Amos 3:6

As it is impossible for two people to share a journey at the same time, he is saying, unless indicating to each other where and why they are traveling, or for a lion to roar if there is no prey, or for a bird to fall without a hunter, or for all the other things mentioned, so it is impossible for any punishment to be imposed without God willing it. He calls punishment "evil," note, by use of a general custom: we are accustomed to use "troubles" of diseases, chastisements, untimely deaths, famines, wars, and the like, not because they are troublesome by nature but because they are troublesome to human beings and the source of distress and grief.

I do not say these things in arrogance, but I have the prophet Amos standing at my side, crying and saying, "There is no evil in the city which the Lord has not done." Now evil is a many-faceted term. I wish that you shall learn the exact meaning of each expression, in order that on account of ambiguity you may not confound the nature of the things and fall into blasphemy. There is then evil, which is really evil; fornication, adultery, covetousness, and the countless dreadful things, which are worthy of the utmost reproach and punishment. Again there is evil, which rather is not evil but is called so, famine, pestilence, death, disease, and other of a similar nature. For these would not be evils. On this account I said they are called so only. Why then? Because, were they evils intended to become the sources of good to us, chastening our pride, goading our sloth and leading us on to zeal, making us more attentive.

It is, then, customary for sacred Scripture to speak of his permission as an action and deed, but even when it goes so far as to say that God "creates evil" and that "there is not evil in a city which the Lord has not done," it still does not show God to be the author of evil. On the contrary, since the word evil is ambiguous it has two meanings, for it sometimes means what is by nature evil, being the opposite of virtue and against God's will, while at other times it means what is evil and painful in relation to our sensibility, which is to say, tribulation and distress. Now while these last seem to be evil, because they cause pain, actually they are good because to such as understand them they are a source of conversion and salvation. It is these last that Scripture says are permitted by God. Moreover, one must know that we too cause them because involuntary evils spring from voluntary ones.