De institutione musica III.5: Quemadmodum Philolaos tonum dividat.
But Philolaus the Pythagorean tried to divide the tone in another way, setting the beginning of the tone at that number which makes the first cube from the first odd number, which was most honored among the Pythagoreans. For since the number three is the first odd, if you take three three times, and that three times, 27 will necessarily arise, which is distant from the number 24 by a tone, keeping the same difference of three. For three is the eighth part of the sum 24, which added to the same gives 27, the first cube from three. From this, then, Philolaus makes two parts, one which is greater than the half, and this he calls the apotome, the other, which is less than the half, and this again he calls the diesis, which later writers called the lesser semitone; and the difference of these he calls the comma. And first he holds that the diesis consists in 13 units, because this difference was perceived between 256 and 243, and because the same number, 13, consists of nine, three and a unit, which unit holds the place of a point, three that of the first odd line, nine that of the first odd square. For these reasons, then, since he places 13 as the diesis, which is called a semitone, he fixes the remaining part of the number 27, which is contained in 14 units, as the apotome. But since a unit makes the difference between 13 and 14, he judges that the unit is to be set in the place of the comma. The whole tone he places in 27 units because between 216 and 243, which are distant from each other by a tone, the difference is 27.