Properly speaking, the unconscious is the real psychic; its inner nature is just as unknown to us as the reality of the external world, and it is just as imperfectly reported to us through the data of consciousness as is the external world through the indications of our sensory organs.

Sigmund Freud (1856–1939) was an Austrian neurologist and the founder of psychoanalysis, a clinical method for treating psychopathology through dialogue between a patient and a psychoanalyst. He developed techniques such as the use of free association and identified the phenomenon of transference. His analysis of dreams as wish-fulfillments provided him with models for the underlying mechanisms of repression, and on this basis he elaborated his theory of the unconscious and went on to develop a model of psychic structure comprising id, ego, and superego. He also postulated the existence of libido, a sexualized energy that generates erotic attachments, and a death drive, the source of compulsive repetition, hate, aggression, and neurotic guilt.