Such doubt would inevitably undermine a sense of faith or trust in the basic goodness of life and the world and would thereby legitimize cynicism and moral skepticism. To say that something exists is to say that it possesses a definable nature. Whatever has a definable nature possesses form, order, harmony. Whatever possesses form, order, harmony is good. Whatever exists, i.e., has a definable nature, is good.
Corruptive evil consists in a deprivation of form, order and harmony.
Deprivation of form, order and harmony can exist only as long as a thing having form, order and harmony exists. Therefore, corruptive evil, as a deprivation of form, order and harmony, exists in a thing only as a lesser degree of goodness and being than the thing might otherwise possess. Evil is nothing in itself, but is only a possible state of a thing that is in itself good, but not supremely and unchangeably good. By showing that corruptive evil lacks positive reality, Augustine shows that all that exists is good, even though not supremely and unchangeably good. If all that exists is good, Augustine can affirm God as both good and all-powerful.