14 Do you dare to laugh at us when we speak of hell, and fires which cannot be quenched, into which we have learned that souls are cast by their foes and enemies? What, does not your Plato also, in the book which he wrote on the immortality of the soul, name the rivers Acheron, Styx, Cocytus, and Pyriphlegethon, and assert that in them souls are rolled along, engulphed, and burned up? But though a man of no little wisdom, and of accurate judgment and discernment, he essays a problem which cannot be solved; so that, while he says that the soul is immortal, everlasting, and without bodily substance, he vet says that they are punished, and makes them suffer pain. But what man does not see that that which is immortal, which is simple, cannot be subject to any pain; that that, on the contrary, cannot be immortal which does suffer pain?
And yet his opinion is not very far from the truth. For although the gentle and kindly disposed man thought it inhuman cruelty to condemn souls to death, he yet not unreasonably supposed that they are cast into rivers blazing with masses of flame, and loathsome from their foul abysses. For they are cast in, and being annihilated, pass away vainly in everlasting destruction. For theirs is an intermediate state, as has been learned from Christ's teaching; and they are such that they may on the one hand perish if they have not known God, and on the other be delivered from death if they have given heed to His threats and proffered favours.
And to make manifest what is unknown, this is man's real death, this which leaves nothing behind. For that which is seen by the eyes is only a separation of soul from body, not the last end— annihilation: this, I say, is man's real death, when souls which know not God shall be consumed in long-protracted torment with raging fire, into which certain fiercely cruel beings shall cast them, who were unknown before Christ, and brought to light only by His wisdom.
Source: Against the Heathen (New Advent)