"To rear a child and give him an education is easy, but to put a good mind into him is impossible... By teaching you will never make the bad man good." — Theognis of Megara (Elegies, lines 429–438) (quoted by both Plato in Meno and Aristotle in Nicomachean Ethics)

Or:

"Naturam expellas furca, tamen usque recurret." ("You may drive out Nature with a pitchfork, yet she will always return.") — Epistles, Book I