WHAT makes an artist great? Brilliant composition, no doubt. Superb draughtsmanship, certainly. Originality of subject or of concept, sometimes. But surely true greatness means that the creator of a painting has brought a certain je ne sais quoi to the work as well.

There is, however, a type of person who seems to sait perfectly well what that quoi is, and can turn it out on demand. In 1945, for example, a Dutchman named Han van Meegeren faced execution for selling a national art treasure, in the form of a painting by Vermeer, to Hermann Göring, Hitler’s deputy. His defence was that it was a forgery he had painted himself. When asked to prove it by copying a Vermeer he scorned the offer. Instead he turned out a completely new painting, “Jesus Among the Doctors”, in the style of the master, before the eyes of his incredulous inquisitors.

Göring, who was facing a little local difficulty at the time, did not sue van Meegeren. But that has not been the experience of Glafira Rosales, an art dealer in New York who admitted this week that she has, over the past 15 years, fooled two local commercial art galleries into buying 63 forged works of art for more than $30m. She is being forced to give the money back, and is still awaiting sentence.

A load of Pollocks

Ms Rosales is guilty of passing goods off as something they are not, and should take the rap for the fraud. But although art forgers do a certain amount of economic damage, they also provide public entertainment by exposing the real values that lie at the heart of the art market.

That art market pretends that great artists are inimitable, and that this inimitability justifies the often absurd prices their work commands. Most famous artists are good: that is not in question. But as forgers like van Meegeren and Pei-Shen Qian, the painter who turned out Ms Rosales’s Rothkos and Pollocks, show, they are very imitable indeed. If they were not, the distinction between original and knock-off would always be obvious. As Ms Rosales’s customers have found, no doubt to their chagrin, it isn’t.

If the purchasers of great art were buying paintings only for their beauty, they would be content to display fine fakes on their walls. The fury and embarrassment caused by the exposure of a forger suggests this is not so.

Expensive pictures are primarily what economists call positional goods—things that are valuable largely because other people can’t have them. The painting on the wall, or the sculpture in the garden, is intended to say as much about its owner’s bank balance as about his taste. With most kit a higher price reduces demand. But art, sports cars and fine wine invert the laws of economics. When the good that is really being purchased is evidence that the buyer has forked out a bundle, price spikes cause demand to boom.

All this makes the scarcity and authenticity that underpin lofty valuations vital. Artists forget this at their peril: Damien Hirst’s spot pictures, for instance, plummeted in value when it became clear that they had been produced in quantities so vast nobody knew quite how many were out there, and when the market lost faith in a mass-production process whose connection with the original artist was, to say the least, tenuous.

Ms Rosales’s career is thus a searing social commentary on a business which purports to celebrate humanity’s highest culture but in which names are more important than aesthetics and experts cannot tell the difference between an original and a fake. Unusual, authentic, full of meaning—her life itself is surely art, even if the paintings were not.