 The promise of behaviorism lay in its conviction that there could be a science of human behavior as objective and explanatory as other “higher-level” sciences such as chemistry and biology. Behaviorism indeed had some early successes, especially in the domain of animal learning, and its principles are still used, at least for heuristic purposes, in various areas of psychology. But as many psychologists (and others, e.g. Chomsky 1959) have argued, the successes of behaviorism seem to depend upon the experimenters' implicit control of certain variables which, when made explicit, involve ineliminable reference to organisms' other mental states. For example, rats are typically placed into an experimental situation at a certain fraction of their normal body weight — and thus can be assumed to feel hunger and to want the food rewards contingent upon behaving in certain ways. Similarly, it is assumed that humans, in analogous experimental situations, want to cooperate with the experimenters, and understand and know how to follow the instructions. It seemed to the critics of behaviorism, therefore, that theories that explicitly appeal to an organism's beliefs, desires, and other mental states, as well as to stimulations and behavior, would provide a fuller and more accurate account of why organisms behave as they do. They could do so, moreover, without compromising the objectivity of psychology as long as the mental states to which these theories appeal are introduced as states that together play a role in the production of behavior, rather than states identifiable solely by introspection. Thus work was begun on a range of “cognitive” psychological theories which reflected these presumptions, and an important strain of contemporary functionalism, “psycho-functionalism” (Fodor 1968, Block and Fodor 1972) can be seen a philosophical endorsement of these new cognitive theories of mind.