Johannes Tinctoris, in his treatise Concerning the Skill of Counterpoint, begins with a quote from Horace's Ars Poetica:
"Scribendi recte sapere est et principium et fons." That is, to understand or to have knowledge (sapere) is the first principle and the fountain (principium et fons) of writing well (scribendi recte).
It's worth noting that Tinctoris is drawing a pretty clear corollary between the skill of poetry (ars poetica) and the skill of counterpoint (ars contrapuncti). He's importing a principle from the one for the other so he can explain to his reader the difference between just scribendum (writing) and scribendum recte (writing well). The difference is sapere.
Understanding this in cultural context is important. In the 15th century, counterpoint is the not-uncertain capital of the Church. The French chansons, the obvious exception to the vast majority of contrapuntal pieces serving a liturgical function, was stylistically not all that different from the many of the liturgical pieces (take, for example, the similarity between Josquin's Absolon fili mi and his Je ne me puis tenir d'aimer). Obviously, because of the lack of commercialization and the printing press, the audiences to this sort of secular music was extremely limited, probably to nobility and their clergy.
So, when Tinctoris starts off saying that knowledge is key to writing well, he's not talking about writing a nice tune that a troubadour might sing or the latest local hit song in the tavern. Tinctoris is talking about High music. And the first thing he points out about this music that distinguishes it from just any music (hence the recte, "well" qualifying scribendi, "writing") is the possession of a body of facts concerning counterpoint.
Folk music is music centered around the village square or the domestic hearth. In the 20th century, with the rise of commercial music, it may center around the LP or the iPod, but I think it's still the same principle. There's a fundamental difference between this and High music. High music can be described as requiring ars or τεχνη or skill. That skill comes only through a knowledge of the theoretical techniques and musical precedent.