On Fugitives (199-201)

But the wicked having fled away, and having passed their time without ever tasting the draught of immortality, have digged, insane persons that they are, for themselves, and not first for God, having preferred their own actions to the heavenly and celestial things, and the things which proceed from care to those which are spontaneous and ready.

Then they dig, not as the wise men Abraham and Isaac did, making wells, but cisterns, which have no good nutritious stream belonging to and proceeding from themselves, but requiring an influx from without, which must proceed from instruction. While the teachers are always pouring into the ears of their disciples all kinds of doctrines and speculations of science altogether, admonishing them to retain them in their minds, and to preserve them when faithfully committed to memory.

But now they are but worn-out cisterns, that is to say, all the channels of the ill-educated soul are broken and leaky, not being able to hold and to preserve the influx of those streams which are able to profit.