 In the last part of the 20th century, functionalism stood as the dominant theory of mental states. Like behaviorism, functionalism takes mental states out of the realm of the “private” or subjective, and gives them status as entities open to scientific investigation. But, in contrast to behaviorism, functionalism's characterization of mental states in terms of their roles in the production of behavior grants them the causal efficacy that common sense takes them to have. And in permitting mental states to be multiply realized, functionalism seems to offer an account of mental states that is compatible with materialism, without limiting the class of those with minds to creatures with brains like ours.