One applied from the outside and in ignorance is, I would suggest an invalid one and more importantly a unhelpful one.

But categories imposed from without DO have value, sometimes. I don't know why The Shambles added "and in ignorance" to my "applied from the outside." I wasn't talking about "categories applied from the outside and in ignorance," but categories applied from the outside for legitimate analytical and descriptive purposes. To wit, the fast food industry may not recognize the category of "fast food," preferring to think of themselves as "good food, served quickly," but we still can usefully apply the category of fast food. The genres of Cowboy Poetry and African-American Poetic Storytelling may have different names for their traditions, but we can create the category of "verse recitation" to include both of these extremely similar art forms. Similarly, musical traditions may not have names for individual genres, but it can be useful to create those categories, as long as you keep in mind that they are your categories, not theirs.

I would essentially agree with Sandy. The category of "traditional musician," even though it wouldn't have occurred to many "traditional musicians," is still helpful to differentiate Frank Profitt from the Kingston Trio. Even if Frank P thought he was just teaching songs to Frank Warner, and that therefore he and Frank W were both just singers, it doesn't mean there is no value in differentiating the two--Frank P as a traditional singer and Frank W as something else--in certain circumstances.

The whole category of "folk music" was by nature an externally-imposed category. Until very recently, nobody would have called themselves a folksinger or folk musician. BUT, many of us are aware that we like folksingers, and can usefully apply this category to people who would not have used it themselves. If Sandy says to me that Jeannie Robertson is an excellent folksinger, I know what he means to convey even if Jeannie wouldn't have used the term. I can therefore take his recommendation to listen to her recordings, proving that the category is not "unhelpful." It helps me and Sandy communicate, and while Jeannie was alive it would have helped her sell records, too.