Macrobius, Saturnalia 1.24.11-12 (Full Latin text on LacusCurtius)
“The letter of Vergil, in which he addresses the emperor Augustus, begins thus: ‘I have indeed received many letters from you.’ Further down, he writes, ‘I would gladly send you something from my Aeneid if I had something worthy of your ears; yet, I have begun such a massive work that I think that I may have been out of my mind by starting it, especially since (as you know) I must apply much other, more serious learning to it.’ The abundance of material in his poem supports these words of Vergil, but almost all of our teachers trample over it with unwashed feet, as though a professor should know nothing more than the explanation of individual words. In this way, those literary swells have placed certain limits or even sacred boundaries upon learning, and have settled that anyone who dares to cross them should be condemned as though he looked into the temple of a goddess from which men are banned.”
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. Ita sibi belli isti homines certos scientiae fines et velut quaedam pomeria et effata posuerunt, ultra quae si quis egredi audeat, introspexisse in aedem deae a qua mares absterrentur existimandus sit.