De institutione musica I.28: Quae sit natura consonantiarum.
Consonance, although the sense of hearing also judges it, is nevertheless weighed by reason. For whenever two strings, one of them lower, are stretched and, struck together, give back a sound in a way mingled and sweet, and the two voices, as if joined into one, coalesce, then there arises what is called consonance. But when, struck together, each wishes to go its own way and they do not blend to the ear a sweet sound, one compounded of two, then there is what is called dissonance.