It is on account neither of God's weakness nor ignorance that evil comes into the world, but rather it is due to the order of his wisdom and the greatness of his goodness that diverse grades of goodness occur in things, many of which would be lacking if no evil were permitted. Indeed, the good of patience would not exist without the evil of persecution; nor the good of preservation of life in a lion if not for the evil of the destruction of the animals on which it lives.

De Potentia (On Power) q. 3, art. 6, ad 4