The fact that more attention is paid by the best writers to the rather pleasing sound of words and phrases—what the Greeks call euphony—instead of rules and custom, things revealed by grammarians
“When Probus Valerius was once asked-and I heard this story from one of his close friends—whether has Urbis or has urbes was correct or hanc turrim or hanc turrem, he said “If you are creating verse or you are writing out prose and you need to use these words, don’t listen to the pedantic and specific precepts of grammarians; but heed your own ear for whichever solution it prefers. What pleases it, that will certainly be the most correct.
When his friend asked in turn “What do you mean “heed my ear”?” And Probus is said to have responded “In the same way that Vergil mixed things up, so he wrote in different places urbis and urbes, obeying the judgment and advice of his ear.”
XXI. Quod a scriptoribus elegantissimis maior ratio habita sit sonitus vocum atque verborum iucundioris, quae a Graecis euphonia dicitur, quam regulae disciplinaeque, quae a grammaticis reperta est.I. Interrogatus est Probus Valerius, quod ex familiari eius quodam conperi, “has” ne “urbis” an “has urbes” et “hanc turrim” an “hanc turrem” dici oporteret. “Si aut versum” inquit “pangis aut orationem solutam struis atque ea verba tibi dicenda sunt, non finitiones illas praerancidas neque fetutinas grammaticas spectaveris, sed aurem tuam interroga, quo quid loco conveniat dicere; quod illa suaserit, id profecto erit rectissimum”. II. Tum is, qui quaesierat: “quonam modo” inquit “vis aurem meam interrogem?” III. Et Probum ait respondisse: “Quo suam Vergilius percontatus est, qui diversis in locis “urbis” et “urbes” dixit arbitrio consilioque usus auris.