According to functionalism, a mental state such as a pain is characterized by its causal roles. But if one physical state plays the causal role of pain, so do many others and so the characterization fails. For instance, if neural state N plays the causal role of pain in me, so does the conjunction of N with my having blue eyes. One could require minimality of the state, but that won't help. First, plausibly, there is no minimal state that plays the role: if a state plays it, so does that state minus a particle. Second, even if there is one, it is very unlikely to be unique. There is likely to be redundancy, and there will be many ways of getting rid of redundancy.
The solution to this problem in the spirit of Lewisian functionalism is to restrict one's quantifiers to natural states. There are two ways of doing this. First, we could restrict the quantifiers to states which are sufficiently natural, whose degree of unnaturalness is below some threshold. (An obvious way to measure unnaturalness is to measure the length of the shortest linguistic expression taht expresses the state in terms that are perfectly natural.) But this is unlikely to work. If mental states have degreed unnaturalness, presumably there will be a lot of variation in the degree of unnaturalness. Some mental states will, for instance fall far below the threshold. Those states could then be made slightly more complicated while still staying below the threshold, so once again we would have a problem.
So we better restrict quantifiers to perfectly natural states, at least in the case of the basic mental states (or maybe protomental states—I won't distinguish these) out of which more complex ones are built. Thus we have our first conclusion:
- If functionalism is true, basic mental states are perfectly natural.
- If functionalism is true, a purely physical computer has no basic mental states, and hence no mental states.
What about organisms? Well, if organisms are purely physical, then their mental states will be biological states (subject to evolution and the like). So:
- If functionalism is true, then some of the biological states of a minded purely physical organism are perfectly natural.
- Functionalism implies that all minded organisms have non-physical states (dualism) or some minded organisms have perfectly natural biological states (antireductionism) (or both).
Moreover, functionalism is the only tenable version of physicalism (I say). Thus:
- Physicalism implies biological antireductionism.