Macrobius, Saturnalia 1.24:
“If you are of the opinion,” said Symmachus, “that Vergil should be considered as having perceived nothing that was not poetic, you would surely begrudge him this title too. But hear what he himself pronounces of the manifold learning of his work. For a letter in Vergil’s own hand, in which he addresses Augustus, begins thus: ‘I receive frequent letters from you,’ and later we read, ‘Indeed, as to my Aeneid, if I had anything – by Hercules – worthy of your ears, I would happily send it. But such a great thing is started here that I seem to myself to have set upon such a great work almost by some fault of my mind, especially since (as you know) I must engage in many more serious studies for that work.’ Vergil’s abundance of matter does not contradict this, but more or less all of our litterateurs pass over this with unwashed feet, as though a grammarian were permitted to know nothing except the explanation of words.”
Si in hac opinione es, inquit Symmachus, ut Maro tibi nihil nisi poeticum sensisse aestimetur, licet hoc quoque eidem nomen invideris: audi, quid de operis sui multiplici doctrina ipse pronuntiet. Ipsius enim Maronis epistola, qua conpellat Augustum, ita incipit: Ego vero frequentes a te litteras accipio, et infra: De Aenea quidem meo, si mehercle iam dignum auribus haberem tuis, libenter mitterem: sed tanta inchoata res est, ut paene vitio mentis tantum opus ingressus mihi videar, cum praesertim, ut scis, alia quoque studia ad id opus multoque potiora inpertiar. Nec his Virgilii verbis copia rerum dissonat, quam plerique omnes litteratores pedibus inlotis praetereunt, tamquam nihil ultra verborum explanationem liceat nosse grammatico.