These are irrelevant to the issue.Find a vaild reason other then being in the same site of manufacturer.
Treaty? no
Same culture? no, I do believe even Europe is capable of change in a 2000 year time span.
Same DNA? No, its not a human artifact.
Same religion(s)? Not really unless Greek Orthodox has disappeared.
It's also clear that many previous posters aren't exactly aware of the current status and precedents, not only regarding the parthenon marbles, but also about the various aspects of the bilateral dialogue and public opinion in Britain and worldwide.
Actually it's not a strictly bilateral issue and UNESCO has also tried to facilitate the talks, Parthenon being a World Heritage monument.
If one is seriously interested in the historical context, then read William St. Clair's "Lord Elgin & The Marbles", which also contains the original italian translation of the 2nd firman(1801), before attempting to present any "facts".
The book additionally presents official documents of the British Museum that outline the attempts made to hide from the public eye the damage perpetrated to the marbles during the 30s, documents that remained hidden until quite recently.
Bookmarks