- Token Monism, Event Dualism and Overdetermination
The argument from causal overdetermination ('the CO-argument') is considered to be the shortest route to token monism. It only assumes that:
1. Efficacy: Mental events are causes of physical events.
2. Closure: Every physical event has a sufficient physical cause (if it has any sufficient cause1).
3. Exclusion: Systematic Causal Overdetermination (CO) is impossible: if an event x is a sufficient cause of an event y then no event x* distinct from x is a cause of y.2 [End Page 63]
4. Identity: Therefore, mental events are physical events.
Exclusion does not deny the possibility of two gunmen that fire at a victim at the same time. But event-dualism is like a systematic firing-squad case — whenever I want to raise my arm, my arm is raised, and that is intolerable. What Davidson gets from dubious principles, such as the nomological character of causation and the anomaly of the mental,3 the argument from overdetermination gets us almost for free, save for the near-triviality that systematic overdetermination is impossible.
However, whenever I want to raise my arm, I do so in virtue of a mental property (the property of being a desire to raise my hand), which is causally relevant to my behavior; and whenever it is causally relevant, we can tell a complete physical story about my arm-raising without mentioning mental properties at all. That physical properties are causally sufficient for the production of behavior seems to exclude the causal relevance of mental properties. One has only to replace 'event x' with 'property X,' and 'cause' with 'causally relevant'4 in order to get a parallel argument ('the PO-argument'), which leads directly to type identity:
1. Efficacy*: Mental properties are causally relevant for the occurrence of physical events.
2. Closure*: Every physical event e is preceded by some other event that has some physical property, which causally suffices for e to occur (if anything causally suffices for its occurrence) just as it does (i.e., with the properties it has).
3. Exclusion*: Systematic Property Overdetermination (PO) is impossible: if a property X is causally sufficient for the occurrence of y, then no distinct property X* is causally relevant for y.5 [End Page 64]
4. Identity*: Therefore, mental properties are physical properties.
The thesis of this paper is that in dispatching the PO-argument against their own position, non-reductive physicalists thereby open the door to event dualism, for the considerations they adduce against the PO-argument work equally well against the CO-argument. So, until they venture some further argument against event dualism, it's hard to see why they aren't led by their rebuttal of the PO-argument to endorse this doctrine.
The conclusion that both arguments are strongly interlinked is not new by now6; what is distinctively new is the argument for it. My argument does not depend on the tight connection between causation and explanation, held by rivals like Kim and Burge. I do not enter the metaphysical versus epistemic controversy about causation and explanation. Nor do I argue that one of the premises in both arguments is unjustified, or that the arguments are invalid.7 My point is to use Lewis's and Bennett's innocuous constraint on how to evaluate certain counterfactual claims, 'the ban of replacement,'8 to show that there is no special overdetermination worry that cuts against (event) dualists but not monists. The contribution of this paper lies in the claim that if we evaluate the counterfactual claims which are necessary for overdetermination [End Page 65] in accordance with 'the ban of replacement,' we get a surprising result: whatever the token monist does to refute the PO-argument is, ipso facto, a refutation of the CO-argument.9 The point is not that a dualist can refute the CO-argument by a strategy similar or close to that used by the monist to refute the PO-argument; rather, my point is that the dualist has nothing else to do: the token monist's refutation of the PO-argument does the job for her too.10
In the second section I...