Early in this century, most empirically oriented psychologists believed that all motivation was based in the physiology of a set of non-nervous- system tissue needs. The theories of that era reflected this belief and used it in an attempt to explain an increasing number of phenomena. It was not until the 1950s that it became irrefutably clear that much of human motivation is based not in these drives, but rather in a set of innate psychological needs. Their physiological basis is less understood; and as concepts, these needs lend themselves more easily to psycho- logical than to physiological...