 Some functionalists, however (e.g. Shoemaker 1984c), have suggested that if a creature has states that approximately realize our functional theories, or realize some more specific defining subset of the theory particularly relevant to the specification of those states, then they can qualify as being mental states of the same types as our own. The problem, of course, is to specify more precisely what it is to be an approximate realization of a theory, or what exactly a “defining” subset of a theory is intended to include, and these are not easy questions. (They have particular bite, moreover, for analytic functionalist theories, since specifying what belongs inside and outside the “defining” subset of a functional characterization raises the question of what are the conceptually essential, and what the merely collateral, features of a mental state, and thus raise serious questions about the feasibility of (something like) an analytic-synthetic distinction. (Quine 1953, Rey 1997)).