Nicene and Post-Nicene Fathers, Vol. IV:
Writings in Connection with the Manichæan Controversy.: Chapter 35

Chapter 35.—Evil Alone is Corruption.  Corruption is Not Nature, But Contrary to Nature.  Corruption Implies Previous Good.

39.  For who can doubt that the whole of that which is called evil is nothing else than corruption?  Different evils may, indeed, be called by different names; but that which is the evil of all things in which any evil is perceptible is corruption.  So the corruption of an educated mind is ignorance; the corruption of a prudent mind is imprudence; the corruption of a just mind, injustice; the corruption of a brave mind, cowardice; the corruption of a calm, peaceful mind, cupidity, fear, sorrow, pride.  Again, in a living body, the corruption of health is pain and disease; the corruption of strength is exhaustion; the corruption of rest is toil.  Again, in any corporeal thing, the corruption of beauty is ugliness; the corruption of straightness is crookedness; the corruption of order is confusion; the corruption of entireness is disseverance, or fracture, or diminution.  It would be long and laborious to mention by name all the corruptions of the things here mentioned, and of countless other things; for in many cases the words may apply to the mind as well as to the body, and in innumerable cases the corruption has a distinct name of its own.  But enough has been said to show that corruption does harm only as displacing the natural condition; and so, that corruption is not nature, but against nature.  And if corruption is the only evil to be found anywhere, and if corruption is not nature, no nature is evil.

40.  But if, perchance, you cannot follow this, consider again, that whatever is corrupted is deprived of some good:  for if it were not corrupted, it would be incorrupt; or if it could not in any way be corrupted, it would be incorruptible.  Now, if corruption is an evil, both incorruption and incorruptibility must be good things.  We are not, however, speaking at present of incorruptible nature, but of things which admit of corruption, and which, while not corrupted, may be called incorrupt, but not incorruptible.  That alone can be called incorruptible which not only is not corrupted, but also cannot in any part be corrupted.  Whatever things, then, being incorrupt, but liable to corruption, begin to be corrupted, are deprived of the good which they had as incorrupt.  Nor is this a slight good, for corruption is a great evil.  And the continued increase of corruption implies the continued presence of good, of which they may be deprived.  Accordingly, the natures supposed to exist in the region of darkness must have been either corruptible or incorruptible.  If they were incorruptible, they were in possession of a good than which nothing is higher.  If they were corruptible, they were either corrupted or not corrupted.  If they were not corrupted, they were incorrupt, to say which of anything is to give it great praise.  If they were corrupted, they were deprived of this great good of incorruption; but the deprivation implies the previous possession of the good they are deprived of; and if they possessed this good, they were not the perfection of evil, and consequently all the Manichæan story is a falsehood.