The lion and the dead lamb: Andy Warhol shocked, but he was a great artist. So far, Damien Hirst is just shocking

THE DEAD, preserved lamb was vandalised on Monday, its formaldehyde bath and fluffy fleece polluted with ink. But the suspended shark, the upside-down version of Rodin's Balzac, the snowstorm paperweight and the drugs cabinet filled with anti-viral agents were untouched. Clearly, the phantom inker was particularly incensed by the lamb, perhaps provoked by the poignant bubbles around its mouth, or its floating, oppressive deadness.

The gesture was, in its way, fair enough. A sign outside the Some Went Mad, Some Ran Away . . . exhibition at the Serpentine Gallery warns that we may be disturbed by what we see. Inside, another sign drives the point home. 'The work here,' it says, 'far from belonging to the closed specialisation contemporary work has so often been seen to have become, reaches out to real life, with all its humour, confusion and difficulty. Sometimes, perhaps, it gets too close for comfort.'

Ink, in the circumstances, might be seen by Damien Hirst, the show's curator and the maker of the lamb, as a disappointingly mild response.

Meanwhile, over at the cool Bond Street salon of Anthony d'Offay, an exhibition of Andy Warhol portraits requires no warnings or explanatory signs. Here there is no reaching out to real life, only d'Offay staff in beautiful suits and, on the walls, blank stares from another world occupied by people called 'Joan Collins', 'Aretha Franklin' or 'Man Ray'. This is, if anything, too distant for comfort. But no ink will be thrown, nobody will feel 'disturbed'.

Yet, in his day, Warhol also provoked. His New York commune of deadbeats, junkies, defectives and wall-eyed hustlers stirred the disgust of honest, God-fearing American folk. The man himself seemed an affront to nature, dignity and health. 'My skin is pale as the outdoor moon,' ran Lou Reed's beautiful song about Warhol, 'My hair is silver like a Tiffany watch.' And, in 1968, Valerie Solanis of the Society for Cutting Up Men shot him in the chest for no clear reason. Damien Hirst is a big man in British art at the moment, but he's got to do better than a little ink-stained lambswool if he wants to keep up with the master.

That the new must shock has become one of the essential beliefs of visual arts in the 20th century. Ever since Ruskin abused Whistler 'for flinging a pot of paint in the public's face' or the Impressionists scandalised Paris, it has been taken as a commonplace within much of the arts brotherhood that the best is most clearly defined by the fury of the bourgeois reaction. Of course you are upset, they say, because upsetting is precisely what art must be. And, runs the unspoken sub- text, because we are artists, we are smarter and better than you.

The bourgeois fury takes two forms. Either the art is seen to be so easy, so devoid of any input of individual talent or craft that anybody could do it. Or, as in Hirst's work, it is seen to be both talent-free and viciously unpleasant - a steel-capped Doc Marten in the face of the public. This, say the provoked, is the state to which we have descended - from the delicacy of a late Titian to the emptiness of a dead sheep.

Underlying this fury is the fear of a loss of sense. If there is no craft involved and no detectable tradition, how can anything be judged? What standards can be applied? Ah ha, say the artists, quite so. There is no fixed point of judgement, there are no standards. We live in a value void, our work makes you confront this fact.

This, however, is no more than a hollow ingenuity and, it has to be said, the bourgeoisie are almost entirely right - 99 per cent of all contemporary art is no more than facile, shallow posturing that will not last into next week, never mind the next century. The Dutch government, for example, have taken a decent-minded, indulgent, liberal view of financing their young artists, and warehouses throughout Holland are full of useless, worthless chunks of what was once seen as very shocking art.

But 99 per cent of all art at all times has probably been worthless. Even Titian and Vermeer float on oceans of rank mediocrity. The good is a fragmentary, miraculous by- product of the bad. And the logic of that is that the production of the bad is necessary for the production of the good. For art to be produced at all, the idea of art must have a place, must be seen to be a worthwhile aspiration even among the talentless, the dull and the offensive.

The intelligent, well-meaning viewer should, therefore, be prepared to take the boot or the paint in the face, knowing that this might be the context, if not the specific occasion, of something that will endure. He or she need not be gullible, nor even blandly open-minded. The viewer need only be prepared to watch and wait.

This is where Andy Warhol comes in. He is seen by sceptics as the first malign harbinger of the decline of postwar art to dead sheep and plastic Balzacs. Certainly there can be little doubt that much that now happens in galleries in New York and London happens because of Warhol. As a further stimulus to doubters, the bleached-blond mumbler has become an established 'great'. The art in his estate, after a long legal battle, is valued at more than dollars 500m and a mighty Warhol museum is to open this month as the new municipal pride of Pittsburgh. For an artist of the ephemeral who was at his peak almost a quarter of a century ago, this represents considerable longevity.

In 1971 I, callow and idealistic about art, saw the big Warhol exhibition at the Tate. I had expected to derive a kind of smart, in-the-know thrill from the soup tins and Brillo boxes. But I came away dazzled by what I was convinced was great art. At the d'Offay this week the conviction was renewed.

For the truth is that Warhol's work has endured, not just in legal fees and big museums, but as art that is moving and intense in the entirely traditional, as opposed to the aggressively avant-garde, sense. The portraits at d'Offay might be his most routine stuff, but they are still extraordinary. Partly because so many of the high society subjects are already dead, but also because of the way Warhol's technique seems to remove their vanity to an immense emotional distance, these are works that can now be seen as genuinely tragic. With the passage of time Warhol has escaped the adoring embrace of the fashionable to become, I think, timeless.

His great contribution to the art of the portrait was the indefinite article. This is not the Joan Collins, it is a Joan Collins, a version as 'real' as the flesh and blood star we assume is the source of this image. So he could produce one Marilyn Monroe or Elvis Presley and then another and then another. Each was as fully Marilyn or Elvis as any other object of canvas or flesh, because the world was now made only of these interminably repeated versions. He painted famous people because they were the most obviously repeated. But the tragedy sprang from the truth that we all feel similarly alienated by the process of repetition, we are all becoming 'a' rather than 'the'. Warhol may have let Truman Capote look smart and pleased with himself, but secretly he was stealing his soul.

The problem for Hirst and the group of artists he has chosen for the Serpentine show is that you can only do this once. Warhol was at one with his age and his city. Few artists have such luck or the gift to exploit it. The flashy post-Warhol drivel of Jeff Koons and others shows clearly enough how easily pop, having once been art, quickly reverts to trash. And the current 'masters' of the contemporary art scene - Julian Schnabel being the most obvious - look like no more than cunning, market-sensitive swimmers in a very shallow pool of meanings and effects.

Perhaps frustrated by a state of affairs that looks suspiciously like decadence, the Hirst generation has combined the manipulation of the market with some aggressive and polemical 'messages' that would have embarrassed Warhol with their directness. The lamb is 'about' death, Kiki Smith's flayed Virgin Mary is 'about' feminine stereotypes and the Balzac is 'about' the oppressions of the fine art tradition. So fierce is this new determination to thrust meaning down the viewer's throat that visitors to the biennial exhibition at New York's Whitney Museum last year were given little badges affirming they were disgusted to be white or delighted to be black. This art was 'about' being dementedly PC.

This will, of course, make the art as temporary as the messages and there is certainly nothing at the Serpentine that looks as though it could survive the trauma of losing its 'relevance' or its carefully contrived 'meaning'. This is bad news for the investors. But at least it all wants to be art, a touching aspiration that will in time sustain another Warhol or even Titian. Not, however, yet.