Nicene and Post-Nicene Fathers, Vol. IV:
Writings in Connection with the Manichæan Controversy.: Chapter 6

Chapter 6.—What Corruption Affects and What It is.

8.  What further does the Catholic light say?  What do you suppose, but what is the actual truth, that it is the created substance which can be corrupted, for the uncreated, which is the chief good, is incorruptible; and corruption, which is the chief evil, cannot be corrupted; besides, that it is not a substance?  But if you ask what corruption is, consider to what it seeks to bring the things which it corrupts; for it affects those things according to its own nature.  Now all things by corruption fall away from what they were, and are brought to non-continuance, to non-existence; for existence implies continuance.  Thus the supreme and chief existence is so called because it continues in itself, or is self-contained.  In the case of a thing changing for the better, the change is not from continuance, but from perversion to the worse, that is, from falling away from essence; the author of which falling away is not He who is the author of the essence.  So in some things there is change for the better, and so a tendency towards existence.  And this change is not called a perversion, but reversion or conversion; for perversion is opposed to orderly arrangement.  Now things which tend towards existence tend towards order, and, attaining order they attain existence, as far as that is possible to a creature.  For order reduces to a certain uniformity that which it arranges; and existence is nothing else than being one.  Thus, so far as anything acquires unity, so far it exists.  For uniformity and harmony are the effects of unity, and by these compound things exist as far as they have existence.  For simple things exist by themselves, for they are one.  But things not simple imitate unity by the agreement of their parts; and so far as they attain this, so far they exist.  This arrangement is the cause of existence, disorder of non-existence; and perversion or corruption are the other names for disorder.  So whatever is corrupted tends to non-existence.  You may now be left to reflect upon the effect of corruption, that you may discover what is the chief evil; for it is that which corruption aims at accomplishing.