The Argument from Mental Causation
I. The Argument from Mental Causation
The third of my arguments is the Argument from Mental Causation. If naturalism is true, even if there are propositional states like beliefs, then these states have to be epiphenomenal, without a causal role. Now careful reflection on rational inference, if we think about it, commits us to the idea that one mental event causes another mental event in virtue of its propositional content.
Now if events are caused in accordance with physical law, they cause one another in virtue of being a particular type of event. A ball breaks a window in virtue of being the weight, density, and shape that it is in relation to the physical structure of the window. Even if it is the baseball that Luis Gonzalez hit against Mariano Rivera that won the 2001 World Series, its being that ball has nothing to do with whether or not it can break the window now.
So let us suppose that brain state A, which is token identical to the thought that all men are mortal, and brain state B, which is token identical to the thought that Socrates is a man, causes the belief that Socrates is mortal. It isn’t enough for rational inference that these events be those beliefs, it is also necessary that the causal transaction be in virtue of the content of those thoughts. If anything not in space and time makes these thoughts the thoughts that they are, and if naturalism is true, then the propositional content is irrelevant to the causal transaction that produces the conclusion, and we do not have a case of rational inference. In rational inference, as Lewis puts it, one thought causes another thought not by being, but by being seen to be the ground for it. But causal transactions in the brain occur in virtue of the brain’s being in a particular type of state that is relevant to physical causal transactions. Only that property of the brain can be relevant to what the brain does, according to a naturalistic account of causation.
What this means is that those forms of substance materialism that accept property dualism invariably render the “mental” properties epiphenomenal. If the physical properties are sufficient to produce the physical effect, then the mental properties are irrelevant unless they are really physical properties “writ large,” so to speak. And mental states that are epiphenomenal cannot really participate in rational inference.
Carrier’s account of mental causation clearly presupposes a reductive, rather than a nonreductive materialism. He writes:
Every meaningful proposition is the content or output of a virtual model (or rather: actual propositions, of actual models; potential propositions, of potential models). Propositions are formulated in a language as an aid to computation, but when they are not formulated, they merely define the content of a nonlinguistic computation of a virtual model. In either case, a brain computes degrees of confidence in any given proposition, by running its corresponding virtual model and comparing it and its output with observational data, or the output of other computations. Thus, when I say I "accept" Proposition A this means that my brain computes a high level of confidence that Virtual Model A corresponds to a system in the real world (or another system in our own or another's brain, as the case may be); while if I "reject" A, then I have a high level of confidence that A does not so correspond; but if I "suspend judgment," then I have a low level of confidence either way. By simply defining "proposition" as I have here, Proposition 3 follows necessarily from Propositions 1 and 2. Therefore naturalism can account for this as well.
But I see a serious problem with this whole concept. In order for the content of the mental state to be relevant to the production of a rational inference, it seems to me that everyone who believes that Socrates is mortal would have to be in the same type of brain state as everyone else who believes that Socrates is mortal. Is this plausible?
But more than that, here again we find Carrier explaining one kind of mental activity in terms of another mental activity and then explaining it “naturalistically” by saying “the brain” does it. My argument is, first and foremost, that something exists whose activities are to be fundamentally explained in intentional and teleological terms. Whether we call it a brain, a part of the brain, a soul, a banana, or a bowling ball is not essential to my argument; if the fundamental explanations are intentional, then I have established all that I am trying to establish.