Freud's "scientific psychology" is traced from 1888 to 1938 in the succession of mental apparatuses he proposed. The key proposition, for which he shared credit with Theodor Lipps after 1898, holds that consciousness is a sense organ for detecting and translating the unknown inner reality of the unconscious. The significance of this approach from the standpoint of the methodology, constructions, and metapsychology of psychoanalysis is considered and compared with other views of metapsychology now current.