This paper begins by outlining the debate at the beginning of the twentieth century between structuralist and functionalist psychology. We examine some of the consequences of emphasizing either the functional or the structural properties of the mental apparatus. The functional explanation finds its most extreme example in Watson’s behaviorism. Then we examine Freud’s notion of the mental apparatus. We find that in the metapsychology of 1915 Freud gives priority to a structural explanation of mental phenomena, while in the metapsychology of 1923 he constructs the mental apparatus as being divided into functional units.