Chapter 13 — That evil has a cause of some kind

It can be shown from what precedes that, although evil has no essential cause, yet every evil must have an accidental cause.

For whatever is in a thing as its subject must have a cause, since it results either from the principles of the subject, or from some external cause. Now evil is in good as its subject, as shown above. (Ch. 11.) Therefore, evil must have a cause.

Again. That which is in potency to either of two opposites is not actuated by either except by some cause, for no potency actuates itself. Now evil is the privation of that which is connatural and due to anyone, (Ch. 7.) since it is on account of this that a thing is said to be evil. Therefore, evil is in a subject that is in potency to evil and its opposite. Therefore, evil must have some cause.

Moreover. Whatever is in a thing in addition to its nature supervenes through some other cause, for whatever is natural to it is permanent unless something else is in the way. Hence a stone is not borne upwards unless someone throws it, and water is not heated unless something makes it hot. Now evil, in whatever subject it is, is always there in addition to the nature of that subject, since it is the privation of what is connatural and due to a thing. Therefore, evil must always have a cause, either essential or accidental.

Further. Every evil is consequent to some good; (Ch. 10.) thus corruption is consequent to generation. Now every good has a cause, except the highest good, in which there is no evil, as proved in the first book. (Ch. 39.) Therefore, every evil has a cause from which it results accidentally.