Needles reviews Freud's concepts related to the pleasure-pain principle and points out that Freud's enigma in this respect continued until the end of his writings. The author further suggests that the law of constancy (Fechner's theory) should be replaced by more current constructs dealing with homeostatic equilibria. It is essential to include in the theory of affects the roles of ego and superego in the pursuit of pleasure and pain whereas Freud's theory is based too much on his earlier instinctual drive and topographical bias. Even with the accumulation of more precise knowledge in psychology and other disciplines, these additional facts do not permit exact definition of the nature of the pleasure-pain enigma. The author thinks that the reason for this is that pleasure and pain are determined biologically and phylogenetically.