Marcel Duchamp's

Reinventing the wheelLove it or hate it, Marcel Duchamp's urinal revolutionised modern culture in 1917. Did the

The object in Tate Modern is white and shiny, cast in porcelain, its slender upper part curving outward as it descends to a receiving bowl - into which I urinate. It's just a brief walk from here in the fifth-floor men's loo to Marcel Duchamp's Fountain, an object sealed in a plastic display case on a plinth that is nevertheless almost identical to the receptacle into which I've just pissed. This museum treasure is no more or less than Duchamp described it to his sister in a letter of spring 1917: une pissotière en porcelaine. Duchamp warned against an attitude of "aesthetic delectation" that would transfigure his urinal into something artistic. Yet, as a visual form, it is bizarrely lovely, so white and incongruously ethereal, and as art it is ... well, there's a question already tripping me up. Is it art?

The eminent New Yorkers who ran the American Society of Independent Artists decided in April 1917 that it wasn't. The Independents congratulated themselves on championing all that was new and progressive in art, and to ensure openness to the new they agreed to the idea of one of their directors, Duchamp himself, that anyone who paid a $6 fee should be able to show in their inaugural exhibition. This meant that technically there were no grounds to refuse the mysterious R Mutt's last-minute entry of a men's urinal entitled Fountain - for he had paid his fee. An emergency meeting nevertheless rejected it.

The next month, a little magazine called The Blind Man, which was co-edited by Duchamp, defended Mr Mutt's Fountain: "Whether Mr Mutt with his own hands made the fountain or not has no importance. He CHOSE it. He took an ordinary article of life, placed it so that its useful significance disappeared under the new title and point of view - and created a new thought for that object."

These words resonate as excitingly, provocatively, philosophically today as they did in the early 20th century. A vast proportion of 21st-century art traces its origins to these words, that pissotière. The editorial in The Blind Man - whose authorship Duchamp never formally acknowledged, any more than he officially owned up to being R Mutt - is actually more important than the urinal itself, which was not his first "readymade" work of art. Rather, it allowed him to make explicit an idea that until then was only a private musing. How did he come up with such a notion?

This, it seems to me, is the question no one asks about Duchamp. His big idea - that any ordinary "readymade" object can be chosen by the artist as a work of art - has sunk so deep into modern culture that he is imagined almost as a biblical prophet, a remote figure of authority. It's as if contemporary art history begins with him. Art is steeped in tradition - today, there is a tradition of the readymade - and to make a painting, a film, a photograph is to know you are contributing to a form that has been shaped and defined by predecessors. Even the most radical film is a film. But Duchamp did something for which there was no precedent. Love or hate the art that claims him as ancestor, you can't deny the originality of the thought itself, which I suspect was all that mattered to Duchamp. The readymade was a new concept of art, rather than just an ingenious and idle way of making it. No wonder that most serious discussions tend to assimilate it to philosophy, from Richard Wollheim's famous 1965 essay that took the urinal as a paradigm of "minimal art" to more recent ruminations on Duchamp and Kant's aesthetics. But I think we need to stick to the simple problem: how did anyone ever have such a wild idea?

With Tate Modern about to open an exhibition on Duchamp and his friendships with the brilliant playboy painter Francis Picabia and the subversive photographer Man Ray, let's try to imagine a time when no one dreamed of separating art from manual labour.

Duchamp grew up with art. He was born in 1887, the youngest son of a prosperous notary in Blainville, Normandy. Duchamp's grandfather had been an artist even as he prospered in business; there was art in the family and, as well as Marcel, two of his brothers and one of his sisters set their hearts on becoming artists. His brother Gaston was a painter who took the name Jacques Villon in homage to Villon the poet; another brother was Raymond Duchamp-Villon, the important cubist sculptor; his sister Suzanne also struggled to become a painter.

Young Marcel's early paintings were unpromising, ordinary enough to give ammunition to hostile critics who portray him as the original conceptualist fraud, a man who couldn't hack it on talent alone so discovered a wheeze to make "talent" seem unsophisticated. Yet such great painters as Piet Mondrian and Kasimir Malevich had equally weak starts. Like them, Duchamp eventually excelled as a painter, and it was his depiction of a body as a fluttering mechanism, Nude Descending a Staircase No 2 (1912), that made him famous in America when it was shown in the Armory show that popularised modern art in New York in 1913.

