Summa Theologica, by St. Thomas Aquinas, [1947], at sacred-texts.com
We next inquire into the cause of evil. Concerning this there are three points of inquire:
(1) Whether good can be the cause of evil?
(2) Whether the supreme good, God, is the cause of evil?
(3) Whether there be any supreme evil, which is the first cause of all evils?
Objection 1: It would seem that good cannot be the cause of evil. For it is said (Mat. 7:18): "A good tree cannot bring forth evil fruit."
Objection 2: Further, one contrary cannot be the cause of another. But evil is the contrary to good. Therefore good cannot be the cause of evil.
Objection 3: Further, a deficient effect can proceed only from a deficient cause. But evil is a deficient effect. Therefore its cause, if it has one, is deficient. But everything deficient is an evil. Therefore the cause of evil can only be 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 source of evil except good."
I answer that, It must be said that every evil in some way has a cause. For evil is the absence of the good, which is natural and due to a thing. But that anything fail from its natural and due disposition can come only from some cause drawing it out of its proper disposition. For a heavy thing is not moved upwards except by some impelling force; nor does an agent fail in its action except from some impediment. But only good can be a cause; because nothing can be a cause except inasmuch as it is a being, and every being, as such, is good.
And if we consider the special kinds of causes, we see that the agent, the form, and the end, import some kind of perfection which belongs to the notion of good. Even matter, as a potentiality to good, has the nature of good. Now that good is the cause of evil by way of the material cause was shown above (Q[48], A[3]). For it was shown that good is the subject of evil. But evil has no formal cause, rather is it a privation of form; likewise, neither has it a final cause, but rather is it a privation of order to the proper end; since not only the end has the nature of good, but also the useful, which is ordered to the end. Evil, however, has a cause by way of an agent, not directly, but accidentally.
In proof of this, we must know that evil is caused in the action otherwise than in the effect. In the action evil is caused by reason of the defect of some principle of action, either of the principal or the instrumental agent; thus the defect in the movement of an animal may happen by reason of the weakness of the motive power, as in the case of children, or by reason only of the ineptitude 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 reason of a defect, either of the agent or of the matter. It is caused by reason of the power or perfection of the agent when there necessarily follows on the form intended by the agent the privation of another form; as, for instance, when on the form of fire there follows the privation of the form of air or of water. Therefore, as the more perfect the fire is in strength, so much the more perfectly does it impress its own form, so also the more perfectly does it corrupt the contrary. Hence that evil and corruption befall air and water comes from the perfection of the fire: but this is accidental; because fire does not aim at the privation of the form of water, but at the bringing in of its own form, though by doing this it also accidentally causes the other. But if there is a defect in the proper effect of the fire---as, for instance, that it fails to heat---this comes either by defect of the action, which implies the defect of some principle, as was said above, or by the indisposition of the matter, which does not receive the action of the fire, the agent. But this very fact that it is a deficient being is accidental to good to which of itself it belongs to act. Hence it is true that evil in no way has any but an accidental cause; and thus is good the cause of evil.
Reply to Objection 1: As Augustine says (Contra Julian. i): "The Lord calls an evil will the evil tree, and a good will a good tree." Now, a good will does not produce a morally bad act, since it is from the good will itself that a moral act is judged to be good. Nevertheless the movement itself of an evil will is caused by the rational creature, which is good; and thus good is the cause of evil.
Reply to Objection 2: Good does not cause that evil which is contrary to itself, but some other evil: thus the goodness of the fire causes evil to the water, and man, good as to his nature, causes an act morally evil. And, as explained above (Q[19], A[9]), this is by accident. Moreover, it does happen sometimes that one contrary causes another by accident: for instance, the exterior surrounding cold heats (the body) through the concentration of the inward heat.
Reply to Objection 3: Evil has a deficient cause in voluntary things otherwise than in natural things. For the natural agent produces the same kind of effect as it is itself, unless it is impeded by some exterior thing; and this amounts to some defect belonging to it. Hence evil never follows in the effect, unless some other evil pre-exists in the agent or in the matter, as was said above. But in voluntary things the defect of the action comes from the will actually deficient, inasmuch as it does not actually subject itself to its proper rule. This defect, however, is not a fault, but fault follows upon it from the fact that the will acts with this defect.
