Nature, in as Far as It is Nature, No Evil
No nature, therefore, as far as it is nature, is evil; but to each nature there is no evil except to be diminished in respect of good. But if by being diminished it should be consumed so that there is no good, no nature would be left; not only such as the Manichæans introduce, where so great good things are found that their exceeding blindness is wonderful, but such as any one can introduce.
Source: Concerning the Nature of Good, Against the Manichaeans (New Advent)