What we know, or think we know, determines what we see. Poor Mr Hope's picture became, in his eyes, a worthless, daubed hunk of wood once he had been told it was not a Rembrandt. Yet it may have been, as other Rembrandts are, one of the great works of the human imagination. There is a fine line between ecstasy and despair and the line is knowledge.
Or take Van Meegeren, the greatest art forger of them all, who fooled the finest experts of his day. He spotted a gap in the Vermeer catalogue and duly filled it. Oddly, however, his fakes would fool nobody today because they are so obviously of their time. For one thing his Mary Magdalene, indeed all his women and even his Christ, bear a striking resemblance to Marlene Dietrich. That was the ideal of ethereal beauty at the time, so that is what people expected and that is what he gave them. A new Van Meegeren would, perhaps, have to paint a Madonna that looked like, well, Madonna.
And now we are in the midst of a whole rash of stories of new art errors. The Tate Gallery archives have been tampered with to provide false provenance for fake paintings by Ben Nicholson and sculptures by Giacometti. A picture in the National Portrait Gallery which Sir Roy Strong insisted was of Lady Jane Grey is now said to be false. And a National Gallery Rubens - Samson and Delilah - may have been knocked off by the young Jacob Jordaens.
The art world shudders. The Tate affair, in particular, has caused a mute, embarrassed closing of ranks. Anybody who has bought a Nicholson in the past six years, like anybody who ate bovine spinal cords in the Eighties, is at risk. Mad Art Disease is on the loose. How many paintings will have to be culled to allay consumer fears?
The primary force at work here is the immense value attached to things we call works of art. The visual arts, unlike any other, depend on specific material objects. We need to know that this is the very paint applied by Rembrandt or Vermeer. A photograph, a copy or a fake is as nothing. The prices are lifted into the millions by the assurance that what is for sale is the direct imprint of genius.
That assurance is provided by expertise. Stupid ages - and we are most definitely one of those - will define that expertise by its quick fixes, its gimmickry. Mr Hope destroyed his probable Rembrandt because some plausible expert came up with an instant test that happened to be completely wrong. And now the experts are going on about wood again. They say they can confirm or deny the National's Rubens by dendrochronology - counting the tree rings in the panel on which it was painted. Maybe they can. But remember, Snake Oil is always being sold, usually by much the same people.
True expertise involves a more subtle, cultivated assessment. Technical fixes can be proved wrong in time, honest evaluations can only be modified by experience. And what the scholar knows does not involve tree-rings, it involves a sense of the entire history of art.
But even this sense is useless when we come to Ben Nicholson. For Nicholson was a modernist and the point about modernism is that it deliberately severed the artistic tradition that defined and placed each artist. In the high modernist works of Marcel Duchamp and Andy Warhol, for example, the identity of the maker is deliberately suppressed. So now you do not know you have got a Nicholson because some expert has looked at the painting. The painting itself is too simple, too free of the identifying techniques of the Old Masters. You know only because of the provenance, the documentation that establishes a true line back to the artist's hand.
This is not to denigrate these painters. I remain convinced that Warhol was a great artist. But it does reveal a crucial change in the status of the hand of genius. How, for example, would you fake a Warhol Marilyn Monroe? Every Warhol is, in some sense, fake because he deliberately employed industrial techniques and materials to suppress the idea of the unique creator.
The problem is that the art world cannot live with Warhol's ego suppression. There would be nothing left to sell if they took him at his word. My Marilyn or yours would be as valuable as his, the imprint of genius would be a worthless technicality. To avoid this financially catastrophic state of affairs, documentation of paintings has become crucial, the one way of showing this coloured surface is worth more than that because this was touched by the artist himself. So the culturally au fait fraudsters who used the Tate got their priorities right. They first fixed the archive, then they painted the pictures.
But I think there is more to this urge for authenticity than simply the dealer's commission. There is a real longing to discover something holy, something magical in material things. Old Master paintings are probably the only objects that our whole secular society regards as sacred. Resplendent and usually over-restored in their galleries, they celebrate a connection to a tradition and set of values. They seem to mean everything to everybody.
The impersonality, the simplicity of much of modernism was an affront to this idealism. It inspired some artists, notably Jackson Pollock, to work furiously against the impersonal grain by painting pictures whose whole essence was defined by the absolute physical presence of the artist. And it has now created the bizarre state of affairs in which artists - say, Damien Hirst or Julian Schnabel - who, though every bit as impersonal as Warhol or Duchamp, take on the highly personalised role of romantic artist for an art world in which the aesthetic of impersonality is highly valued but is obliged to co-exist with the cult of personality, the yearning, financial and spiritual, for the presence of the authentic hand of genius.
This is a very contemporary contradiction. We want authenticity but we also want the convenience, simplicity and clear, strong meanings of the mass-produced. Sir Roy Strong's identification of Lady Jane Grey would, I would guess, have sold a lot of postcards. This was, for a while, the real Lady Jane and you could buy her picture, as clear as any photograph. But it wasn't and you couldn't. And, meanwhile, Samson and Delilah may not be Rubens. Knowledge, as Mr Hope found out, changes everything. Except, perhaps, the need to know.Reuse content