The type identity theory of physicalism is the idea, as outlined by David Lewis (Journal of Philosophy, 63, pp 17-25), that "every experience is identical with some physical state." Experiences and states in this sense are to be taken as universals, and not as particulars, which is where the type identity theory differs from the token identity theory.
In this essay I will look at the type identity theory as outlined by Lewis, and also the problem of whether this theory can accommodate the idea that mental states are variably realisable in animals of different physical natures. This problem is talked about in the article "The Nature of Mental States" by Hilary Putnam (1979).
[...] And so while Lewis's theory does imply that the appropriate population for our ordinary Martian is the Martian population, it has to be because and outweigh and together, and not simply by itself as Lewis suggests. But how can outweigh itself? There seems to be a slight inconsistency in the method used by Lewis. However, the real problems occur when looking at the case of a mad Martian. The example is as follows: X is a mad Martian, I would be inclined to say that he is in pain when the cavities in his feet are inflated; and so says our theory provided that criteria and together outweigh either or by itself.” (Lewis p220). [...]
[...] This is not compatible with the identity theory because, according to the identity theory, pain is whatever state occupies the pain role for us. It seems that the identity theory is too chauvinistic, in that it does not allow animals that clearly have mental states to in fact truly have mental states. However, Putnam's theory seems far too liberal, in that it will allow virtually anything that can be in a state of some functional kind to have a mental state, even something like a tin can! [...]
[...] So, why if this is also an idea that the mental states we have are physical in some way, does it pose a problem for the type identity theory of physicalism? Well, the reason is that in the type identity theory, we have to say that if you have a mind, you must have a brain. This discounts the possibility that we can have brains made of other things, such as silicon, or alien brains. It also leaves us with a problem of how we describe something like the pain of an animal. [...]
[...] But second, unless outweighs there will be no grounds for taking the appropriate population for, for example, an artificially intelligent robot to be the population of similarly designed robots (no natural kind) as opposed to the human population. It seems to me that all of the theories I have mentioned have serious problems. The identity theory is flawed because it cannot accommodate the realisability of mental states within creatures of different physical natures. The attempt to solve this through Putnam's functionalist model also fails because it cannot accommodate mental states such as mad pain, where the pain is not caused by the normal pain causal role. Finally, Lewis' attempt to combine [...]
[...] In other words, the first premise of Lewis' argument is that all experiences are defined by their causal roles. This principle, as Lewis says, is not in itself a materialist principle. It is merely saying that experiences are in some way real and that they are efficacious outside their own realm. It does not say anything about the actual nature of the reality of the experience, or the nature of its efficacy. The second premise that Lewis talks about is the idea that in today's world, we have a set of scientific theories that provide a true account of all physical phenomena. [...]
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