Psychiatric Diagnosis and Classification - ResearchGate

4 PSYCHIATRIC DIAGNOSIS AND CLASSIFICATION
provide a ``natural'' classification. In medicine, an essentialist view of diseases
as independently existing agents causing illnesses in individuals was
proposed by Sydenham in the eighteenth century [11]; its vestiges survive
into the present in some interpretations of the notion of ``disease entity''.
A radically different philosophy of classification evolved more recently in
biology as a way out of certain difficulties in applying the Darwinian
phyletic principle to the systematics of bacteria and viruses. In contrast to
the essentialist strategy, this approach, known as numerical taxonomy, shifts
the emphasis to the systematic description of the appearance of objects hence
the approach is also called phenetic) and treats all characters and attributes
as having equal weight [6]. Groups are then identified on the basis of the
maximum number of shared characteristics using statistical algorithms. An
approximation to such a strategy in medical classification would be the
empirical grouping of symptoms and signs using cluster or factor analysis.
Another recent taxonomic strategy, based on the analysis of ``folk'' systems
of categories referred to above, is the prototype-matching procedure
[12, 13]. In this approach, a category is represented by its prototype, i.e. a
fuzzy set comprising the most common features or properties displayed
by ``typical'' members of the category. The features describing the prototype
need be neither necessary nor sufficient, but they must provide a theoretical
ideal against which real individuals or objects can be evaluated. Statistical
procedures can be used to compute for any individual or object how closely
they match the ideal type.
The taxonomic strategies described above employ different rules for identifying
taxon membership. Thus, the classical phyletic strategy presupposes
a monothetic assignment of membership in which the candidate must meet
exactly the set of necessary and sufficient criteria that define a given class. In
contrast, both numerical taxonomy and the prototype-matching approach
are polythetic, in the sense that members of a class ``share a large proportion
of their properties but do not necessarily agree on the presence of any one
property'' [6]. The periodic table of the elements, where atomic weight and
valence are the only characteristics that are both necessary and sufficient for
the ordering of the entire chemical universe, is a pure example of a monothetic
classification. DSM-IV and ICD-10 research criteria are examples of a
polythetic classification where members of a given class share some, but not
all, of its defining features.
THE NATURE OF PSYCHIATRIC CLASSIFICATION:
CRITIQUE OF THE PRESENT STATE OF NOSOLOGY
No single type of classification fits all purposes. It is unlikely that the
principles underlying the classification of chemical elements, or subatomic