Stephen Fry has lent his support to the campaign to restore the Parthenon marbles to the Acropolis in Athens. I find it interesting that when this topic comes up most commentators avoid the P word, i.e. property. Ultimately, the rights of property are what the British Museum is defending: we own them, so sod off.
The argument in favour of the return of the marbles usually boils down to "they belong here", which is a more polite version of "they're ours, give 'em back". The many and various arguments against restitution are mostly specious, their number being a sign of uncertainty and the reluctance to mention property - i.e. if you had one good argument ("we own them") you wouldn't bother thinking up others. They tend to cluster around four main strands of thought.
1. You cannot undo history. This is the conservative argument. It is perhaps regrettable that things have turned out this way, but it would be better to accept them as they are and not compound matters.
This is an argument in conflict with itself. If you believe that we have a duty to honour history, in the Burkean sense, then you tend to accept the historical significance of specific artefacts. You cherish ruins and join the National Trust. You believe that artefacts possess a historical charge. But unless you believe their historical significance is primarily as the spoils of war, or as a commodity bought on the market, then their true historical value is revealed only when they are reintegrated into the organic environment from which they came.
2. This sets a precedent. This is the pragmatic argument. Where do we stop? What if every artefact in every museum had to be returned?
This is not really pragmatic, as the extreme case of every artefact winging its way home is clearly unlikely, though the net effect would probably be healthy: a wider distribution of artefacts generally and greater concentrations that would aid scholarship. In fact, we can be confident that only artefacts of major cultural significance (a subjective characteristic that can change over time) would be requested for return. The British Museum isn't going to ask that every Victorian steam engine exported to India is returned, nor is Sutton Hoo parish council going to insist on the return of its Saxon hoard.
3. Here is better than there. This is the elitist argument. The artefact will receive better care where it is now. We have the best expertise and facilities.
The unspoken assumption is that if the artefact is repatriated it will end up being damaged or neglected because the owning country is incompetent or doesn't really care. This is essentially the same argument that was used to justify the removal of the marbles in the first place, which is enough reason to reject it. Nobody would argue that we should have forcibly retained the treasures of Afghanistan, which were on display at the British Museum last year, because London offers a more politically stable home than Kabul.
4. More people will see it here. This is the utilitarian argument. We're a global transport hub and tourist destination, which means the artefact is easily accessible to experts and non-experts alike.
This is actually an argument for abolishing every local museum in the country and concentrating everything in Bloomsbury, or even for insisting that the Rijksmuseum should be transferred to London (assuming an extra runway at Heathrow). It's also vulnerable to the counter-argument that moving the marbles to Athens would actually boost the numbers who see them and better serve Greek experts..
Most artefacts have been removed from where they were found. To an archaeologist this is problematic as their value is in the context of their use, e.g. their position in strata or their relationship to specific buildings, which is why "treasure hunting" that produces the artefact without detailed provenance is frustrating.
So why don't the British Museum simply concentrate on the defence of property rights? Perhaps it was the dubious legal basis on which Elgin spirited the marbles away, however I think it worth noting that the Greek government has never formally requested their return (despite Melina Mercouri's urging), and nor has there been a rash of governmental claims for other artefacts elsewhere in the World.
I can't help feeling that governments and museums are tiptoeing around the issue because the only logically consistent basis for the status quo is the brute fact of possession. This is, of course, the actual basis for all property rights, but it isn't something we like to admit publicly. We're keeping them because we have them.