The Enchiridion by St. Augustine
Chapter 11.--What is Called Evil in the Universe is But the Absence of Good.
And in the universe, even that which is called evil, when it is regulated and put in its own place, only enhances our admiration of the good; for we enjoy and value the good more when we compare it with the evil. For the Almighty God, who, as even the heathen acknowledge, has supreme power over all things, being Himself supremely good, would never permit the existence of anything evil among His works, if He were not so omnipotent and good that He can bring good even out of evil. For what is that which we call evil but the absence of good? In the bodies of animals, disease and wounds mean nothing but the absence of health; for when a cure is effected, that does not mean that the evils which were present -- namely, the diseases and wounds -- go away from the body and dwell elsewhere: they altogether cease to exist; for the wound or disease is not a substance, but a defect in the fleshly substance, -- the flesh itself being a substance, and therefore something good, of which those evils -- that is, privations of the good which we call health -- are accidents. Just in the same way, what are called vices in the soul are nothing but privations of natural good. And when they are cured, they are not transferred elsewhere: when they cease to exist in the healthy soul, they cannot exist anywhere else.