Phaedrus 273c7-e3

Socrates: Gosh! Tisias, or someone else, whoever he is, and whatever he likes to be named after, has discovered a skill that has been most ingeniously hidden! Yet, my friend, there is something we should say to him, or perhaps not …

Phaedrus: What?

Soc: This: “Dear Tisias, a while ago, before you joined us, we happened to be saying that this notion of likelihood tends to arise in most people because of resemblances to the truth. But we explained earlier that the person who knows the truth is best equipped in every respect to discover the resemblances. So if you say anything else about the skill of speaking we shall listen, but if not, we shall believe what we have already explained; that unless someone enumerates the various natures among his prospective audience and distinguishes things that are on the basis of forms, and unless he is capable of comprehending each particular through a single characteristic, he will never be skilled at speaking to the extent that a human being can be. But these abilities will never be acquired without a great deal of effort which a wise man should undertake, not for purposes of speaking and acting towards his fellow men, but in order to be capable of speaking what is acceptable to the gods, and of acting to his utmost capacity in a manner acceptable to them.

Translation: David Horan