In keeping with prior authority in the church, Augustine maintains the rigid distinction between creature and Creator that is demanded by the terms of definition one finds in creationist ideology, namely that God is All-Powerful and man is certainly less than that. Exactly how and why this perception of reality matters becomes clear as Augustine pursues his refutation of the Manichaeans. In Chapter 6, for instance, he discusses the concept of corruption by arguing that only a good thing can actually be corrupted, not an evil one, because he defines evil as an absence of the good.
Corruption is anything that diminishes the good.
Because evil does not contain any element of the good in the first place, it cannot be corrupted since it would be impossible to diminish evil by removing from it that which it does not possess from the beginning. He then qualifies this assertion, in Chapter 7, by noting that God does not allow anything He has created from becoming so corrupt that it ceases to exist altogether. Thus, when rational souls fall away from God, although they possess the greatest amount of free-will, He ranks them in the lower grades of creation, where their proper place is. So they suffer misery by the divine judgment, while they are ranked suitably to their deserts.