What is the Type Identity Theory of Physicalism?

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! [...]