Bricks, tins of excrement, pickled animals and unmade bedsI Anything can be passed off as art, says Robert Dixon, and London's Tate Gallery is largely to blame.

Whoever gets the Turner prize on Tuesday, serious questions will remain over the Tate Gallery and the art that it champions. Never mind that this year's shortlist contains (surprise) two painters, nor even that one of them, Glenn Brown, displays (shock) sharp pictorial skills. Last year's record attendance for Tracey Emin's unmade bed should leave no one in doubt about the nature of the game that is modern art: shock, surprise, challenge.

Modern art may have begun with the earnest attempts of certain painters in 1860s Paris to devise looser, easier, more colourful styles of picture making, vindicated by subsequent popularity with viewers and amateur practitioners, but long ago it became a pain instead of a pleasure. The camera called time on art as surely as the engine did on the horse.

But art was not put out to grass. Too much was at stake. The power to turn cheap matter into fabulous treasure and a blossoming new role as public shrine helped it to survive and extend itself. This was done by a complete inversion of principles. In the modern world, art must become neither picture nor ornament. It has to be something else. Those impressionist - or surrealist or abstract expressionist - pictures, which many of us might enjoy hanging on our walls, are not valued by the art machine for their visual delights, but for how they depart from the visual experience. They are rarities that create spurious myths of genius. But once created, they do not lead anywhere. A state of permanent innovation, innovation without application, is pathological. It was bound to exhaust all possibilities and result in a process of eliminating, not developing art. This terminus was reached about 30 years ago, since which time installations have staged a takeover. Bricks, string, broken crockery, dirty lino, tins of excrement, glasses of water, pickled animals, elephant dung and unmade beds have all famously carried the day.

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One function dutifully carried out by the Tate for the past half-century has been the annual purchase of modern art, almost all of which in the past remained hidden in storage for lack of wall space. When it was announced that Bankside Power Station, with its giant nave, was to become the new Tate Gallery, I felt an impulse to see all of it laid out in full public view, in order to take stock. Surely we would find that hoard self-evidently so drab or doubtful that we would at once cry out in genuine surprise: "Oh, what a dreadful idea it was from beginning to end!" And yet, it is sad to say, I guess that when it comes to art, we no longer believe our eyes. The placebo effects of grand architecture, institutional magnificence, and an ingrained sense of submissiveness towards culture, defeat both judgement and common sense. If the Tate has proved one thing beyond doubt, it is that anything, especially if it is nothing, can be passed off as art. There is no holding it to account. It has a mandate to go on spending more and more on hoax art of fake profundity. Weird and inscrutable artefacts fulfil a role, just like the holy relics of medieval pilgrimage.

In an era of denationalising industries, and of cuts in spending across a swath of public services, it is remarkable that the public funding of modern art has continued to grow with unbroken cross-party support. Or is it silence? The figures show that since its inception in the immediate postwar years, public funding of the arts has grown steadily and exponentially.

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True, the pot was pegged to inflation in the Thatcher years, but the arrival of the National Lottery doubled the cash available to restore that upward curve. And the sums have increased since. In the case of the Tate Gallery, this flow of fortune is particularly surprising given its persistent record of outraging public opinion. Time and time again, the Tate has not only failed to please, but has pointedly offended the general public.

To understand why the rising tide of public funds for modern art does not reflect public needs or feelings, one has to see how funding of the arts is structured. Two basic tenets apply. First, all arts are treated as a good cause. A single cake is given to the arts and then divided in roughly fixed proportions. Second, each art combines two quite different causes: preservation of antique works and the commission of avant-garde. In the case of visual art, there is an obvious contrast in merit between collecting old works and making new ones - the difference between a museum and a mint. While music might spend heavily to uphold classical performance, traditional visual art mainly bankrolls the cult of modern art. So, by a process that regards all arts as one worthy service, modern art is guaranteed a steady and growing supply of money.

When the government decreed that the National Lottery hand over money to the arts, the Tate Gallery was automatically in line for a fixed percentage of the good causes cash, regardless of its product. The Tate, as the flagship institution of its division of the arts, is funded directly by the government in parallel with Arts Council funding. Funding for the Tate equals all other visual art funding combined. The rest of the money mostly goes to the other seven public modern art galleries, which act in close concord with Tate taste. So what happens at Tate really is more or less the be-all and end-all of public policy and spending on visual art. What began a century ago as a private collection of Victorian paintings and sculpture has slowly turned into the powerhouse command centre of a modern art industry.

Public art selection - for exhibition, purchase or Turner prize shortlisting - is an endlessly incestuous procedure. Artists who make a splash in art galleries are promoted in art galleries. This, in turn, makes their splashes bigger, creating a self-confirming effect and raising at least two important questions. If the splash happens in a private gallery, why should we agree that this is a matter of artistic quality or value? If in a public gallery, one must ask how it came to be exhibited in the first place?

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Then there is the question of public promotions oiling the pumps of private promotions. As well as the collusive arrangement between a small clique of directors, dealers, critics and artists to fix who and what will get promoted, we must surely be concerned about the financial interests of all those involved. All these questions lie outside the remit of the National Audit Office, leaving an unchecked private party to run an endless charade at public expense. This state of production without quality controls on the art or scrutiny of commodity dealing is unhealthy. And consider this. As well as three government departments funding modern art - Arts, Education and the Foreign Office - so do three National Lottery good causes: arts, heritage and millennium. National Lottery funding is in effect triple what it may have been widely supposed to be.

So, given agreement that modern art is an expensive nonsense, where do we go from here? To begin with, we should avoid the false dichotomy of Tate art versus traditional art. Remember that one led to the other. There may be a case for nurturing painting and sculpture, but only if we can drop the habit of being snobbish about styles. Art school training has a long history of imposing its affectations and pictorial inhibitions. The conceptual challenge is to break the language that replicates the failings in thought, to recognise quite simply that "art is good which is good of its kind" and that it comes in all kinds and all media. In the modern world, we are richly surrounded by pictures and ornaments as never before.

Second, the recent refurbishment of Giles Gilbert Scott's building - surely a temple by stealth - is not itself a cause for alarm. The building is value for money, even if the art is not. It is a great public space in a key pedestrian location of the capital with free entry for all. It would be easy to think up better content. I suggested one idea some time ago: a centre of science images with, occupying the great hall, a huge project in using satellite data to construct an image of the whole earth. And I mean, strictly high resolution, honest and without a trace of art.

Robert Dixon is author of The Baumgarten Corruption: From Sense to Nonsense in Art and Philosophy , Pluto Press, ?13.99.

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