Summa contra Gentiles3.13 Chapter XIII. That evil has a cause of some kind

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

For whatever is in a thing as its subject, must needs 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.! Therefore evil must have a cause, Again. That which is in potentiality to either of two opposites, is not actuated by either except by some cause: for no potentiality actuates itself. Now evil is the privation of that which is connatural and due to anyone:? since it is on account of this that a thing is said to be evil. Therefore evil is in a subject that is in potentiality to evil and its opposite. Therefore evil must have some cause.

Moreover. Whatever isin a thing in addition to its nature, supervenes through some other cause; for whatever is natural to it is permanent, unless something else be in the way: wherefore a stone is not borne upwards, unless someone throw it, and water is not heated unless something make it hot. Now evil, in whatever subject it be, 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 per se or accidental.

Further. Every evil is consequent to some good;? thus corruption is consequent to generation. Now every good has a cause, except the Sovereign Good, in which there is no evil, as proved in the First Book. Therefore every evil has. a cause, from which it results accidentally.