Beauty isn't in the eye of the beholder

The story, reported in various newspapers earlier this week, that a Rembrandt etching had been discovered in a charity store dump bin, obeyed nearly all the genre rules for anecdotes of this sort. There was a sense of narrowly averted loss (the picture had been in the chuck-out box, having failed to find a buyer). There was the lottery-win element, with an item that could have been bought for mere pennies proving to be worth around £800 (although the woman who spotted it selflessly told the charity what it had overlooked). And there was the sense that expertise had passed a real-world test. A trained eye had spotted something in the undergrowth, swooped and come up with the prey missed by countless others.

There is always another unspoken pleasure to these stories, though - and it comes from what's implicit in the plight of indifference from which these works are "rescued". Stripped of their attribution, they have - for most people at least - melted into the crowd of artistic also-rans. And since genius always contains an element of rebuke to the talentless there is something mildly comforting about being able to turn the tables. "Oh, yes - you're very grand when you can call on your connections," we think to ourselves, "but you're not so clever without them." There is a distant kinship with all those stories in which power adopts a humble disguise - and hears unvarnished truth for the first time.

Two exhibitions which opened in London this week offer another angle on this ancient theme: the Royal College of Art's annual RCA Secret show, in which art by unknowns and art-world luminaries is shown anonymously, and the Victoria and Albert's refurbished picture galleries, which display paintings from three important 19th-century bequests. In both places the question of how we judge a painting's value - aesthetic and monetary - is uncomfortably pointed. This is most conspicuously the case at RCA Secret, since an ostentatious egalitarianism is the raison d'être of the affair. All the pictures exhibited are the same size (standard postcard format), the same price (£35) and they are hung - or rather, perched - in military ranks which privilege none of them. Though the work of some of the big names seems easy to identify - a pair of Julian Opie female torsos, say - you can't be sure that a student hasn't entered an Opie pastiche and that the man himself is somewhere else in the crowd, perhaps pretending to be Patrick Caulfield for the day. What's more the crowd is vast - some 2,600 works, all of them clamouring for your approval like a litter of puppies in a pet shop. Not quite as noisily, obviously - but, in many cases, just as indistinguishably. It is, you could argue, an artificially constructed version of the charity shop serendipity - with a few precious objects concealed in a great mass of the negligible - and though the official line is that this is a level playing field - on which tyros can compete with stars on equal footing - most of the public treat it as an aesthetic lucky dip.

The odd thing about RCA Secret is that it levels down rather than levelling up. Deprived of the normal guide- ropes for taste - track-record and name and contemporary fashion - you're as likely to find yourself thinking that there's not much to choose between acknowledged talent and unknowns as you are to be reassured that quality will always shine out. And since contemporary art is currently intrigued by the offhand, the machine-made and the flippant, it makes things even harder. It really is quite difficult to tell the difference between incompetence and a sophisticated imitation of incompetence when you're deprived of the signatures that would help you to judge. What price the connoisseur's eye here - until the postcards are turned over and we see who did them?

The new Paintings Galleries at the Victoria and Albert offers one conventional story of connoisseurship rewarded. One of the paintings that forms part of the Ionides Bequest is a Tintoretto self-portrait which the collector picked up for just two guineas, a steal even at the time. The triumph is vitiated a little bit by the fact that he actually thought he was buying a Titian - but even as a Tintoretto it's worth a lot more than it was before he bought it - and our reaction to it is inseparable from our knowledge of its provenance. Just around the corner from the Tintoretto there's evidence that nobody is exempt from the partiality of fashion: Constable's painting of Water Meadows Near Salisbury. The artist entered this work for a Royal Academy Summer Show and was mortified to overhear the hanging committee, unaware that it was by a fellow academician, dismiss it as, "a nasty green thing." They, one assumes, were equally mortified to learn of their faux pas. One can easily imagine similar embarrassments occurring at RCA Secret, but for the fact that any sensible person will be on their guard against honesty.

The temptation in all such cases is to think of the prevarication and prejudice, the altered valuations and the hastily reconsidered judgements as evidence of cultural corruption. The fantasy is that beneath the distortions of money and fame and social networking there lies a pure hierarchy of artistic value. But there isn't. A culture consists in part of the lies we tell ourselves about art that we don't, deep down, like very much. It consists of capitulating to other people's convictions - and the proof lies in these odd moments when negligible pictures become estimable ones in an instant.