De institutione musica V.6: Quae voces armoniae sint aptae.
Since, then, of voices that are not unisonous some are continuous and others separated, the continuous ones are such that their difference is joined by a common boundary, and the high and low voice have no designated place which they hold. The discrete ones, however, have their own places, like unmixed colors, whose difference is seen established in a certain place of its own. The continuous voices, which are not equal in sound, are set apart from the harmonic faculty. For they are unlike themselves and sound no one thing. But the discrete voices are subject to the harmonic art. For the difference of voices that are distant and unlike one another can be grasped; and of these, those which when joined can make a melody are called ἔμμελεῖς, but those which when joined cannot make a melody are called ἐκμελεῖς.