Objection 1: It seems that the good cannot be the cause of evil. For it is written (Mt 7:18): “A good tree cannot bear bad fruit.”
Objection 2: Further, one contrary cannot be the cause of another. But evil is the contrary of good. Therefore, good cannot be the cause of evil.
Objection 3: Further, a deficient effect can only arise from a deficient cause. But evil is a deficient effect. Therefore, if it has a cause, its cause is deficient. But everything deficient is an evil. Therefore, the cause of evil can only be an evil.
Objection 4: Further, Dionysius says (Div. Nom. iv) that evil has no cause. Therefore, good is not the cause of evil.
On the contrary, Augustine says (Contra Julian. i, 9): “There is no possible origin of evil except from the good.”
I answer that it must be said that every evil has a cause in some way. For evil is the absence of good, which is natural and due to a thing. But that something falls away from its natural and due disposition can only arise from a cause that draws it away from its own disposition. For a heavy thing is not moved upward except by a driving force; nor does an agent fail in its action except through an impediment. But only the good can be a cause; for nothing can be a cause except insofar as it exists, and every being as such is good.
And if we consider the specific kinds of causes, we see that the efficient cause, the form, and the end all involve a kind of perfection that belongs to the notion of the good. Even matter, as potency to the good, has the nature of the good. Now, that the good is the cause of evil in the manner of the material cause has been shown above (Question [48], Article [3]). For it was shown that the good is the subject of evil. However, evil has no formal cause, but is rather a privation of form; likewise, it has no final cause, but is rather a privation of ordination to its own end; since not only the end has the nature of the good, but also the useful, which is ordered to the end. However, evil has a cause in the manner of the efficient cause, not directly, but accidentally.
To prove this, it must be known that evil is caused in action differently than in effect. In action, evil is caused by the deficiency of a principle of action, either of the principal or the instrumental agent; thus, a deficiency in the movement of an animal can occur due to weakness of the moving power, as in the case of children, or only due to the unfitness of the instrument, as in the lame. On the other hand, evil is caused in a thing, but not in the proper effect of the agent, sometimes by the power of the agent, sometimes by a deficiency, either of the agent or of the matter. It is caused by the power or perfection of the agent when, upon the form intended by the agent, the privation of another form necessarily follows; as, for example, when upon the form of fire follows the privation of the form of air or water. Thus, the more perfect fire is in its strength, the more perfectly it impresses its own form, and the more perfectly it destroys the contrary. Thus, the evil and destruction that befall air and water come from the perfection of fire: but this is accidental; for fire does not aim at the privation of the form of water, but at the introduction of its own form, although it thereby accidentally causes the other. But if there is a deficiency in the proper effect of fire—for example, that it fails to heat—this comes either from a deficiency of action, which implies a deficiency of a principle, as said above, or from the indisposition of the matter, which does not receive the action of fire, the agent. But the very fact that it is a deficient being is accidental to the good, to which it belongs in itself to act. Therefore, it is true that evil in no way has any other than an accidental cause; and thus, the good is the cause of evil.
Reply to Objection 1: As Augustine says (Contra Julian. i): “The Lord calls an evil will the bad tree and a good will the good tree.” Now, a good will does not bring forth a morally bad action, since a moral action is judged good from the good will itself. Nevertheless, the movement of an evil will is itself caused by the rational creature, which is good; and thus, the good is the cause of evil.
Reply to Objection 2: The good does not cause that evil which is contrary to itself, but another evil: thus, the goodness of fire causes an evil for water, and man, who is good by nature, causes a morally bad action. And as explained above (Question [19], Article [9]), this happens accidentally. Moreover, it sometimes happens that one contrary accidentally causes the other: for example, external surrounding cold warms (the body) by concentrating the internal heat.
Reply to Objection 3: Evil has a deficient cause in voluntary things in a different way than in natural things. For the natural agent produces the same kind of effect as it is itself, unless it is hindered by something external; and this comes down to some deficiency belonging to it. Therefore, evil never follows in the effect unless another evil already exists in the agent or in the matter, as said above. But in voluntary things, the deficiency of action comes from the actually deficient will, insofar as it does not actually subject itself to its own rule. However, this deficiency is not guilt, but guilt follows from the fact that the will acts with this deficiency.
Reply to Objection 4: Evil has no direct cause, but only an accidental cause, as said above.