De institutione musica V.3: Quid sit armonica regula vel quam intentionem armonicae Pythagorei vel Aristoxenus vel Ptolomaeus esse dixerunt.
An instrument of this kind, therefore, in which the differences of sounds are investigated with the application of the measure of reason, is called the harmonic rule. On this matter there was much disagreement among the opinions of the learned. For some, who put most trust in the Pythagorean disciplines, said that the aim of harmonics is that all things should follow what is consonant with reason. For sense, they said, gives certain seeds of knowledge, but reason brings it to perfection. Aristoxenus, on the contrary, said that reason is only an attendant and secondary, but that all things are determined by the judgment of sense, and that one must aim at its melody and its agreement. By Ptolemy, however, the aim of harmonics is defined in another way, namely, that nothing may be contrary to the ears and to reason. For according to Ptolemy the harmonician seems to aim at this, that what sense indicates reason also should weigh, and that reason should find the proportions in such a way that sense does not protest, and that the aim of every harmonician be composed of the concord of these two. And in this he chiefly reproves Aristoxenus and the Pythagoreans: Aristoxenus, because he trusts nothing to reason but only to the senses; the Pythagoreans, because they give least heed to the senses and most to the proportions of reason.