Nicene and Post-Nicene Fathers: Series I/Volume IV/Manichaean Controversy/On the Morals of the Manichaeans/Chapter 5
Chapter 5.—If Evil is Defined to Be Corruption, This Completely Refutes the Manichæan Heresy.
7. I ask a third time, What is evil? Perhaps you will reply, Corruption. Undeniably this is a general definition of evil; for corruption implies opposition to nature, and also hurt. But corruption exists not by itself, but in some substance which it corrupts; for corruption itself is not a substance. So the thing which it corrupts is not corruption, is not evil; for what is corrupted suffers the loss of integrity and purity. So that which has no purity to lose cannot be corrupted; and what has, is necessarily good by the participation of purity. Again, what is corrupted is perverted; and what is perverted suffers the loss of order, and order is good. To be corrupted, then, does not imply the absence of good; for in corruption it can be deprived of good, which could not be if there was the absence of good. Therefore that race of darkness, if it was destitute of all good, as you say it was, could not be corrupted, for it had nothing which corruption could take from it; and if corruption takes nothing away, it does not corrupt. Say now, if you dare, that God and the kingdom of God can be corrupted, when you cannot show how the kingdom of the devil, such as you make it, can be corrupted.