Nothing seems to expose the cleavage between progressives and conservatives better than opinions on modern art. While the left is likely to lavish praise on the latest Turner Prize winner for his or her "challenging" and "thought-provoking" message, the right tends to lament the absence of the aesthetic in contemporary non-representational art and cackle despairingly at the charlatanism of its practitioners.
So when a panel of experts voted Marcel Duchamp's Fountain (1917), a simple urinal, the world's most influential piece of modern art, the reaction was predictable. The Guardian lauded "the direct link" between Duchamp and Tracey Emin; the Daily Telegraph groaned that the vote "explains an awful lot about today's art".
But both camps were missing the point. Duchamp, through his urinal, Bicycle Wheel (1913), Bottle Rack (1914) and other works, sought not solely to challenge the concept of art but to mock the art establishment itself. While certainly no apologist for the "that's not art; my granny could do better" school of thought, he would have despaired of Emin and her contemporaries for their unimaginative and derivative work.
After he had given up cubist painting, Duchamp wanted to poke fun at galleries that deemed it their prerogative to decide what was art and what was not. He sought to undermine the notion of the sacred uniqueness of the artistic object, asking: If you put a mass-reproduced toilet in a gallery and sign it "R Mutt", does that make it art? It was a paradoxical gesture - "can one make a work of art that is not a work of art?" he asked - yet the irony was that few got the joke, and in due course his Fountain acquired the aura of the artistic object. Today, at Tate Modern, a replica of the urinal attracts thousands of pilgrims who gaze at it with gushing reverence. Exasperated by how his "ready-mades" had become fetishised, Duchamp gave up art in 1923 to play chess, and later represented France in four Olympiads.
In many respects, installation art ever since has been a footnote to Duchamp. In 1917, to question "what is art" by placing a prosaic object in a gallery was brave and novel; to do so in 2004 is tiresomely predictable. Piero Manzoni's Artist's Shit (1961) was memorable less for its philosophical insights than for its scatological shock value, while Equivalent VIII (1966), Carl Andre's infamous pile of bricks, was really beginning to labour the point. A joke that is repeated gets less funny with every retelling.
Many of today's modern artists possess the attitude Duchamp strove to challenge. Emin was incandescent when two Chinese artists, Yuan Cai and Jian Jun Xi, stripped off and started jumping on her bed at the Tate in October 1999. She was guilty of fetishising an object as art just because she and Tate said it was.
In 2000, Cai and Xi urinated in Fountain at Tate Modern. They said it was a derisive gesture against the excessive importance attached to works of art. After all, is not a urinal merely something one urinates in?
Thank goodness some of us still appreciate that Duchamp was, well, just taking the piss.