 The identity theory faces the problem of multiple realizability—the idea that it is possible for physically diverse creatures to be in the same kind of mental state. It is possible, for example, that Martian neurophysiology might be entirely different from that of humans while Martians nonetheless have the same kinds of mental state as humans have; if a Martian’s body is injured, it writhes, groans, and avoids the causes of such injuries—the Martian is in pain (Lewis 1980). Indeed it is possible that cognitive scientists might design a machine that is capable of higher-level thought (implemented on, for example, a silicon architecture). It is even possible that the same mental kinds be realized by distinct neurophysiological systems in one and the same organism over time. Consequently, it is at least an open empirical possibility that mental kinds may correspond to a widely disjunctive and heterogeneous set of neurophysiological kinds, and hence that there is no one-to-one correlation between them.