W. R. D. Fairbairn's singularly original contributions to an understanding of psychoanalytic object-relations theory are grounded upon what he termed the “basic endopsychic situation.” Fundamental to that formulation are: (1) there exist from the beginning an “original object” and an “original ego” which form the matrix of later-developed mental structure, (2) the process of splitting is responsible for that development, (3) splitting and repression are essentially equivalent, (4) the dynamic of repression is aggression, and (5) schizoid and hysterical phenomena are dynamically similar. It is the author's conclusion that these and other related formulations lead ineluctably to a concept of mental structure that necessarily incorporates elements of both “deficit” and “drive-defense” models and the paper that follows attempts to develop and explicate that conclusion by means of a detailed examination of relevant parts of Fairbairn's writings.