My god, what a lot of got air generated by you guys! I tend to agree with what Howard and Steve G are saying, but for me by far the best in-depth and no-nonsense discussions of what constitute 'folk song'' are in Steve Roud's introductions to both 'The New Penguin Book of English Folk Song' and 'Street Ballads in Nineteenth-Century Britain, Ireland, and North America'. Read those and then resume your argument. And before anyone says they're prohibitively expensive - there are free public libraries, and if necessary inter-library loans at very little cost.