Was just reading the cover notes on the Waterson/Carthy "Common Tongue" CD.
Martin Carthy writes "... the musical instinct is far too inclusive to allow any notions of 'purity' to survive for long, and it is neither possible nor desirable to set up musical border checkpoints anywhere. However, there is that elusive and ever changing thing called identity."
The notes arise in the context of devoting the CD to English music, as opposed to Irish and Scots. The point, in the context of the CD, is that identifying "English music" is as ambiguous as any other identification, but that there is some elusive form of English "identity".
In the discussion in this thread, of course, the point is that "folk music" is undefinable as a distinct notion, but that there is some thing that one might call the folk "identity".