  The issue comes down to one concerning the individuation of mental states. How do we determine what is and is not the “same” belief? Fodor begins by introducing the constraint that he calls “methodological individualism,” viz., “the doctrine that psychological states are individuated with respect to their causal powers” (1987, 42). This implies, among other things, that if one psychological state is incapable of causing anything different to happen than some other psychological state, then the two must be the same. “Methodological solipsism” is the stronger claim that “psychological states are individuated without respect to the semantic evaluations” (1987, 42). This implies, among other things, that even if one state is “true” in some context and another is “false,” the two may still turn out to be the same. As Fodor goes on to point out, the semantic evaluation of a mental state will typically be relational, e.g. whether certain beliefs about water are true will depend upon how things happen to stand with water in the world; thus methodological solipsism has the consequence of precluding one type of relational property from playing a role in the individuation of mental states. It is therefore “individualistic” in the everyday sense of the term, since it suggests that what’s going on in the agent’s head does most or all of the work in the individuation of mental states.  Methodological individualism, on the other hand, “does not prohibit the relational individuation of mental states; it just says that no property of mental states, relational or otherwise, counts taxonomically unless it affects causal powers”(1987, 42). Thus it is very unclear why Fodor chooses to call this a form of “individualism,” since these relations could also be relations to other speakers, and not just the physical word.