In Fux's study of First Species modal counterpoint, why is it permitted to raise the 7th as he approached the final cadence. I realize that allowing that creates a leading tone which sounds better, but why is that ALLOWED in MODAL counterpoint? What rule permits that?
In doing research (on this site), I came across Knud Jeppesen's statement in his book that the "the seventh degree is raised at the cadence in all ecclesiastical modes except the Phrygian, in which half steps do not already exist between the seventh and eighth degrees." BUT there isn't a half-step between 7 and 8 in the Phrygian mode - so now I'm really confused. Thoughts? Thank you.