What Is 'A Song'?

It seems that BBC 6 Music recently produced a listener-voted poll of the Top 100 songs of the last ten years (the time the station has been in air). I'm not going to dispute the choices (de gustibus non est disputandum), but it did seem odd to me that the Top Ten included at least one cover version, specifically Johnny Cash's version of 'Hurt' (originally by Nine Inch Nails).

I don't know what the identity conditions, or ontological status, of a song are. But ordinarily I'd describe Cash's 'Hurt' as a new version of the same song, not as a new song. And, if it is the same song as the one NIN released in 1994, then it doesn't seem that it should be eligible.

Perhaps this is over-restrictive. Perhaps new (re)interpretations should be counted as new songs, rather than variations on old ones (though I don't think it's how we ordinarily speak). But, if this is so, I wonder how much has to change for the song to become a new one.

Nine Inch Nail's And All That Could Have Been live album was apparently released in March 2002, so also too long ago, but had that been a year later would that have counted as a new song of the last ten years? Taken to the extreme, is every new performance a new song? If so, I'd suggest that there are some real classics missing from this list of songs...