Imagine, then, that a man should need to get fire from a neighbour, and, upon finding a big bright fire there, should stay there continually warming himself; just so it is if a man comes to another to share the benefit of a discourse, and does not think it necessary to kindle from it some illumination for himself and some thinking of his own, but, delighting in the discourse, sits enchanted; he gets, as it were, a bright and ruddy glow in the form of opinion imparted to him by what is said, but the mouldiness and darkness of his inner mind he has not dissipated nor banished by the warm glow of philosophy.

Plutarch, "De Auditu" (On Listening), a chapter of his Moralia.

This essay is also the original source of the much-quoted line "The mind is not a pot to be filled, but a fire to be ignited." It is variously attributed, but is a fair distillation of the original passage, which comes directly before the quote above:

For the mind does not require filling like a bottle, but rather, like wood, it only requires kindling to create in it an impulse to think independently and an ardent desire for the truth.