As I mentioned in my last post, some philosophers are identity theorists, that is, they hold that every type of mental state is identical to a specific type of physical state. For example, they might hold that pain is identical to C-fiber stimulation.

Any identity theorist is committed to the claim that every type of mental state is necessarily a specific type of physical state. However, there can still be debate about what kind of necessity this is. Some forms of necessity seem to be a priori, like 1+1=2, and some seem to be a posteriori, like the fact that water is necessarily H2O. So, which kind of necessity holds of the connection between mental and physical states?

The issue might at first seem easy to decide, since “mental states are necessarily physical states” is not just blindingly obvious from the concepts involved like 1+1=2. However, the issue is not whether the necessity of identity theory is obvious to us right now, but whether it would be obvious if we had all of the physical facts available to us. If we had to do more research to find out whether mental states were necessarily physical states at that point, then identity theory would be necessary a posteriori, and if we did not have to do any more research, then identity theory would be necessary a priori.

The a priori side has to defend the counterintuitive claim that it could just be obvious that mental states are physical states given enough information. The a posteriori side also has problems, since it’s not clear what sort of further research we could do if we already had all of the physical facts in our possession.

So, there appear to be three options:

  1. Don’t accept identity theory.
  2. Accept identity theory as necessary a priori and explain how it could just be obvious that mental states are physical states given enough information.
  3. Accept identity theory as necessary a posteriori and explain what sort of further research we could do after obtaining all of the physical facts.

My inclination is to say that the a priori side has a more plausible position than the a posteriori side, but I’m not an identity theorist.

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