De institutione musica V.10: Quae sit proprietas diapason consonantiae.
He contends that this happens because the diapason is almost a single little voice, and is such a consonance that it forms, in a way, one sound; and just as with the number ten, whatever is added within it is kept whole and inviolate, while in other numbers this does not at all happen, so too in this consonance. For if you add two to three, you at once produce five and the species of the number is changed; but if you add the same to ten, you will have made twelve, and the two joined to ten is preserved. Likewise the three and the rest in the same way. So the diapason symphony, whatever other consonance it takes up, preserves the consonance, and neither changes it nor makes a dissonance out of a consonance. For as the diapente symphony, joined to the diapason consonance in the triple proportion, preserves the diapason and diapente consonance, so also the diatessaron, when joined as a consonance with the diapason, renders another consonance; and, according to Ptolemy, the addition of the other consonance to that which is the diapason and diatessaron is constituted in the multiple superpartient, and that proportion is the double superbipartient, as eight to three. For eight has three twice and two parts of it, that is, two units.