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 Sirach 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 (2, 3; 6, 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 (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 (2, 3; 6, 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 (6, 3,4). But nothing can be essentially bad. For it was shown above that every being, as such, is good (5, 3); and that evil can exist only in good as in its subject (48, 3).
Secondly, because the first principle of good is the highest and perfect good which pre-contains in itself all goodness, as shown above (Question 6, Article 2). But there cannot be a supreme evil; because, as was shown above (Question 48, Article 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 (Article 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 (47, 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 (Question 2, Article 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 (Question 48, Article 3). Hence, since evil is privation of good, as appears from what was said above (48, 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 (Article 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.
Source: Summa Theologica (New Advent)