Reply to Objection 4: Evil has no direct cause, but only an accidental cause, as was said above.
Objection 1: It would seem that the supreme good, God, is the cause of evil. For it is said (Isa. 45:5, 7): "I am the Lord, and there is no other God, forming the light, and creating darkness, making peace, and creating evil." And Amos 3:6, "Shall there be evil in a city, which the Lord hath not done?"
Objection 2: Further, the effect of the secondary cause is reduced to the first cause. But good is the cause of evil, as was said above (A[1]). Therefore, since God is the cause of every good, as was shown above (Q[2] , A[3]; Q[6], AA[1],4), it follows that also every evil is from God.
Objection 3: Further, as is said by the Philosopher (Phys. ii, text 30), the cause of both safety and danger of the ship is the same. But God is the cause of the safety of all things. Therefore He is the cause of all perdition and of all evil.
On the contrary, Augustine says (QQ. 83, qu. 21), that, "God is not the author of evil because He is not the cause of tending to not-being."
I answer that, As appears from what was said (A[1]), the evil which consists in the defect of action is always caused by the defect of the agent. But in God there is no defect, but the highest perfection, as was shown above (Q[4], A[1]). Hence, the evil which consists in defect of action, or which is caused by defect of the agent, is not reduced to God as to its cause.
But the evil which consists in the corruption of some things is reduced to God as the cause. And this appears as regards both natural things and voluntary things. For it was said (A[1]) that some agent inasmuch as it produces by its power a form to which follows corruption and defect, causes by its power that corruption and defect. But it is manifest that the form which God chiefly intends in things created is the good of the order of the universe. Now, the order of the universe requires, as was said above (Q[22], A[2], ad 2; Q[48], A[2]), that there should be some things that can, and do sometimes, fail. And thus God, by causing in things the good of the order of the universe, consequently and as it were by accident, causes the corruptions of things, according to 1 2:6: "The Lord killeth and maketh alive." But when we read that "God hath not made death" (Wis. 1:13), the sense is that God does not will death for its own sake. Nevertheless the order of justice belongs to the order of the universe; and this requires that penalty should be dealt out to sinners. And so God is the author of the evil which is penalty, but not of the evil which is fault, by reason of what is said above.
Reply to Objection 1: These passages refer to the evil of penalty, and not to the evil of fault.
Reply to Objection 2: The effect of the deficient secondary cause is reduced to the first non-deficient cause as regards what it has of being and perfection, but not as regards what it has of defect; just as whatever there is of motion in the act of limping is caused by the motive power, whereas what there is of obliqueness in it does not come from the motive power, but from the curvature of the leg. And, likewise, whatever there is of being and action in a bad action, is reduced to God as the cause; whereas whatever defect is in it is not caused by God, but by the deficient secondary cause.
Reply to Objection 3: The sinking of a ship is attributed to the sailor as the cause, from the fact that he does not fulfil what the safety of the ship requires; but God does not fail in doing what is necessary for the safety of all. Hence there is no parity.
Objection 1: It would seem that there is one supreme evil which is the cause of every evil. For contrary effects have contrary causes. But contrariety is found in things, according to Ecclus. 33:15: "Good is set against evil, and life against death; so also is the sinner against a just man." Therefore there are many contrary principles, one of good, the other of evil.
Objection 2: Further, if one contrary is in nature, so is the other. But the supreme good is in nature, and is the cause of every good, as was shown above (Q[2], A[3]; Q[6], AA[2],4). Therefore, also, there is a supreme evil opposed to it as the cause of every evil.
Objection 3: Further, as we find good and better things, so we find evil and worse. But good and better are so considered in relation to what is best. Therefore evil and worse are so considered in relation to some supreme evil.
Objection 4: Further, everything participated is reduced to what is essential. But things which are evil among us are evil not essentially, but by participation. Therefore we must seek for some supreme essential evil, which is the cause of every evil.
Objection 5: Further, whatever is accidental is reduced to that which is "per se." But good is the accidental cause of evil. Therefore, we must suppose some supreme evil which is the "per se" cause of evils. Nor can it be said that evil has no "per se" cause, but only an accidental cause; for it would then follow that evil would not exist in the many, but only in the few.
