The action of repression creates a repository of thoughts and impulses of which we remain unaware but that nonetheless influence our conscious thought and behavior. The delineation of that repository forms Freud's signature contribution to psychology, the designation of an unconscious portion of the mind.

In “The unconscious” (1915c), his landmark paper on the subject, Freud justifies the existence of the unconscious, maps out its characteristics, and situates it in our broader mental ecology. He proposes a mental “topography” that includes unconscious, preconscious, and conscious processes, importantly clarifying that consciousness is tantamount to a sensory organ that detects thought, which is itself unconscious. Within this frame he draws an intriguing comparison between thought and emotion, deepens his analysis of the dynamics of repression, compares the characteristics of conscious and unconscious mentation, and addresses the communication between the two systems.

Psychoanalysis after Freud depends on this paper in spirit if not in detail given that the existence of the unconscious in mental life is foundational to all schools. Fields well beyond psychoanalysis, as well as intellectual and popular culture, accept the major divisions laid out here. The relation between thought and emotion remains an active topic in modern psychological and philosophical discourse.

JUSTIFICATION FOR THE UNCONSCIOUS

Freud marshals several lines of evidence to document the existence of unconscious mentation. These include dreams, psychoneurotic symptoms, and the small mental errors of daily life, like slips of the tongue, he calls parapraxes. They also encompass the normal popping up of thoughts whose source we cannot identify, the capacity of hypnosis to reveal memories to which people have no conscious access, and our very susceptibility to hypnotic suggestion. Psychoanalysis shows individual happenings of these kinds to have had motives unavailable to consciousness at the time the happenings occurred. Indeed, Freud maintains, conscious processes account for only a small percentage of our mental activity.