1Five stages in the development of Freud's efforts to supply a metapsychological explanation of the pleasure-unpleasure experience are traced, the basis for each successive shift in formulation indicated, and Freud's dissatisfaction with the end result of his labors noted.

2Attempts of subsequent investigators to amplify and refine the theory are delineated. Certain modifications were seen to derive from more precise phenomenological analysis, the application of more recent concepts of ego psychology, and the replacement of the constancy principle in psychic functioning by the homeostatic principle.