Some might call Manzoni's art shit, but, argues Tom Lubbock, once you accept that his shit is art, an art gallery is perhaps the last place you should expect to find it
THIS is a curious business. The Serpentine Gallery has just reopened, and it's looking pretty good. It's had a comprehensive renovation, which leaves its exhibition space larger, lighter, more open-plan - leaves it, that is, as a more efficient version of the all-white modern gallery it was before, a better ideal home for the modern art object. And what does it open with? A show of Piero Manzoni.

The Italian artist died exactly 35 years ago, aged 29, after an alcoholic binge. Yet in the last six years of his life, he produced a body of work that puts him second only to Marcel Duchamp as an artist who revolutionised the nature of art. His influence on contemporary art, not least Young British Art, has been enormous, but his name hasn't had a lot of public recognition lately. In those terms a retrospective exhibition seems timely. On Manzoni's own terms, however, it can only look like an open contradiction.

It's good to see a photo of the artist, to catch the spirit - in place of Duchamp's knowing, enigmatic smile, you see a chubby, mischievous grin. Look, for instance, at the photo of Manzoni in front of a lavatory, holding up a can of Merda d'artista - artist's shit. This was his most famous production, a series of 90 tins, each preserving 30 grams of his own stool, and priced by Manzoni at the same rate as 30 grams of gold (the price fluctuated daily with the markets).

Or again, he designated humans as artworks, by signing them, or by issuing them with certificates. The critic and novelist Umberto Eco was one who got a chit making him a work of art for life; others got more temporary status. In Fiato d'artista (artist's breath), he inflated balloons, the shrunken remains of which are shown here, as are some of the Uove con Impronte, hard-boiled eggs bearing the artist's thumbprint, and examples from the Linea series, sealed cardboard tubes containing scrolls of paper on which Manzoni had drawn lines of various lengths, never to be opened.

The mischief is clear. With works like these, Manzoni put in question some basic notions about the artwork. For example: that it's a unique, permanent, physical, visible thing, a commodity with a market price, a vehicle for self-expression with a premium put on the artist's original handiwork. He challenged these notions by stretching them to the limit. (What is a thumbprint but the most literal kind of "handiwork"; what are breath or excrement but the most literal forms of "self-expression"?) He pursued, in short, a hopeful anarchic dream: the dissolution of that special category of things traditionally called art. Odd then to find him celebrated in a one-man retrospective of precious, unique originals, in an art gallery rebuilt the better to perform its traditional function.

As the Serpentine's director, Julia Peyton-Jones, asks (rhetorically): "How could a gallery present such works as the Merda d'artista without a self-consciousness that its very tenets are being questioned?" And I would answer: this self-consciousness is a meaningless bluff. For it is plainly true that Manzoni and other artists have questioned art's tenets, and been much praised for doing so, without it making the slightest difference to the values of the art world.

Everything stayed in place. All that happened was that Manzoni's "questions", rather than achieving art's dissolution, enlarged art's repertoire. Performance art, conceptual art, body art: his legacy in the past 30 years is great. But what was meant to burst art's limits has only extended the kinds of objects and activities that can count as art - new ways of doing the same old business. (I don't complain, the results of this extension have often been good.) As for his own works, they now seem to stand as relics of a high-minded, historic failure, with gallery exhibition as the conclusive stamp of this failure.

This is felt most piquantly with Magic Base, a wooden pyramid structure with a flat top, on which are stuck a pair of shoe insoles. Manzoni would invite people to stand on this plinth: they would thus - hey presto - briefly, themselves, become artworks. But as it's exhibited here... well, you can guess, can't you? It has a cordon round it. So the work transmits a contradictory message. Step up, good people, become art. Oh, no you don't, keep back. It would have been possible, if the original was too fragile or too expensive, to make a practical replica. But that wouldn't do, would it? The way the value of the original art object remains undisturbed couldn't be shown more clearly.

But maybe the cordon should be seen as really part of the work, a conscious editorial correction to it. It would be a way of saying: of course Manzoni's "magic" doesn't work any more, we know better now; all these ideas about the dissolution of art, nice anarchic hopes at the time, but quite unrealistic, quite impossible.

Certainly he was attempting the impossible, for the blunt reason that artists cannot unilaterally revolutionise the nature of art. The art world, and the world generally, have to agree. And it's hard to imagine, say, the present world agreeing not to trade in anything that's sellable. Today 30 grams of gold costs about pounds 200: rather less, presumably, than the amount a can of Merda d'artista would fetch under the hammer. (Splat! That auction is a sight I'd like to see.) Meanwhile, the distinction between what's art and what's not is as rigid as ever. Manzoni's certificates are collectables, but no collector has tried, so far as I know, to purchase Umberto Eco.

Perhaps Manzoni was aware of the impossibility. The "magical" dimension of his work - turning a person or a pocket of breath into art, turning shit into gold - seems ambiguous. It exposes the old idea of the great artist's transforming, value-creating touch. Yet this critical gesture also returns ironically on to Manzoni himself: a hint that his whole attempt to break the boundaries of art was itself no more than wishful conjuring.

But then Manzoni's "magical" turns have yet another side: not critical or ironic, but performing an act of simple praise for the world and the human. It's the most lasting and attractive aspect of his work. Our breath, our shit, our bodies, our most basic marks - these things are worthy of all the honour we have given to the greatest masterpieces. Seen like that, Manzoni's work itself stays well within the confines of art, tries to change nothing, offers only a vision, an elevating vision that radiates here from one work in particular.

Socle du Monde (world plinth) is a large iron cuboid, on one side of which the title is embossed in bronze classical lettering - upside-down. And, with that inversion, gravity is reversed and the world itself turned upside-down. This block at our feet is the planet's own pedestal, and for a moment we stand towards the whole earth both in awe and as its makers. It's a noble emblem of humanism. Praise to the Serpentine for exhibiting it. In a few square feet it does what a certain dome, now under construction, can only dream of doing.

To 26 April, Serpentine Gallery, Kensington Gardens, London W2 (0171- 402 6075). Sponsored by BMW Financial Services Group and Selfridges & Co.