Duchamp, the founding father of Dadaism, the absurd Surrealist par excellence, did more than any other single artist to irrevocably transform 20th- century notions of art and aesthetics. The most astonishing thing is that he managed to do it with such brevity of means. His works may have been few, but such is their individual monumental perfection that each one has the capacity to haunt our subconscious. Ask yourself how he achieved this immortality and you stumble upon the ultimate triumph of Duchamp's intellect - the fact that he knew that some 30 years after his death we would in fact be asking the question itself. Obligingly, Duchamp left us the answer. On New Year's Day 1941, he summarised his art in his Box in a Valise, the portable museum - a limited-edition box containing miniaturised versions of the major works he had created over the past 30 years. Seven of these boxes are now on show at the Entwistle Gallery, in one of the most useful small exhibitions of recent years. If you were unlucky enough to miss the Duchamp retrospective which inaugurated the Pompidou Centre in Paris in 1977, then here is the next best thing.
These are the works of art which define our century: from the proto-Futurist Nude Descending a Staircase, of which the original was painted in 1912, to a papier-mache model of the work with which Duchamp defined his position, when, in 1917, he submitted it for exhibition in New York. Fountain is a urinal signed "R Mutt" - in sculptural terms, it was the latest in a series of "ready-mades", which included Bicycle Wheel (1913) and Bottle Rack (1914). As a gesture it opened the floodgates to the future of conceptualism. Simultaneously, Duchamp was working on his "masterpiece" Large Glass: the Bride Stripped Bare by her Bachelors, Even, which is also presented here in an exquisite miniature. In a characteristic statement against convention, this bizarre painting on glass was left incomplete when, in 1923, the artist decided to cease production, convinced that to play chess would be altogether more creative than making works of art.
Like all great collections, Duchamp's portable museum is selective, and a notable omission is the work of 1920 in which he drew a moustache on a reproduction of the Mona Lisa and added the inscription "L.H.O.O.Q." (phonetic French for "elle a chaud au cul" - "she's got a hot arse").
This single shortcoming notwithstanding, there is enough in the box to tell you all you need to know to engage with the basic truth of Marcel Duchamp - that in an age of mechanical reproduction, the work of art exists only in the mind. Duchamp anticipated that in the post-Modern, post-Pop world, where repetition guarantees reputation, it would be the ease with which we can call it to mind that would create a masterpiece. In making his boxes Duchamp was subverting the power of his own works of art, just as an art postcard, reproduced and disseminated in its thousands by a public gallery, in some way lessens the power of the original. The valise is the ultimate act of art sabotage by the ultimate cynic.
Duchamp's apparent pranks and his self-created infamy prompted the late, great and at times reactionary critic Peter Fuller to dub him simply: "The start of all the trouble." But it is impossible to deny that Duchamp, Warhol and, most recently, Damien Hirst exemplify a noble tradition of autogenous, and, to an extent, implicitly self-destructive artists who challenge the boundaries of aesthetics as much as they do those of public credibility. All, in effect, declare, quite rightly, that any art that we see defined as "art" is necessarily flawed - economically led by professional taste-makers.
While saleroom prices for Duchamp and Warhol, as much as Hirst's current lionisation by the chattering classes, demonstrate that no artist can ever be entirely above involvement in that process, to be able to scoff at it with such wit and intelligence must surely be a sign of some greatness.
n 'Marcel Duchamp: sculptures and boxes' is at the Entwistle Gallery, 6 Cork Street, London W1 (0171-734 6440) to 27 JulyReuse content