If then evil is caused by anything, what causes it must be good.
Every cause is either material, formal, efficient, or final.
In like manner evil cannot be an efficient cause, since everything acts according as it is actually and has a form. But since good and evil are opposites, and one opposite cannot be cause of another except accidentally, it follows that good cannot be the active cause of evil except accidentally. In physics, this accident may happen either on the part of the agent or on the part of the effect. On the part of the agent, when the agent suffers from a lack of power, whence it follows that the action is defective and the effect deficient. On the part of the effect evil is caused accidentally as well in respect of the matter of the effect as also in respect of its form.