De institutione musica V.4: In quo Aristoxenus vel Pythagorici vel Ptolomaeus gravitatem atque acumen constare posuerint.
Since all agree that sound is a striking of the air, Aristoxenus and the Pythagoreans, who followed him, laid down the difference of lowness and highness on different principles. Aristoxenus thought that the differences of sounds according to lowness and highness lie in quality, whereas the Pythagoreans placed them in quantity. Ptolemy seems closer to the Pythagoreans because he too holds that lowness and highness are constituted not in quality but in quantity; for denser and finer bodies give out highness, looser and larger ones lowness, to say nothing now of the manner of tightening and loosening; although even when something is loosened it becomes, as it were, looser and thicker, whereas when it is tightened it is rendered denser and drawn out finer.