Marcel Duchamp (18871968) invented the idea of the "ready-made" around 1915. If you decided something was art, then it was art, he said, and went on to exhibit a bottle rack, a snow shovel and, most infamously, in 1917, a urinal (left) which he signed with the manufacturer's name, R. Mutt. After Duchamp, everyone could be their own art movement, a situation that currently prevails in the art world. He and Picasso have been described as the most significant artists of the 20th century but, if the first 50 years are Picasso's, the last 50 belong to the father of conceptual art, Duchamp, with his attitude of cool, ironic detatchment.