The big art idea of the 20th century, courtesy of the Dadaist Marcel Duchamp in 1917, was the "readymade". You find a mass-produced industrial object (in his case a urinal), sign it (any name will do, Duchamp choosing ' R. Mutt' for the purpose) give it a provocative title ('Fountain', he called it) and declare it to be art. Today's conceptual artists, the ones who surface most years as contenders for the Tate's Turner Prize, owe everything to Duchamp. It was a wheeze that worked: it changed the course of art. In fact, as the French philosopher Jean Baudrillard has observed, it did more: it entirely replaced art with "the idea of art". Where does the Tate's new gallery fit into this?

The found object: before conversion

At the start of the 21st century, the Tate has stumbled upon the biggest "readymade" of all - the gargantuan, formidable Bankside power station. Some unlikely buildings have been converted into art museums before - most notably the old railway terminus of the Musee d'Orsay in Paris - but never on this heroic scale. Big though this place was, it used to be virtually invisible - just an old power station. Nobody paid it any attention, just as nobody used to think about an everyday urinal until Duchamp made people think differently. Today, the power station is still in the same place, is still almost the same size, but now the magic-dust of art has made it into a landmark. True to the spirit of Duchamp, its new owners have signed it and given it a title: 'Tate Modern'. But they have gone further still: they have put all the other art inside it. This amounts to what Tony Blair would doubtless call a "step change" in our cultural perceptions. Art placed within a huge readymade art object? What does that do to the insides of our heads?