The boundaries of psychology have never been very distinctly defined and, as a consequence, science has witnessed frequent border incidents. But it obviously is not psychology alone which suffers from such lack of delineation, but its neighbors, the biological and social sciences, do as well. Cooperation between sciences becomes difficult under these conditions. All agree that psychology is the science of mind, but few agree to what “mind” is. At least within our century “mind” has been taken to be “behavior”, for the present day outlook is that mind is behavior rather than that which is responsible for behavior (3). The metaphysical dynamism of a prime mover has been replaced by an experimental concept, i.e., one that is susceptible to scientific investigation. There is no need to review the complex history of behavioristic psychology in its many ramifications, ranging from Watson's mechanism to the various vitalistic teleologies. We turn instead to a fundamental problem on which there is little agreement: “What kind of behavior should be called mind?”