De institutione musica I.30: Quemadmodum Plato dicat fieri consonantiam.
Plato says that consonance arises in the ear in this way. The higher sound, he says, must be the swifter. This, therefore, when it has gone before the low, enters the ear swiftly, and having struck the farthest part of the same body, as if repelled, returns by a repeated motion. But now it is slower and does not run as swiftly as when first sent forth, and therefore is also lower. When, therefore, it returns already lower and meets the low sound that is just then arriving, it is like it, is mixed with it, and, as he says, makes one consonance in the mixing.