De institutione musica I.9: Non omne iudicium dandum esse sensibus, sed amplius rationi esse credendum; in quo de sensuum fallacia.
But we propose these matters in such a way that we do not give the whole judgment to the senses, although the whole beginning of this art is taken from the sense of hearing. For if there were no hearing, there would have been no discussion of sounds at all. But hearing holds in a certain way the place of a beginning and, as it were, of a prompting, while the final perfection and the force of knowledge rest in reason, which, holding to fixed rules, never slips into any error. For why must more be said about the error of the senses, when neither do all have the same power of sensing, nor is it always equal in the same man? It is vain, moreover, for anyone to entrust to a changeable judgment what he truly desires to investigate. Therefore the Pythagoreans are said to have gone a middle way. For they did not give all judgment to the ears, and yet some things are tested by them by the ears alone. For they measure the consonances themselves by the ear, but by what distances the consonances differ from one another, this they leave not to the ears, whose judgments are dull, but to rules and to reason, so that the sense is as an obedient servant, while reason is the judge and ruler. For although the moments of almost all the arts and of life itself have been brought forth by occasion of the senses, yet in these there is no sure judgment and no grasp of truth if the decision of reason is absent. For the sense itself is corrupted alike by the greatest things and by the least. It can neither perceive the least, because of the smallness of the sensible things themselves, and it is often confounded by the greater, as in sounds: if they are very small, hearing catches them with difficulty; if they are very great, it is deafened by the very intensity of the sound.