Duchamp settled in Paris in 1906 at a moment when decades of courageous experiment by French artists were about to ignite modernism. It was the year Cézanne died, the year Picasso finished his portrait of Gertrude Stein with a carved wooden mask for a face. Duchamp - six years younger than Picasso - was part of the generation who were influenced and inspired by this revolution. His two older brothers lived in the suburb of Paris that gave its name to the Puteaux group, the minor wing of the cubist movement.

Picasso and Georges Braque, the originators of cubism, were relearning the idea of painting with every new canvas. Their cubist paintings are to this day impossible to summarise or to explain away. The Puteaux cubists, by contrast, used shards and planes of fragmented colour to convey the drama of modern life in an easy-to-decode way. Jean Metzinger's 1913 painting The Cyclist, for instance, clearly and iconically portrays a cyclist hunched forward on his racing bike. It's a celebration of the modern world, of the clean-minded technology of the bicycle, and points immediately to the ideas that were in Duchamp's mind when he dreamed up his first "readymade" work of art - a spoked wheel suspended in a metal fork fixed to the seat of a wooden stool. It might be the eye of a cyclops, or an astronomical model, or a bizarre evocation of a nude. You can't help thinking it means something, but interpretation is vain. It just is. It simply stands there in its light-hearted, lovable glory.

Bicycle Wheel was recognised later by Duchamp as his first "readymade", though he hadn't yet come up with the term or finalised the idea. Nor did he think of exhibiting the piece. He just liked to have it in his studio: "To set the wheel turning was very soothing, very comforting, a sort of opening of avenues on other things than the material life of every day ... I enjoyed looking at it, just as I enjoy looking at the flames dancing in a fireplace."

This toy was a step into intellectual realms no one had entered before. And yet, it is so charmingly of its time, celebrating the very machine Metzinger praises in his cubist painting. The long thin spokes and unbending mount of Bicycle Wheel vividly evoke the Paris of 1913: look into those spokes and it's not hard to picture the iron lattices of the Eiffel Tower. A graceful modernity, optimistic and young, is balanced on that stool. And what of the stool itself? It might even make you think of Van Gogh's painting of his wooden chair. Perhaps the walk from Van Gogh's studio to Duchamp's is not so far.

The idea of the readymade seems to have something to do with the idea of the studio as a utopian refuge from the workaday world. In 19th- and early 20th-century France, this idea of the studio was fundamental to the new way of life of the avant garde. You can see it in Cézanne's early painting The Stove in the Studio, and read it in Zola's novel The Masterpiece - this sense of the studio as a hideout where the artist is free to explore non-bourgeois habits and dreams. Picasso's studio in the ramshackle Bateau-Lavoir was a workshop of cubist experiment: photographs of Picasso and other artists of this time in their studios, surrounded by African masks and their unexhibited paintings, depict magic worlds.

In the studio, sex, drink and drugs contributed to a hallucinatory focus on ordinary things that suddenly seemed fascinating. Duchamp's discovery of the readymade is deeply rooted in the cubist obsession with real, tangible, solid things that are close to hand. Picasso and Braque rediscovered one of the oldest and humblest painting genres: the still life. In a cubist masterpiece such as Picasso's Absinthe Glass, Bottle, Pipe and Musical Instruments On a Piano (1910-11), it is the immediate world of objects that is broken and shattered and strained for.

It was actually Picasso who first used found objects in art. In 1912 he stuck a piece of oilcloth, with a printed design imitating a caned chair-seat, on to a cubist canvas. Still Life with Chair Caning gave him and Braque a new weapon in their struggle to give art the force of reality. Duchamp's Bicycle Wheel is a cubist masterpiece, rivalling the complexity and resistance of Picasso's and Braque's art. It belongs recognisably to the Paris of that time, which is even more true of Duchamp's Bottle Dryer (or Bottle Rack) - a spiky metal tower widely used in France to dry used wine bottles.

Yet Duchamp didn't name these objects that he had in his Paris studio as "readymades" until later. Nor did he show them as "art". The "readymade", a word he only ever used in his second language, English, came about when he made the leap from one culture to another. It is an idea found in translation.

In 1915 Duchamp sailed for New York. He felt instantly liberated by America. "For a Frenchman, used to class distinctions, you had the feeling of what a real democracy could be," he later said. Learning a new language set his mind completely free of all influences, to achieve a simple, relaxed revolution quite unconnected with anything in the art of cubism.

In January 1916 he wrote to his sister Suzanne asking her to preserve what he now regarded as two important works left in Paris: "Now if you went up to my place you saw in my studio a bicycle wheel and a bottle rack. I had purchased this as a sculpture already made. And I have an idea concerning this and the bottle rack: Listen. Here in NY I bought some objects in the same vein and I treat them as 'readymade'. You know English well enough to understand the sense of 'readymade' that I gave these objects ..."

