Pseudo-Plutarch, De musica
Goodwin, Ed.

But our musicians nowadays have so utterly exploded the most noble of all the moods, which the ancients greatly admired for its majesty, that hardly any among them make the least account of enharmonic distances. And so negligent and lazy are they grown, as to believe the enharmonic [p. 131] diesis to be too contemptible to fall under the apprehension of sense, and they therefore exterminate it out of their compositions, deeming those to be triflers that have any esteem for it or make use of the mood itself. For proof of which they think they bring a most powerful argument, which rather appears to be the dulness of their own senses; as if whatever fled their apprehensions were to be rejected as useless and of no value. And then again they urge that its magnitude cannot be perceived through its concord, like that of the semitone, tone, and other distances; not understanding, that at the same time they throw out the third, fifth, and seventh, of which the one consists of three, the other of five, and the last of seven dieses. And on the same principle all the intervals that are odd should be rejected as useless, inasmuch as none of them is perceptible through concord; and this would include all which by means of even the smallest diesis are measured by odd numbers. Whence it necessarily follows, that no division of the tetrachord would be of use but that which is to be measured by all even intervals, as in the syntonic diatonic, and in the toniaean chromatic.