Leader: Sunflowers, saints' bones and Spice Girls CDs

``Well, he was a nutter, wasn't he; chopped his ear off and then topped himself. Still, if you like that sort of thing, put one in a nice frame and it's better than a blank wall. It's a free country. But what's crazy is paying millions for the real thing when with modern technology you can copy all the bumps in 3D; you'd need to be a rocket scientist to tell the difference. And then - this is the really funny bit - hundreds of them turn out to be fakes anyway and that Japanese company that paid pounds 25m for Sunflowers finds it's got some worthless bit of canvas daubed by Van Gogh's doctor.''

There is some truth in the rant of the caricature Philistine (although, to be historically accurate, it is an insult to Philistines to tar them with the brush of modern British anti-intellectualism). There is indeed something strange about the valuation of works of art, but the strangeness is much more interesting than the boorish utilitarian will allow. Art fakery is one of those subjects, like gruesome murders and the sexual proclivities of vicars, that seems to hold an endless fascination for people. But what would it mean if more than a hundred paintings and drawings attributed to Vincent Van Gogh were actually fakes? Certainly it would mean some Japanese investors would suddenly find themselves several billion yen poorer.

But would it mean that Van Gogh was a lesser artist than we thought - ho, ho, anyone can do them? Of course not: a faker is simply manipulating the creative ideas and techniques of a greater human. Van Gogh changed the way we see: but the moment his vision had been realised, it was possible to copy it and mimic it. More than that, it was inevitable that it would influence art that followed it. That was its power.

But this brings us back to the question of authenticity. Why should a painting of sunflowers by Van Gogh be worth, say, a thousand times more than a very similar-looking painting that turns out not to be by Van Gogh - and a million times more than a very handsomely produced reproduction? This is beyond reason, but not because it's silly. Valuing the authentic works of genius is a human instinct that is strongly related to the cherishing of religious trophies in earlier ages: Dr Gachet's Garden is, in part, for us what the toe-bone of St Thomas was for a 14th-century merchant - a physical link with the sublime, or miraculous. Mankind has long yearned for the touch of genius, to have bodily connection with something greater than ourselves.

Once, that was largely a religious connection, though there were always collectors for the great works of antiquity. Today, though, many of us worship the purest or bravest expressions of human creativity instead. And indeed, the reason why so many people want, and so few can afford, to own the actual bits of canvas on which great artists painted is not so far from the reason why other people want to own Princess Diana's dresses. It is the primitive impulse that drives the autograph hunter, that lights up the faces of the voter who says he won't ever wash his hand after shaking Tony Blair's. It is in the teenage scream of "I touched him!" when the hem of a pop idol has been felt. Its magic is in dubious grey lumps sold as moon rock, or bits of Berlin Wall, or relics.

Now part of that, of course, is merely about scarcity and market value: art works are also valued because they have a rarity that can be compared to gold. They are useful price-fixers. And at a lower level, signed books are rarer than unsigned ones and therefore, perhaps, a little more valuable. But when it comes to art we are talking about quasi-religion, not simply the market. Look at the formulaic reverence in the dazed faces of pilgrims filing past the Mona Lisa in the Louvre. Like any human impulse, it can be exploited for commercial gain - there is not much difference between the sale of papal indulgences in the 16th century and of individually signed Hockney prints in the 20th. Yesterday we reported that George Michael, along with Inspiral Carpets, Skunk Anansie, the Boo Radleys and Bryan Ferry had complained that record companies were putting out all manner of remixed offcuts under their names and debasing their artistic integrity. The same thing again: authenticity worshipped, then exploited. This is only a trivial example of the observation that the third-rate works of first-rate artists are worth more than the occasional great works of lesser names.

But simply because valuations are irrational does not make them wrong. Reaching out and touching what awes us is an essential part of being human; so, in the age of the market, it is tradeable. For metropolitan sophisticates, their irrationalism is dressed up as Art with a capital A, swooning over paintings they don't understand, or really like, while they patronise the superstitions of so-called "ordinary" people - the yearning to meet a Royal, or possess a signed Spice Girls CD. It is easy for confident, well-connected people to sneer at the desire to touch and collect. But it is not fundamentally different from the impulse that values a Van Gogh so highly, or treasures those few meaningless words exchanged with a Pavarotti or a Mandela. In both cases, it is touch and authenticity that matter.

It was not rational for medieval travellers to bring back bits of bones from the Middle East, and it was not rational to build incredible structures - both the stories and the huge stonework cathedrals - over them. It is equally irrational to build huge monetary values around paintings that have become familiar and are endlessly reproduced. But rip out that irrationalism and you rip out our very essence.