If this is great art, how can it be reduced to a joke?

THE TROUBLE with a lot of conceptual art, you may say, is that it remains a concept. Although the best conceptual art comes up with ideas or paradoxes that continue to develop richly in the viewer's mind, such as Damien Hirst's dead shark swimming in formaldehyde, a lot of it remains just an idea, without any kind of resonance.

It's often thought that conceptual art is the most rarefied form of art imaginable. But I wonder whether this is really the case; whether most of it, in fact, pursues a blunt and ordinary idea which might make it an art form most appropriate for universal consumption, for distribution to the lowest common denominator of intelligence.

Other highly conceptual art forms, of course, aren't thought of by anyone as being abstruse or inaccessible. Rather than thinking of conceptual art as a branch of philosophy, we might do better to compare one high concept with another; to compare the mass of practitioners with the vicious blandness of Hollywood's "high-concept" movies. Just as you can repeat the idea of this summer's hit comedy in two sentences to all your friends, it's often possible to come out of some SoHo gallery with a neat summary of the fashionable conceptualist of the moment, and in neither case do you feel that very much needs to be omitted in the retelling.

The gorgeous bookshop at the ICA does a roaring trade in postcards of conceptual art, and sometimes, looking at a work of art which reads I SHOP THEREFORE I AM, you can't help thinking that the work of art was really always a template for the postcard; that it has found its true level when it is handed over as a greetings card, and understood by both giver and recipient as a joke. First-rate conceptual art remains what it is, and, like all art, somewhat resistant to reproduction; the great mass of it, however, has become an idea, and readily turns into a slogan.

All the same, even if they are not much more than ideas, the works of conceptual artists are their own, and they have a right to make money out them. You have a bit of sympathy when you read about the artist Gillian Wearing's battles with the advertising industry, which has pretty clearly nabbed ideas from her pieces, and used them to flog cars and satellite television.

The first case occurred about a year ago. Wearing had attracted a good deal of attention with her photo-pieces of strangers in the street, holding up cards on which they had written whatever they wanted to write. As insights into their character, the curiosity was in the gap between their appearance and what, with reflection, the cards seemed to say about them. It passed the time, a neat idea in an art gallery between more profound or complex work.

Someone at Volkswagen's advertising agency must have seen the idea, and an obvious borrowing from Wearing's idea surfaced in their next campaign, as middle-aged men in pinstriped suits held up placards reading "At weekends my name is Brenda" - that sort of thing.

Anyone who knew Wearing's work would have had no doubt where the idea had come from, and it wasn't a surprise to hear that she was mounting a legal challenge for plagiarism.

And now they're at it again, it seems; there is an advertisement for satellite television which stresses the choice available by having children's voices dubbed on to adults' faces, with the idea that "you never stop wanting to have your own way". This, too, looks perilously close to a piece of Wearing's, and her attack on the campaign looks pretty serious.

I can't help thinking, however, that Gillian Wearing is on a bit of a hiding to nothing by going round complaining that her ideas have been stolen by the advertising industry. Of course, it's deplorable that some idle creative can't think up his own ideas, and has to borrow the ideas of other people - and anyone, having seen Wearing's placard piece and the VW campaign, would very readily come to the conclusion that here was, at the very least, a case of direct inspiration.

Advertising, however, has never been a highly original medium. What it does is to steal the sort of ideas that can be packed into a 30-second slot, and tack on a slogan. It hardly matters where these ideas come from, though they all seem to watch exactly the same films - I often think I'll scream if I see just one more advert that tries to sell a car with the last scene of The Graduate - and it was really only to be expected that, sooner or later, conceptual art was going to be raided.

You can see, of course, that these adverts do damage Wearing's art, and damage her capacity to pursue her own ideas. The placard piece, for instance, was not originally a joke, or not really; although it sometimes moved towards an ironic humour, it didn't aim primarily to amuse.

The VW advert that it turned into, on the other hand, was nothing but a joke, and it was very striking that after it began to be aired, people started to respond to Wearing's work as if it were meant to make you laugh. Not only is that kind of thinning of emotional response pretty damaging, but it also made it difficult for her to carry on with the project. Anyone who was subsequently approached in the street and asked to be photographed with their thoughts on a card would generally remark, "Oh, like that car advert", and make an attempt to come up with a joke.

All the same, I think that if I were Gillian Wearing I might give a moment's thought to the value of practising an art that is so easily condensed into 30 seconds of air time and can be so slickly given a neat slogan at the end.

Of course, in the short term the firms concerned ought to pay her for what is an indisputable use of her ideas. But a much more valuable solution would be for conceptual artists to work at levels that cannot be boiled down to the intellectual level of the snake-oil salesmen; to produce works of art that are too rich and ambiguous in meaning for any hired mediocrity to snaffle for the benefit of a new chocolate bar.