Pseudo-Plutarch, De musica
Goodwin, Ed.

Then again, that the ancients did not through ignorance abstain from the third string in the spondaic style, their use of it in play makes apparent. For had they not known the use of it, they would never have struck it in harmony with parhypate; but the elegancy and gravity that attended the spondaic style by omitting the third string induced them to transfer the music to paranete. The same reason may serve for nete; for this in play they struck in concord to mese, but in discord to paranete, although in song it did not seem to them proper to the slow spondaic motion. And not only did they do this, but they did the same with nete of the conjunct heptachords; for in play they struck it in concord to mese and lichanos, and in discord to paranete and parhypate;¹ but in singing those touches were no way allowable, as being ungrateful to the ear and shaming the performer. As certain it is from the Phrygians that Olympus and his followers were not ignorant of the third string; for they made use of it not only in pulsation, but in their hymns to the Mother of the Gods and several other Phrygian songs. Nor is it less apparent, with regard to the ὑπάται, that they never abstained for want of skill from that tetrachord in the Dorian mood; indeed in other moods they knowingly made use of it, but removed it from the Dorian mood to preserve its elegant gravity.