How long do you have to possess a stolen object before you can keep it for ever? The instinctive reply might be that, even if there is a statute of limitations on the crime, no amount of time will legitimately transfer title to the proceeds. But if this is true, and we decide to act on it, we are in big trouble. Imagine the cultural exodus that would ensue - not just the Elgin Marbles to be returned to Athens but the treasuries of the Victoria & Albert and British Museum catalogued and sorted for repatriation - an airlift of cargo planes darkening the sky, an armada of container ships spreading out from the constriction of the Thames estuary to return precious objects to their ancestral homes. For some objects - twice or thrice stolen over the millennia - the final destination might be difficult to work out, though one presumes that native origin would be the final arbiter in really heated custody battles. And once this global migration was over (a few small ships would come back up the Thames, riding fairly high in the water), what would we be left with? A world compartmentalised into purely parochial cultures, an artistic ethnic cleansing. It seems to me that it would also be a much poorer world. There is a good case for a mongrel culture, one in which the bloodlines have been inextricably mixed.

Of course this argument might carry more weight if I were writing from the perspective of one of the dispossessed nations; if, for example, almost all the Turners in existence were concentrated in a Tokyo gallery. Given that British museums and galleries groan with the loot of empire and exploration, any suggestion that the existing status quo might have its advantages is bound to look expedient rather than principled. If a burglar was to steal a favourite watercolour, you could be forgiven for resisting his description of the act as cultural exchange, even if he insisted that you weren't looking after the picture properly and he had saved it from inevitable decay. In this regard, Britain is a burglar that has never been burgled - or at least when we were, by the Romans, they were the sort of intruder who broke in, fitted central heating and tidied the kitchen before leaving empty-handed.

It might also be objected that, if past theft is ratified, there can be no logical barrier against present theft. At which point it is perhaps best to come clean. While there can be no defence for the sort of outright expropriation that characterised some kinds of colonialism, it's at least arguable that the definition of theft has now been extended so far that it inhibits useful acts of exchange. I might even go further and say that, provided you take a long view of these matters, a certain amount of theft may even be a good thing, allowing for a cross-pollination of national cultures. These issues are in the air again - not just because of recent scandals over art-house "smuggling" of national heritage (aka selling mediocre paintings for very good prices to foolish millionaires) but also because of the suggestion that the Benin bronzes in the British museum should be returned to Nigeria.

On one level the moral issue is fairly clear-cut - these objects were not even the result of some dubious purchase, in which a discrepancy of power supported a manifestly unfair exchange. They were simply taken from their place of origin as the spoils of a punitive expedition. So, given that we would now disavow the exercise of power that secured them, why should we not restore the objects to their rightful owners? One answer would be that they would have much less value in Lagos than they do here, that to send them home (if you accept that Lagos is their home) would be to deplete their power. Even if the object of the exercise is to reinforce a sense of Nigerian patriotic pride, the bronzes might serve their purpose better from the cabinets of the British Museum. Over here, they remain an object of pining regret, a focus for feelings of past historical grandeur and unifying grievance. Over there, they could very well end up as an under-visited asset - a complacent addition to what may in any case be a mendacious account of the national culture, undemocratic governments being all too prone to arranging national treasures in a way that suits their own political ambitions.

In the British Museum, they are possessed because of their intrinsic artistic worth, the universality of their artistic quality confirmed by the extraordinary company they keep. In the patriotically homogeneous surrounds of a Nigerian museum, they would be present simply because they were part of the family - the joy of reunion would surely be followed by a much longer period in which their virtues were taken for granted. It's arguable, too, that a sense of history may be a brake on a developing country, rather than the accelerator it is always fondly imagined to be.

And if we do concede that dues are owed, how much better to send back a decent Turner (we have thousands of them, after all), an object of undeniable preciousness to our own national heritage but one which would have a new and unpredictable life in the surroundings of an African museum.

Such a thing is unthinkable, of course, but only because British museum curators are possessed by the same stifling instinct for completeness and heritage that provokes such repatriation demands in the first place. Besides, the argument that the British Museum should not be obliged to return the Benin bronzes has an important corollary - it should not be obliged to keep them either, if the day comes when their intellectual value falls below the cost of retaining them. The destination of great works of art should not be decided by reasons of politics or nationalism or simple tradition, but by a free competition of reverence. To do anything else is to deny the very roots of the word culture - which grows out of the Latin colere - to honour with worshipn