It is fairly widely believed that felt bodily sensations (aches, pains, itches, tickles, throbs, cramps, chills, and the like) have physical or at least functional correlates. That is to say, the following thesis is fairly widely believed:
Correlation Thesis. For every type of sensation state, S, there is a type of physical or functional state, P/F, such that it is nomologically necessary that for any being, x, x is in S if and only if x is in P/F.
The Correlation Thesis, if it is true, would not, of course, solve the mind-body problem for sensations. For the Correlation Thesis is compatible with virtually every theory of mind: not only with noneliminativist materialism and functionalism, but also with Cartesian Dualism, dual-aspect theory, neutral monism, and panpsychism.
Some philosophers, however, myself included, maintain that the following thesis would offer the best explanation of the correlation thesis:
Type Identity Thesis. For every type of sensation state, S, there is a type of physical or functional state, P/F, such that S = P/F2.
This thesis implies the Correlation Thesis. For, as Saul Kripke (1971) demonstrated,
The Necessity of Identity. For any A and B, if A = B, then necessarily A = B.3
The kind of necessity in question is the strongest sort: if A = B, then there is, unqualifiedly, no way the world could be such that A is not identical with B. Thus, given the necessity of identity, if the Type Identity Thesis is true, then the Correlation Thesis is true as well.