'"We generally don't depend on definitions to understand or use words correctly."
We dio if we wish to talk to each other intelligently.'

Jim, I don't mean to be rude, but that is simply not the case. People can talk to one another intelligently who have never heard of definitions.

(And inversely, people using definitions can and do talk to one another incoherently if they rely on different definitions, or if they interpret a shared definition differently.)

'"exclude all non-chairs."
Nobody is suggesting anything like that'

Maybe I wasn't clear: Isn't a definition supposed to distinguish between what a word refers to and what it does not? Shouldn't a definition of "folk song" not only include all folk songs, but exclude non-folk songs? Otherwise, why not "I never heard a horse sing it"? That certainly applies to all folk songs--and a good deal else.