“The correct analogy for the mind is not a vessel that needs filling, but wood that needs igniting — no more — and then it motivates one towards originality and instills the desire for truth. Suppose someone were to go and ask his neighbors for fire and find a substantial blaze there, and just stay there continually warming himself: that is no different from someone who goes to someone else to get to some of his rationality, and fails to realize that he ought to ignite his own flame, his own intellect, but is happy to sit entranced by the lecture, and the words trigger only associative thinking and bring, as it were, only a flush to his cheeks and a glow to his limbs; but he has not dispelled or dispersed, in the warm light of philosophy, the internal dank gloom of his mind.”

οὐ γὰρ ὡς ἀγγεῖον ὁ νοῦς ἀποπληρώσεως ἀλλ' ὑπεκκαύματος μόνον ὥσπερ ὕλη δεῖται ὁρμὴν ἐμποιοῦντος εὑρετικὴν καὶ ὄρεξιν ἐπὶ τὴν ἀλήθειαν. ὥσπερ οὖν εἴ τις ἐκ γειτόνων πυρὸς δεόμενος, εἶτα πολὺ καὶ λαμπρὸν εὑρὼν αὐτοῦ καταμένοι διὰ τέλους θαλπόμενος, οὕτως εἴ τις ἥκων λόγου μεταλαβεῖν πρὸς ἄλλον οὐχ οἴεται δεῖν φῶς οἰκεῖον ἐξάπτειν καὶ νοῦν ἴδιον, ἀλλὰ χαίρων τῇ ἀκροάσει κάθηται θελγόμενος, οἷον ἔρευθος ἕλκει καὶ γάνωμα τὴν δόξαν ἀπὸ τῶν λόγων, τὸν δ᾽ ἐντὸς: εὐρῶτα τῆς ψυχῆς καὶ ζόφον οὐκ ἐκτεθέρμαγκεν οὐδ᾽ ἐξέωκε διὰ φιλοσοφίας.
On Listening to Lectures, Plutarch, Moralia 48C (variously called De auditione Philosophorum or De Auditu or De Recta Audiendi Ratione)
Moralia, Others