Ed Feser has been giving Paul Churchland a well-deserved drubbing over at his blog and I should like to join in on the fun, at least in the in the first main paragraph of this post.
One of the standard objections to substance dualism in the philosophy of mind is that the substance dualist cannot account for mind-body and body-mind causal interaction. I have already quoted Dennett and Searle to this effect. Here is Paul M. Churchland repeating for the umpteenth time a standard piece of materialist boilerplate:
How is this utterly insubstantial 'thinking substance' to have any influence on ponderous matter? How can two such different things be in any sort of causal contact? (Matter and Consciousness, p. 9)
Churchland apparently thinks that a substance, to be 'substantial,' must be material. Churchland thereby betrays his inability to conceive of (which is not the same as to imagine) an immaterial substance. Note that 'immaterial substance' is not an oxymoron like 'immaterial matter.' Feser in his series of posts shows just how ignorant Churchland is of the history of philosophy, so it is no surprise that he cannot wrap his eliminativist head around the concept of substance as used by Descartes et al. But let that pass. The issue for now is simply this: How can two things belonging to radically disjoint ontological categories be in causal contact? But here again, Churchland seems to be laboring under a false assumption, namely, that causation must involve contact between cause and effect. But why should we think that this 'billiards ball' model of causation fits every type of causation? Why must we think of causation as itself a physical process whereby a physical magnitude such as energy is transferred from one physical object to another? On regularity and counterfactual theories of causation there is no difficulty in principle with the notion of a causal relation obtaining between two events that do not make physical contact.
But today I have different fish to fry. I want to see if the materialist's conception of mind-body interaction is free of difficulty. If the materialist view of mind were without difficulty I would be a materialist, on grounds of parsimony.
Suppose that every token mental state is identical to some token brain state. (Token as opposed to type.) This token-token identity will be held both by type-type identity theorists as well as by functionalists who reject type-type identity. Accordingly, my pain sensation after touching a hot stove is strictly identical to (not merely correlated with) a state of my brain. If so, there would seem to be no difficulty in principle with this state's being brought about by other physical states. For then physical-mental causation would just be a special case of physical-physical causation — which we assume to be unproblematic. (The general problem of causation is not now on the table.) At first glance, then, it appears that physicalism does not face the sort of interaction objection that substance dualism faces. To put it graphically, there is no 'ontological chasm' to 'jump over.' The mind-body problem gets traded in for the brain-body problem which presents no special philosophical difficulties, as difficult as it may be to work out the scientific details.
But why does the brain state which is (identically!) my pain sensation cause aversive behavior such as the withdrawal of my hand from the stove? Because it has the property of being painful. It is the painfulness of the sensation that is causally relevant. Why did I remove my hand from the stove? Because touching the stove hurt. A causal explanation of my aversive behavior (the yanking back of the hand) cannot merely invoke a brain state such as the firing of C-fibers (or whatever a completed neuroscience would specify): it must make reference to a brain state that has the property of being painful. This is a qualitative property or a quale to use a piece of phlosophical jargon. It is the peculiar first-person feel of the experience, the what-it-is-like of it. This raw feel (to us an old expression of Herbert Feigl) has what Searle calls a "first person ontology": its esse is its percipi. One cannot distinguish its appearing from its being.
Now here is the problem: if the pain sensation is identical to a brain event, then there is no place for the felt painfulness. For the brain event has a third person mode of existence and among the properties of this brain event you will not find anything that has a first person mode of existence. A brain event is purely physical and has only physical properties.
But even if you insist that the phenomenal property of painfulness -- the sensory quale -- is a property of the brain state, this phenomenal property surely plays no causal role in bringing about the aversive behavior. For it is only the physical aspects of the brain state that play a causal role in bringing about the the other physical states that comprise aversive behavior.
The materialist seems to be no better off than the substance dualist. Both face the problem of interaction since common sense strongly suggests that mind acts on body, and body acts on mind. (Parallelists and occasionalists deny this, but they move quite a distance from common sense.) The problem for the dualist is the problem of bridging the gap between two disjoint ontological categories, while the problem for the materialist is explaining the causal relevance of a pain sensation, say, to other physical events when the very painfulness which accounts for the aversive behavior cannot play any causal role if the pain sensation is identical to a brain state.
Although the materialist avoids the dualists' gap problem, he faces a different problem which is just as bad: the problem of explaining how a brain state can bring about aversive behavior when the property of this brain state that ought to be causally relevant, namely, the phenomenal painfulness of it, cannot be causally relevant.
In sum, the materialist solves the gap problem by construing mental-physical interaction as physical-physical interaction, but he can do this only by rendering the very phenomenal properties that first give rise to the mind-body problem causally inefficacious.
Why then the double standard? Why is the substance dualist taxed with a problem of interaction, when the materialist faces an equally difficult problem? My conjecture is that there is an ideological bias in favor of materialism that is at the root of this double standard.
Recommended Reading: Kenneth Einar Himma, "What is a Problem for All is a Problem for None: Substance Dualism, Physicalism, and the Mind-Body Problem," Amer. Phil. Quart. (April 2005), 81-92.