Petrarch on the proper way to imitate the Ancient writers, so as to bring to your own age from the writers of old somethign new:
An imitator must see to it that what he writes is similar, but not the very same; and the similarity, moreover, should be not like that of a painting or statue to the person represented, but rather like that of a son to a father, where there is often great difference in the features and members, and yet after all there is a shadowy something,—akin to what our painters call one’s air,—hovering about the face, and especially the eyes, out of which there grows a likeness that immediately, upon our beholding the child, calls the father up before us. If it were a matter of measurement every detail would be found to be different, and yet there certainly is some subtle presence there that has this effect.
In much the same way we writers, too, must see to it that along with the similarity there is a large measure of dissimilarity; and furthermore such likeness as there is must be elusive, something that it is impossible to seize except by a sort of still-hunt, a quality to be felt rather than defined. In brief, we may appropriate another’s thought, and may even copy the very colours’ of his style, but we must abstain from borrowing his actual words. The resemblance in the one case is hidden away below the surface; in the other it stares the reader in the face. The one kind of imitation makes poets; the other—apes. It may all be summed up by saying with Seneca, and with Flaccus before him, that we must write just as the bees make honey, not keeping the flowers but turning them into a sweetness of our own, blending many very different flavours into one, which shall be unlike them all, and better. [Source]
Interestingly, he goes on to feign astonishment that someone has found in his own works direct citations of Ancient writers. Just an oversight, really. One must only to borrow the words of Ancient writers, not steal them. A little humor there from “the father of the Renaissance.”