Thomas Sutcliffe: Nice one, Pablo; can I copy it?

Why is it that artists and painters are so rarely accused of plagiarism? There are exceptions to this rule, of course - though almost all the ones I can think of are recent, and parasitic on a particular type of art practice; the visual-arts equivalent of sampling, in which a pre-existing image is co-opted for a new purpose. It happened to Jeff Koons, for example, who copied a sentimental photograph of puppies by Art Rogers for a sculpture he called String of Puppies and eventually lost his case - the court having decided that what was being parodied was not the original work but the social sentiment that had given rise to it. The fact that Koons's physical interaction with the work didn't go much further than tearing the copyright symbol off the postcard he sent to the Italian artisans who actually made his sculpture didn't exactly help his case.

Damien Hirst suffered a similar problem after the designer of the educational toy on which he modelled his huge bronze sculpture Hymn suggested that he might be in line for a copyright fee. And earlier this year another claim surfaced, with a graphic artist pointing out that Hirst's polka-dot work Valium bore a striking resemblance to his much earlier computer graphic True Daisy. But these are rare examples, clouded in any case with a misunderstanding of how collage and reproduction work in contemporary art.

What is slightly odd, though, is that artists have always been able to borrow and imitate to an extent that would be regarded as dubious if practised in literature. This was underlined for me by a coincidence; the fact that as I walked round the Whitney's fascinating exhibition Picasso and American Art, the factitious row about Ian McEwan's "plagiarism" of a book of wartime memoirs was fresh in my mind.

What the Whitney show in New York does is simple; it takes Picasso paintings and hangs them alongside the American works they inspired. In some cases, what you see on the wall is a conversation in paint; one artist responding to the challenge of another, either repudiating or applauding him - just as Picasso frequently did with his own great forebears (Picasso's version of Velazquez's Las Meninas obviously isn't an act of plagiarism but of respect). In other cases, though, you can't help but feel that admiration has gone a little too far. An artist called Stuart Davis follows Picasso's stylistic twists and turns with such fidelity that he turns himself into an aesthetic stalker (or, at least, he does in this selective representation of his work). The trademarks of Picasso's style are borrowed wholesale - pointillist dots, Cubist shifts of register, simplifications of landscape - the only component that Davis can't quite emulate being the talent.

Clearly such discipleship has its literary equivalents. Think of the innumerable bad imitations of The Waste Land spawned by TS Eliot's original. But the interesting thing about Davis's paintings is that he clearly never felt the slightest anxiety that he might cross the ill-defined border between imitativeness and outright plagiarism. He even adopts Picasso's props - so that the pipe that features in so many early Cubist still lifes pops up in Davis's painting Lucky Strike as well.

There's no copyright on the contents of the world, of course, but I think this insouciance of allusion is qualitatively different to what happens in print. Think of Glenn Brown - a Turner-nominated artist who paints meticulous reproductions of other people's paintings, skewing the dimensions a bit or introducing variations of colour. Could anyone do the same thing with words, printing off their own version of McEwan's Saturday with a few tenses changed and marketing it as their own? I think it's clear that the lawyers would be on the case pretty soon - not least because book-buyers wouldn't easily be able to distinguish between the two.

And that example is surely a clue to the distinction. Plagiarism is a charge that has less to do with the morality of unacknowledged borrowing and more to do with potential infringement of trade. And, in an art form that is largely a matter of one-offs (and, as my colleague Tom Lubbock recently pointed out, largely the preserve of the very rich), the possibility of lost revenue is far less acute. All kinds of cultural history play their part in this distinction - from the way the Renaissance studio system encouraged imitation to the long tradition of compositional and thematic reference. But profit and loss is the true key. If Stuart Davis had ever stood the remotest chance of affecting Picasso's saleroom figures - or diluting the market for Picasso's work with his own awestruck imitations - I have a feeling there would be less talk of "homage" and more of plagiarism.