In psychoanalysis, the part of the psychic apparatus that does not ordinarily enter the individual's awareness but may be manifested by slips of the tongue, dreams, or neurotic symptoms (see neurosis). The existence of unconscious mental activities was first elaborated by Sigmund Freud and is now a well-established principle of psychiatry. The origin of many neurotic symptoms is said to depend on conflicts that have been removed from consciousness by repression and maintained in the unconscious through various defense mechanisms. Recent biopsychological explorations have shed light on the relationship between brain physiology and the levels of consciousness at which people retain memories.

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