Theories of pain, like all scientific theories, evolve as a result of the accumulation of new facts as well as leaps of the imagination (Kuhn, 1970). The gate control theory's most revolutionary contribution to understanding pain was its emphasis on central neural mechanisms (Melzack & Wall, 1965). The theory forced the medical and biological sciences to accept the brain as an active system that filters, selects, and modulates inputs. The dorsal horns, too, were not merely passive transmission stations but sites at which dynamic activities-inhibition, excitation, and modulation-occurred. The great challenge ahead of us is to understand how the brain functions.