While the First World War tore Europe apart, Zurich was a small neutral island. The Cabaret Voltaire, opened by the poet Hugo Ball and the singer Emmy Hennings, quickly became an anti-war refuge where Tristan Tzara, Marcel Janco, Richard Huelsenbeck and Hans Arp converged. Even the origin of the word "Dada" is disputed — said to have been picked at random from a dictionary; nonsense itself became the programme.

Dada's target was the "rational" European culture that had produced the war. If reason had marched men into the trenches, the artist's duty was to rebel against reason, logic and bourgeois taste. In 1916 Hugo Ball stepped onto the cabaret stage in a cardboard cubist costume and recited sound poems like "Karawane", woven from meaningless syllables. Tzara proposed building poems from words drawn at random; Arp made collages from torn paper dropped with his eyes closed. In 1917 the movement leapt to New York: Marcel Duchamp signed a porcelain urinal "R. Mutt", submitted it to an exhibition as "Fountain", and shifted the terms of the twentieth-century debate about art by declaring that the artist decides what art is.

Dada was short-lived as a movement. It scattered through Berlin, Cologne, Hannover, Paris and New York and dissolved as a formal group around 1923. But its inheritance is vast: a year later Breton's Surrealism stepped into the space Dada had cleared; Pop Art fed on Duchamp's ready-made; Fluxus and performance art are the children of the Dada cabaret evenings; and conceptual art is the direct heir of the way of thinking that "Fountain" opened up.

Perhaps Dada's deepest importance is this: it pushed the question of what art is to the centre of art itself. Is it the craftsmanship, the material, the artist's intention, the institution's approval that makes an object art? Once that question had been posed, the history of Western art became another kind of history — a history not of works but of ideas.