Objection 6: Further, the evil of the effect is reduced to the evil of the cause; because the deficient effect comes from the deficient cause, as was said above (AA[1],2). But we cannot proceed to infinity in this matter. Therefore, we must suppose one first evil as the cause of every evil.
On the contrary, The supreme good is the cause of every being, as was shown above (Q[2], A[3]; Q[6], A[4]). Therefore there cannot be any principle opposed to it as the cause of evils.
I answer that, It appears from what precedes that there is no one first principle of evil, as there is one first principle of good.
First, indeed, because the first principle of good is essentially good, as was shown above (Q[6], AA[3],4). But nothing can be essentially bad. For it was shown above that every being, as such, is good (Q[5], A[3]); and that evil can exist only in good as in its subject (Q[48], A[3]).
Secondly, because the first principle of good is the highest and perfect good which pre-contains in itself all goodness, as shown above (Q[6], A[2]). But there cannot be a supreme evil; because, as was shown above (Q[48], A[4]), although evil always lessens good, yet it never wholly consumes it; and thus, while good ever remains, nothing can be wholly and perfectly bad. Therefore, the Philosopher says (Ethic. iv, 5) that "if the wholly evil could be, it would destroy itself"; because all good being destroyed (which it need be for something to be wholly evil), evil itself would be taken away, since its subject is good.
Thirdly, because the very nature of evil is against the idea of a first principle; both because every evil is caused by good, as was shown above (A[1]), and because evil can be only an accidental cause, and thus it cannot be the first cause, for the accidental cause is subsequent to the direct cause.
Those, however, who upheld two first principles, one good and the other evil, fell into this error from the same cause, whence also arose other strange notions of the ancients; namely, because they failed to consider the universal cause of all being, and considered only the particular causes of particular effects. For on that account, if they found a thing hurtful to something by the power of its own nature, they thought that the very nature of that thing was evil; as, for instance, if one should say that the nature of fire was evil because it burnt the house of a poor man. The judgment, however, of the goodness of anything does not depend upon its order to any particular thing, but rather upon what it is in itself, and on its order to the whole universe, wherein every part has its own perfectly ordered place, as was said above (Q[47], A[2], ad 1).
Likewise, because they found two contrary particular causes of two contrary particular effects, they did not know how to reduce these contrary particular causes to the universal common cause; and therefore they extended the contrariety of causes even to the first principles. But since all contraries agree in something common, it is necessary to search for one common cause for them above their own contrary proper causes; as above the contrary qualities of the elements exists the power of a heavenly body; and above all things that exist, no matter how, there exists one first principle of being, as was shown above (Q[2], A[3]).
Reply to Objection 1: Contraries agree in one genus, and they also agree in the nature of being; and therefore, although they have contrary particular cause, nevertheless we must come at last to one first common cause.
Reply to Objection 2: Privation and habit belong naturally to the same subject. Now the subject of privation is a being in potentiality, as was said above (Q[48], A[3]). Hence, since evil is privation of good, as appears from what was said above (Q[48], AA[1], 2,3), it is opposed to that good which has some potentiality, but not to the supreme good, who is pure act.
Reply to Objection 3: Increase in intensity is in proportion to the nature of a thing. And as the form is a perfection, so privation removes a perfection. Hence every form, perfection, and good is intensified by approach to the perfect term; but privation and evil by receding from that term. Hence a thing is not said to be evil and worse, by reason of access to the supreme evil, in the same way as it is said to be good and better, by reason of access to the supreme good.
Reply to Objection 4: No being is called evil by participation, but by privation of participation. Hence it is not necessary to reduce it to any essential evil.
Reply to Objection 5: Evil can only have an accidental cause, as was shown above (A[1]). Hence reduction to any 'per se' cause of evil is impossible. And to say that evil is in the greater number is simply false. For things which are generated and corrupted, in which alone can there be natural evil, are the smaller part of the whole universe. And again, in every species the defect of nature is in the smaller number. In man alone does evil appear as in the greater number; because the good of man as regards the senses is not the good of man as man---that is, in regard to reason; and more men seek good in regard to the senses than good according to reason.
Reply to Objection 6: In the causes of evil we do not proceed to infinity, but reduce all evils to some good cause, whence evil follows accidentally.