After the First World War, saying no with Dada was no longer enough for a young Paris generation; the time had come to look for something to say yes to. André Breton, who had studied medicine and worked with shell-shocked soldiers, was deeply moved by Freud's theory of the unconscious. His Surrealist Manifesto, published in October 1924, defined Surrealism as "pure psychic automatism — the dictation of thought in the absence of any control exercised by reason and outside any aesthetic or moral concern."

The programme weaves three strands together. From Freud, dream, association and repressed desire; from Marx, a revolutionary stance against bourgeois order; from Dada, the courage to mock reason and tradition. Its methods — automatic writing, dream diaries, group games such as the cadavre exquis — soon produced Max Ernst's frottage and Dalí's "paranoiac-critical method". An extraordinary roster gathered around the group: Salvador Dalí, René Magritte, Max Ernst, Joan Miró, Yves Tanguy, Man Ray, and the long under-recognised women Surrealists Méret Oppenheim, Leonora Carrington, Dorothea Tanning, Remedios Varo. In 1929 Luis Buñuel and Dalí's film "Un Chien Andalou" carried Surrealism into cinema; from then on dream logic could speak through the camera as well.

The Second World War scattered most Surrealists out of Europe; Breton, Ernst and Tanguy took refuge in New York, where they introduced the young American painters — Pollock in particular — to the ideas of the unconscious and automatism. Abstract Expressionism is Surrealism's transatlantic aftershock. Magic Realism, Latin American literature, advertising imagery, music videos, digital art: all inherited the dream-logic vocabulary the Surrealists made legitimate.

The importance of Surrealism is that it opened a door rather than fixed a style. The twentieth century first looked into its own dreams through the eyes of the Surrealists; then it learned to think in a dream-like way. That "surreal" has settled into everyday speech is the measure of how wide that door swung open.