In metaphysics, the doctrine that all of reality is essentially of the nature of matter. In the philosophy of mind, one form of materialism, sometimes called central-state materialism, asserts that states of the mind are identical to states of the human brain. In order to account for the possible existence of mental states in creatures that do not share the human nervous system (e.g., octopuses and Martians), proponents of functionalism identified particular mental states with the functional or causal roles those states play with respect to other physical and mental states of the organism; this allows for the multiple realizability of the same mental state in different physical states. (Strictly speaking, functionalism is compatible with both materialism and non-materialism, though most functionalists are materialists.) As a form of materialism, functionalism is nonreductive, because it holds that mental states cannot be completely explained in terms that refer only to what is physical. Though not identical with physical states, mental states are said to supervene on them, in the sense that there can be no change in the former without some change in the latter. Eliminative materialism rejects any aspect of the mental that cannot be explained wholly in physical terms; in particular, it denies the existence of the familiar categories of mental state presupposed in folk psychology. See alsoidentity theory; mind-body problem.