In 1915 he saw a pile of snow shovels with big square steel scoops lined up in a shop for the New York winter. He bought one and hung it from the ceiling of his studio. It was inscribed "In Advance of the Broken Arm, from Marcel Duchamp, 1915". This signature was a way of thinking about the readymade as art - it was not by, but "from" the artist. The object - which exists, like most of Duchamp's long-lost readymades, only in replica - is menacing, with its sharp metal edges and title prophesying injury. It is as uncomfortable as the bicycle wheel is likeable. Perhaps this is the art of it - that undeniable atmosphere.

A spikiness of feeling similarly clings to the steel comb that in February 1916 Duchamp inscribed "Three or four drops of height have nothing to do with savagery". In December 1916 he enclosed a hollow ball of twine inside two metal plates and asked his friend and patron Walter Arensberg to hide a small object inside without letting him see; it is called With Hidden Noise. This and the comb survive as originals.

All this was a private game, until in April 1917 Duchamp, Arensberg and the artist Joseph Stella visited the Mott ironworks in New York to purchase a porcelain urinal that would shock the worthies of the Manhattan art world and give Duchamp the opportunity to explain Mr Mutt's philosophy of art: "He CHOSE it ... and created a new thought for that object."

Their fight to raise themselves above the status of mere craftsman has led artists, since the 15th century, to seek to be seen as intellectuals. In the Renaissance, Leonardo da Vinci typified the artist as thinker. Duchamp identified with Leonardo, a fandom you might not guess from his infamous 1919 "rectified readymade", when he drew a moustache on the Mona Lisa. In all the years during which he was evolving the idea of the readymade, Duchamp was working on an immensely ambitious masterwork, The Bride Stripped Bare By Her Bachelors, Even, which involved elaborate research, starting in a Paris library with his reading treatises on perspective. Like Leonardo, he made copious, quasi-scientific notes, many of which he published in The Green Box.

Duchamp's idea of the readymade is the final, triumphant endgame in western art's long campaign to establish the intellectual status of the artist (Duchamp, who officially gave up art to play tournament chess, was an authority on endgames). In this, his predecessors are not just Leonardo, but Sir Joshua Reynolds and all those academicians who insisted that theirs was a mental calling.

And yet, he didn't just select any object and call it art. One of his notes in The Green Box reminds himself to limit his output of readymades. There are not that many. Something connects them all; their meaning and purpose are clear. They are all manufactured objects: he never named a landscape or a natural object as a work of art. He saw art not just anywhere in the stuff of everyday life, but specifically in the capitalist industrialised world. Anthropology was a passion of artists at the beginning of the 20th century, and photos of Duchamp's studio show totems of mass production. They find magic in the modern world, and their key is The Bride Stripped Bare By Her Bachelors, Even.

The great work he eventually abandoned as "definitively unfinished" in 1923 is an allegory of virtual desire in a machine world. It imagines two realms, defined by separate glass panels. In the upper panel is the "bride", like a Renaissance Madonna ascending to a paradise supported not by angels but by some kind of metallic insect. Below are the "malic moulds", bachelors who vainly send emanations up towards the object of their desires (or prayers). Duchamp's bachelor machines may be happy enough grinding chocolate, but they will never get the bride.

Richard Hamilton, one of Marcel Duchamp's most profound interpreters, made the copie conforme of The Bride Stripped Bare that is on permanent view at Tate Modern. In their preface to The White Box, an English edition of some of Duchamp's more abstruse notes, Hamilton and his co-translator, Ekke Bonk, compare Duchamp's masterpiece with the magazine Astounding Science Fiction - an idea that helps us understand the idea of the readymade. New York must have struck Duchamp as a spectacle from the future. From Otis elevators and skyscrapers to ultra-modern bathrooms and steel combs, "all the great modern things", as Andy Warhol was to call them, were already in place in America in 1915 while Europe was still struggling free of its ancien régimes. It must be this translation to the future that gave Duchamp his unparalleled intellectual freedom, an alienated liberation familiar to travellers. He sought the estrangement to which his readymades are eerie monuments.

There is not really any other art like his, despite his infamous "influence". He discovered something new; no one can discover it again. The cleverest artist of the 20th century played a great joke on history, for Duchamp, who sanctioned and signed replicas of his works and is the prophet of the simulacrum, is in truth inimitable.

· Duchamp, Man Ray, Picabia is at Tate Modern, London SE1, from February 21 to May 26. Details: 020-7887 8888; tate.org.uk