Artistic and archaeological treasures are scattered throughout the world, often quite far from their places of origin. Is there something wrong with that? Who really owns such cultural treasures as the Elgin Marbles (sculptures from the Athens Parthenon) or Vermeer's paintings (17th century Dutch master, whose paintings are scattered through W. Europe and the USA)? Post-colonial nations in particular are quick to argue that such works were stolen from them (as indeed so much else certainly was), that their continued place in museums in New York, Paris or London is a continuation of colonial attitudes, and that they must be returned to their country of origin. But how are such claims justified? There seem to be 3 main ways of staking a claim on such works: national identity, law, and the "cultural heritage of all mankind"
1. The national identity claim has the strongest intuitive force but is also the least convincing. On the one hand if a modern nation declares that an object taken from its territory really belongs to it there is a intuitive reasonableness to that claim. On the other hand, the claim seems to depend on a robust concept of national identity through time, which so far as I know does not exist. It is one thing to be able to say that a country, or a city state like Athens, can be historically distinguished in terms of a unique way of life and culture of the people within its borders (objectively identified); that that identity was felt and affirmed by the people living there (self-identified); and that those people produced cultural artefacts which had a particular centrality in their way of life and hence for their identity (as with the Parthenon sculptures). But for that historical identity to be relevant here and now, continuity must be shown: we must be able to re-identify the same culture in terms of the same features that originally made it unique.
Sometimes continuity is relatively easy to demonstrate - for example for some native American artefacts - and then the national identity claim can work well. But generally it is extremely difficult to re-identify that same unique identity over time, certainly over centuries, when the religion, language, ethnic groups, political borders, political constitution, and even the popular memory, interest, and valuation of the artefacts have all gone through numerous changes. Indeed it is often the case that the cultural artefacts are themselves the only enduring feature of previous historical nations. The national identity claim may then derive from a modern nation coming to feel that that they have a connection to the group that produced the artefacts (consider how the pyramids came to shape and focus modern Egyptians' sense of historical identity only after being 'discovered' by Europeans in the 19th century). Often the only enduring re-identifiable feature from past societies is the physical geography itself and that seems a weak basis for a claim based on national identity, even if modern countries like Egypt suddenly want to reassert, and seem to truly believe in, a claim to the history of the geographical location they occupy.
Additional problems are that various modern countries may have a claim over the same history (e.g. claim the same identity, e.g. Macedonia and Greece), or over the same artefacts because of their significance in different histories (e.g. is the Rosetta stone Greek or Egyptian?). There is also the problem of assigning rights over historical artefacts to particular current governments as true representatives and guardians of the entire national history rather than merely of the current population (and for non-democracies not even that). What kind of legitimacy can such government claims or decisions have, especially given that the same artefacts currently being demanded back were often neglected, and sometimes even sold, by previous governments?
2. Legal claims emphasise the procedural justice of a series of legitimate transactions. The idea is that if the ownership of an object was transferred justly (i.e. a legally valid voluntary transaction or gift by persons who already had legitimate ownership of the object), then its present ownership must also be just. Claims here depend on legal provenance, and while some artefacts have a fairly comprehensive and uncontentious provenance, like Vermeer's paintings, many others in international museums have murkier provenances or fairly obviously were acquired in violation of even contemporary laws (e.g. colonial confiscation, war booty, privateering). Various aspects of the legal provenance of the Parthenon sculptures, supposedly bought from the ruling Ottoman authorities of Athens, are disputed.
One of the problems with the legal approach is that anything can be disputed if you feel strongly enough, and people often feel strongly when their perceived national identity is at stake. So perhaps it isn't surprising that the provenance of the Parthenon sculptures is disputed, but not that of the Mona Lisa (though it was once stolen by an Italian patriot). On the other hand, while the legal case for keeping such artefacts in western museums may be incomplete (and so there is room for dispute), it is still generally stronger than any other legal claim and so has some argument for default status (if we accept the weakness of the national identity claim). The legal perspective sees nothing surprising about the unequal global distribution of cultural treasures, since those with the most wealth, and often legal authority over colonial provinces, when the museum fever began would obviously end up with more (cf capitalism in general: equal rights + unequal endowments lead via just transactions to even more unequal outcomes).
3. The final claim is based exactly on the universal value of great cultural artefacts as belonging to the joint history of all humanity. On this perspective, no-one can be said to own such artefacts in the normal way, only to hold certain responsibilities for their preservation, and exhibition, on behalf of all. This is the modern position, increasingly fixed in international law through various treaties from the 19th century onwards and protected even in time of war because "...damage to cultural property belonging to any people whatsoever means damage to the cultural heritage of all mankind, since each people makes its contribution to the culture of the world... " (Hague Convention for the Protection of Cultural Property in the Event of Armed Conflict, 1954).
Of course the modern position doesn't say much about which countries should look after which artefacts. But since it gives absolute priority to preservation it definitively undercuts the relevance of claims based on either national identity or legal provenance, even where these can be substantiated. It certainly implies that artefacts shouldn't be returned to countries where they might be at risk from conflict, incompetent preservation, or theft (although that probably wouldn't apply to contemporary Greece).
In fact it also seems to implicitly support the status quo placement of large numbers of artefacts from many different historical cultures together in public museums (not private collections) at central locations where tens of millions of international tourists can and do regularly visit (like Paris, London, and New York). These artefacts are deracinated from their context except for a little information plaque: culture, this suggests, inheres in the object rather than to its context, so there is nothing strange about going from ancient Egypt to ancient Greece to Viking Scandinavia in a few steps.
Here a standard argument about common standards applies. It is extremely convenient and beneficial for all to have a standard global language, even though it just happens to be even more convenient for native English speakers (unfair) and the historical circumstances (colonialism) whereby it became so are murky at best and despicable at worst (unjustifiable). It is still convenient even if most speakers will use only an extremely simplified grammar and vocabulary ('Globlish') that treats the language only as a means to communication and strips it of its cultural context and capacity for beauty (like poetry). Likewise, having lots of world-class artefacts publicly available close to major air-traffic hubs probably benefits more people than any other arrangement, even though the precise historical paths that have led to all that cultural heritage being in certain places rather than others generally falls short of the highest standards of justice. And even though the artefacts we see are deracinated to make them as accessible as possible to the busy Japanese tourist with camera phone who wants to know all about the history of western Europe in an afternoon.