Viennese neurologist turned founder of psychoanalysis, and — whatever the scientific status of his specific theories — one of the three or four most culturally consequential thinkers of the modern era. Freud's central claim, developed from the 1890s onward, is that the mind is radically divided: what we think of as the conscious self is only a small island atop a vast unconscious in which desires, fears, and memories continue their work outside awareness, surfacing disguised in dreams, slips of the tongue, jokes, symptoms, and the patterns of a life.
His mature structural model (The Ego and the Id, 1923) divided the psyche into id (drives, especially sexual and aggressive), superego (internalized parental and cultural prohibition), and ego (the mediator trying to negotiate between them and reality). Repression — the keeping of unacceptable material out of consciousness — is the engine of neurosis and the price of civilized life. Psychoanalysis as a clinical practice aims to bring the repressed into speech: "where id was, ego shall be."
Applied to culture, Freud's framework produced a distinctive 20th-century pessimism. Civilization and Its Discontents (1930) argued that the renunciations required to live in society make a certain measure of unhappiness structural, not accidental — civilization is built on damming up the drives and can never fully compensate for what it costs. This picture was reworked in various directions by his successors: Carl Gustav Jung broke away early to develop his own analytical psychology; later dissidents (Adler, Horney, Erikson) softened or socialized the theory; the The Frankfurt School and Herbert Marcuse welded Freud to Marx; existentialist and humanist psychology (Rollo May) took what was useful and left the metapsychology.
Scientific psychology has largely moved on; cultural theory, literary criticism, and everyday self-understanding have not.