That Evil is an Accidental Cause

Running through all the species of causes, we find that evil is a cause accidentally. In the species of efficient cause, since through the deficiency of power in the active cause there follows defect in the effect and action. In the species of material cause, since through the indisposition of the matter there follows a defect in the product. In the species of formal cause, since to one form there is always annexed the privation of another form. In the species of final cause, since the evil annexed to an undue end means the hindering of the end that is due.*

3.12 : That Evil does not entirely swallow up Good
3.15 : That there is not any Sovereign Evil, acting as the Principle of all Evils