7 Hellfire, moreover, and the torments with which holy scripture threatens sinners he explains not as external punishments but as the pangs of guilty consciences when by God's power the memory of our transgressions is set before our eyes. “The whole crop of our sins grows up afresh from seeds which remain in the soul, and all our dishonourable and undutiful acts are again pictured before our gaze. Thus it is the fire of conscience and the stings of remorse which torture the mind as it looks back on former self-indulgence.”
And again: “but perhaps this coarse and earthly body ought to be described as mist and darkness; for at the end of this world and when it becomes necessary to pass into another, the like darkness will lead to the like physical birth.” In speaking thus he clearly pleads for the transmigration of souls as taught by Pythagoras and Plato. And at the end of the second book in dealing with our perfection he has said: when we shall have made such progress as not only to cease to be flesh or body but perhaps also to cease to be souls our perfect intelligence and perception, undimmed with any mist of passion, will discern reasonable and intelligible substances face to face.
Source: Letters (New Advent)