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CAUSATION, DISPOSITIONS AND MATHEMATICAL PHYSICS 169
the joint manifestation of the graviational mass dispositions of both bodies. Alternatively, one
can introduce forces as intermediate entities in the causation complex. A realism with respect
to classical forces has been convincingly argued for by Jessica Wilson (Wilson 2007).
Causation by forces can be integrated into a dispositional framework. Forces are treated as a
special type of dispositions which have accelerations as their manifestations. So here I depart
from the view that dispositions are directly connected to processes; forces are intermediaries
between the causal powers of things and the accelerations which characterize the processes
these things undergo.
I suggest to understand the relation between the causal power of an entity and the executed
forces as a relation of disposition and manifestation. The gravitation disposition D 1 of a heavy
body K 1 is to act by force on a second gravitating body according to Newton’s law.
F(K 1,K 2) acting on a second body (patient) K 2 is a manifestation of the disposition D 1 of
body K 1
F(K 1,K 2) is a manifestation of a disposition of K 1, but the force itself has dispositional
character as well. It is a disposition of the second body K 2 for a change of its motion
(acceleration). The manifestation of the acceleration is a change of the body’s
motion, that is a process or a change in a process parameter.
Schematically: D 1(K 1) → M 1(K 2) = D 2(K 2) → M 2
As we could give the same description from the other bodies’ perspective a (central) force is a
joint manifestation of mutually dependent dispositions of two bodies. And the manifestation
(acceleration) of a force-disposition of the body K 2 is dependent on all other forces acting on
this body. In this way we can also describe forces in equilibrium where no motion takes place,
although forces are active.
4.4 Quantum Propensities
Finally, I consider briefly the case of propensities in quantum mechanics (QM), an approach
originally suggested by Popper (Popper 1959). In QM we find a similar divergence of two
apparently conflicting descriptions, one in terms of a systems’ continuous time evolution and
one in terms of events of measurement and their probabilities, and this seems to lie at the
heart of the notorious interpretational difficulties. On the one hand the “Schrödinger” time
evolution of quantum states according to Schrödinger’s equation is continuous and
deterministic. On the other hand the “Von Neumann” state change is discontinuous,
probabilistic and seems to involve the collapse of the developed state into an eigenstate upon
the triggering of a measurement-like event. This is of course a highly contested terrain, but
one can interpret the quantum superposition state as exhibiting stochastic dispositions,
“propensities” for the possible values of measurement results. The trigger for this projection
onto an eigenstate and the manifestation of a definite value would be the causal influence of
the measuring device. In the decoherence approach (cf. Giulini et al. 1996) the apparent
collapse is due to the interaction with the environment. This could be taken as the deeper
analysis of a propensity model of the collapse, so in this case propensities would probably not
correspond to fundamental, irreducible features. Or one could subscribe to the
Ghirardi/Rimini/Weber model (Ghirardi/Rimini/Weber 1986) with a modified dynamics
that leads to untriggered, spontaneous collapses of the superposition state which are real, not
merely apparent. In this case quantum propensities are fundamental dispositions without the
necessity of a trigger. While this is an ongoing debate, it seems that propensities can capture
important aspects of the ontology of quantum mechanics, and are considered as a serious
option for an ontology of quantum field theory (cf. Kuhlmann 2010).