . . . 19<br />
infér<strong>en</strong>ce classique . . . . . . . . . . . . . . . . . 18<br />
infér<strong>en</strong>ce <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce . .18<br />
infér<strong>en</strong>ce exist<strong>en</strong>tielle . . . . . . . . . . . . . . . 19<br />
infér<strong>en</strong>ce libre . . . . . . . . . . . . . . . . . . . . . .19<br />
infér<strong>en</strong>ce non monotone . . . . . . . . . . . . .18<br />
infér<strong>en</strong>ce universelle . . . . . . . . . . . . . . . . 19<br />
J<br />
jeux <strong>de</strong> coalitions . . . . . . . . . . . . . . . . . . . 72<br />
L<br />
L-infér<strong>en</strong>ce . . . . . . . . . . . . . . . . . . . . . . . . 19<br />
logique FOUR . . . . . . . . . . . . . . . . . . . . 23<br />
logique SEVEN . . . . . . . . . . . . . . . . . . . 23<br />
logique <strong>de</strong> Belnap . . . . . . . . . . . . . . . . . . 23<br />
logique <strong><strong>de</strong>s</strong> défauts supernormaux . . . .23<br />
logique LP . . . . . . . . . . . . . . . . . . . . . . . . . 22<br />
logique LP m . . . . . . . . . . . . . . . . . . . . . . . 22<br />
logique multi-valuée . . . . . . . . . . . . . . . . 23<br />
logique possibiliste . . . . . . . . . . . . . . 41, 64<br />
logique quasi-classique . . . . . . . . . . . . . 21<br />
logique quasi-possibiliste . . . . . . . . . . . .21<br />
logique tri-valuée . . . . . . . . . . . . . . . . . . . 22<br />
logiques <strong>à</strong> base <strong>de</strong> bi-treillis . . . . . . . . . 23<br />
logiques non monotones . . . . . . . . . . . . .40<br />
logiques paraconsistantes . . . . . . . . . . . .21<br />
M<br />
manipulabilité . . . . . . . . . . . . . . . . . . . . . .61<br />
marchandage . . . . . . . . . . . . . . . . . . . . . . .71<br />
maxcons . . . . . . . . . . . . . . . . . . . . . . . 19, 52<br />
MC-infér<strong>en</strong>ce . . . . . . . . . . . . . . . . . . . . . . 19<br />
mesure d’incohér<strong>en</strong>ce basée sur les formules<br />
. . . . . . . . . . . . . . . . . . . . 25<br />
mesure d’incohér<strong>en</strong>ce basée sur les variables<br />
. . . . . . . . . . . . . . . . . . . . . . 26<br />
mesure d’information . . . . . . . . . . . . . . . 24<br />
mesure <strong>de</strong> contradiction . . . . . . . . . . . . . 24<br />
mesures d’incohér<strong>en</strong>ces . . . . . . . . . . . . . 28<br />
mesures d’incohér<strong>en</strong>ce <strong>à</strong> base <strong>de</strong> plans <strong>de</strong><br />
tests . . . . . . . . . . . . . . . . . . . . . .27<br />
minimisation absolue . . . . . . . . . . . . . . . 43<br />
mon<strong>de</strong> possible . . . . . . . . . . . . . . . . . . . . .38<br />
N<br />
η−cohér<strong>en</strong>ce . . . . . . . . . . . . . . . . . . . . . . .25<br />
η−cohér<strong>en</strong>ce maximale . . . . . . . . . . . . . 25<br />
négociation . . . . . . . . . . . . . . . . . . . . . . . . 69<br />
O<br />
opérateur d’arbitrage . . . . . . . . . . . . . . . .49<br />
opérateurs d’amélioration . . . . . . . . . . . 45<br />
opérateurs <strong>de</strong> combinaison . . . . . . . . . . 53<br />
opérateurs <strong>de</strong> conflu<strong>en</strong>ce . . . . . . . . . . . . 71<br />
opérateurs <strong>de</strong> révision dynamique . . . . 45<br />
P<br />
paradoxe <strong>de</strong> la loterie . . . . . . . . . . . . . . . 25<br />
primauté <strong>de</strong> la nouvelle information . . 32<br />
principe <strong>de</strong> changem<strong>en</strong>t minimal . . . . . 32<br />
principe <strong>de</strong> cohér<strong>en</strong>ce . . . . . . . . . . . . . . . 32<br />
principe <strong>de</strong> succès . . . . . . . . . . . . . . . . . . 32<br />
profil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10<br />
(pseudo-)distance . . . . . . . . . . . . . . . . . . .51<br />
R<br />
révision . . . . . . . . . . . . . . . . . . . . . . . . 32, 34<br />
révision AGM . . . . . . . . . . . . . . . . . . . . . .34<br />
révision <strong>de</strong> Darwiche et Pearl . . . . . . . . 43<br />
révision DP . . . . . . . . . . . . . . . . . . . . . . . . 43<br />
révision itérée . . . . . . . . . . . . . . . . . . . . . . 41<br />
révision itérée <strong>à</strong> mémoire . . . . . . . . . . . .45<br />
révision naturelle . . . . . . . . . . . . . . . . . . . 43<br />
révision non prioritaire . . . . . . . . . . . . . . 45<br />
relation <strong>de</strong> nécessité qualitative . . . . . . 41<br />
264
S<br />
sous-<strong>en</strong>semble maximal cohér<strong>en</strong>t . . . . 19<br />
sous-théories maximales <strong>de</strong> K n’impliquant<br />
pas α . . . . . . . . . . . . . . . . . 36<br />
succès . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32<br />
système <strong>de</strong> sphères . . . . . . . . . . . . . . . . . 38<br />
T<br />
théorème <strong>de</strong> représ<strong>en</strong>tation . . . . . . . . . . 32<br />
