In chapter 2 of his book Sensations: A Defense of Type Materialism, Christopher Hill presents an argument in favor of type materialism – the thesis that mental states are brain states and that mental types are brain types – with regard to sensations. Importantly, Hill’s argument applies only to those mental states which have an accompanying ‘feel’ to them, such as pain (or, the mental states which have a qualitative aspect to them).
Hill’s argument presupposes that what he calls the ‘psychophysical correlation thesis’ is correct: the thesis that for any qualitative aspect of a sensation S, it is a law of nature that neural event N always accompanies any occurrence of S. Thus, a creature x is experiencing S iff (if and only if) N is occurring in x’s brain. Supposing this is true, we can allow Hill to advance his argument: the identity theory (which says that sensations just are neural events) best explains the psychophysical correlation thesis. That is to say, the identity theory provides a good explanation of the correlation thesis and this explanation is better than any rival theory.
The rival theories Hill mentions are dualism and what he calls the ‘double aspect theory.’ Dualism is the view that mental events are non-physical, perhaps occurring in a separate mental substance of some sort. Hill avoids delimiting the varieties of dualism and says that the whole lot of them falls victim to not being able to explain the correlation thesis at all: why should neural event N always accompany sensation S? The dualist has no answer at all except to appeal to mysterious forces like God; and even after such an appeal is made, more questions have to be raised: why should God have made it such that there are two types of thing rather than one? Why did God correlate N with S rather than S2? These questions are unanswerable and thus the dualist is stuck with saying that these correlations are simply brute facts, not explicable by any further reasoning. And this lack of explanation is clearly inferior to the positive explanation proffered by the identity theory.
Meanwhile, Hill thinks that the double aspect theory (which, by the way, is a form of token physicalism), which says that mental events are physical events but also that qualitative properties are not equivalent to physical properties but rather are intrinsic properties, also runs into the problem of failing to provide an explanation for the psychophysical correlation thesis. While the double aspect theory is in better shape than dualism because it can say that a conscious experience E is identical to a neural event N. Where it runs into an explanatory impasse, however, is when one asks why the property of ‘being an experience of type E’ is always correlated with the property of ‘being neural event N.’ The identity theorist can answer this question by explaining that the property of ‘being an experience of type E‘ is actually identical with the property of ‘being a neural event N.’ The double aspect theorist, meanwhile, cannot explain this correlation. Thus, the identity and double aspect theory can both explain the correlation of mental and physical events, but only the identity theory can explain the correlation of their accompanying properties.
Argument: The Identity Theory Best Explains the Psychophysical Correlation Thesis (the major premises are right out of the book, whereas the minor premises are the result of my usual argumentative reconstructions):
1. If a theory provides a good explanation of a set of facts, and the explanation is better than any explanation provided by a competing theory, then one has a good ans sufficient reason to believe that the theory is true.
2. Type materialism provides a good explanation of the psychophysical correlations that are claimed to exist by the psychophysical correlation thesis.
3. Moreover, the explanation that it provides is superior to the explanations provided by all competing theories.
S1. Type materialism provides a superior explanation than dualism.
SS1. Type materialism successfully explains the correlations posited by the psychophysical correlation thesis.
SS2. Dualism does not explain the correlations posited by the psychophysical correlation thesis, but takes them to be inexplicable, brute facts.
SS3. Therefore, type materialism provides a superior explanation than dualism. [SS1, SS2]
S2. Type materialism provides a superior explanation than the double aspect theory.
SS1. Type materialism can explain the correlation of sensations and brain processes and the correlation of their accompanying properties.
SS2. The double aspect theory can only explain the correlation between sensations and brain processes, not the correlation of their accompanying properties.
SS3. Therefore, type materialism provides a superior explanation than the doublr aspect theory. [SS1,SS2]
S3. Therefore, the explanation type materialism provides is superior to the explanations of all competing theories. [S1 & S2]
4. Therefore, provided that the psychophysical correlation thesis is true, we have good and sufficient reason to suppose that type materialism is true.