théorème du jury . . . . . . . . . . . . . . . . . . . 60<br />
théorème du jury sous incertitu<strong>de</strong> . . . . 60<br />
théorie du choix social . . . . . . . . . . . . . . 65<br />
U<br />
unanimité sur les formules . . . . . . . . . . .55<br />
unanimité sur les interprétations . . . . . .55<br />
V<br />
valeur d’incohér<strong>en</strong>ce basées sur les minimaux<br />
incohér<strong>en</strong>ts . . . . . . . . . .29<br />
valeur d’incohér<strong>en</strong>ce <strong>de</strong> Shapley . . . . . 29<br />
valeurs d’incohér<strong>en</strong>ces . . . . . . . . . . . . . . 28<br />
valeurs <strong>de</strong> vérité désignées . . . . . . . . . . 22<br />
vecteur <strong>de</strong> conflit . . . . . . . . . . . . . . . . . . . 56<br />
vote par approbation . . . . . . . . . . . . . . . . 60<br />
265
266
RÉSUMÉ<br />
Ce travail se situe dans le domaine <strong><strong>de</strong>s</strong> logiques pour l’intellig<strong>en</strong>ce artificielle, et concerne<br />
plus précisém<strong>en</strong>t la modélisation et la représ<strong>en</strong>tation <strong><strong>de</strong>s</strong> connaissances et <strong><strong>de</strong>s</strong> raisonnem<strong>en</strong>ts<br />
(KR).<br />
Nous nous intéressons au problème du raisonnem<strong>en</strong>t <strong>en</strong> prés<strong>en</strong>ce d’incohér<strong>en</strong>ce. Selon<br />
les informations supplém<strong>en</strong>taires dont on dispose, cela conduit <strong>à</strong> <strong><strong>de</strong>s</strong> cadres différ<strong>en</strong>ts,<br />
nécessitant <strong><strong>de</strong>s</strong> métho<strong><strong>de</strong>s</strong> <strong>de</strong> raisonnem<strong>en</strong>t spécifiques :<br />
• Incohér<strong>en</strong>ce : on ne dispose d’aucune information supplém<strong>en</strong>taire. Il est donc nécessaire<br />
d’obt<strong>en</strong>ir <strong><strong>de</strong>s</strong> conclusions cohér<strong>en</strong>tes (raisonnables) <strong>à</strong> partir d’un <strong>en</strong>semble<br />
d’information incohér<strong>en</strong>t.<br />
• Révision : une <strong><strong>de</strong>s</strong> formules est plus importante que les autres. Il faut donc gar<strong>de</strong>r<br />
cette formule, tout <strong>en</strong> éliminant les incohér<strong>en</strong>ces.<br />
• Fusion : les formules provi<strong>en</strong>n<strong>en</strong>t <strong>de</strong> sources différ<strong>en</strong>tes. Il faut alors définir une<br />
base cohér<strong>en</strong>te <strong>à</strong> partir <strong>de</strong> ces informations, <strong>en</strong> pr<strong>en</strong>ant <strong>en</strong> compte la localisation <strong>de</strong><br />
ces informations (avec <strong><strong>de</strong>s</strong> argum<strong>en</strong>ts majoritaires par exemple).<br />
• Négociation : les formules provi<strong>en</strong>n<strong>en</strong>t <strong>de</strong> sources différ<strong>en</strong>tes, comme pour la fusion,<br />
mais les sources gar<strong>de</strong>nt la maîtrise <strong><strong>de</strong>s</strong> modifications <strong>de</strong> leurs formules. Il faut<br />
donc t<strong>en</strong>ir compte <strong>de</strong> possibles interactions (coalitions, etc.).<br />
Notre démarche est principalem<strong>en</strong>t axiomatique, c’est-<strong>à</strong>-dire que nous t<strong>en</strong>tons <strong>de</strong> trouver<br />
<strong><strong>de</strong>s</strong> caratérisations logiques pour les différ<strong>en</strong>tes métho<strong><strong>de</strong>s</strong> <strong>de</strong> raisonnem<strong>en</strong>t. Nous<br />
prés<strong>en</strong>tons nos travaux concernant ces quatre cadres, <strong>en</strong> particulier les caractérisations<br />
obt<strong>en</strong>ues pour les mesures d’incohér<strong>en</strong>ce, la révision itérée, et la fusion ; ainsi que les<br />
premiers résultats concernant la négociation.<br />
267
268
Raisonnem<strong>en</strong>t & Incohér<strong>en</strong>ce<br />
<strong>Habilitation</strong> <strong>à</strong> <strong>diriger</strong> <strong><strong>de</strong>s</strong> <strong>recherches</strong><br />
Sébasti<strong>en</strong> Konieczny<br />
Composition du jury<br />
Gerhard Brewka (rapporteur)<br />
Professeur <strong>à</strong> l’Université <strong>de</strong> Leipzig<br />
Marie-Christine Rousset (rapporteur)<br />
Professeur <strong>à</strong> l’Université <strong>de</strong> Gr<strong>en</strong>oble<br />
Torst<strong>en</strong> Schaub (rapporteur)<br />
Professeur <strong>à</strong> l’Université <strong>de</strong> Potsdam<br />
Salem B<strong>en</strong>ferhat<br />
Professeur <strong>à</strong> l’Université d’Artois<br />
Andreas Herzig<br />
Directeur <strong>de</strong> <strong>Recherche</strong> au CNRS - IRIT<br />
David Makinson<br />
Professeur <strong>à</strong> la London School of Economics<br />
Pierre Marquis<br />
Professeur <strong>à</strong> l’Université d’Artois