Objection 1. It would seem that evil is not found in things. For whatever is found in things, is either something, or a privation of something, that is a "not-being." But Dionysius says (Div. Nom. iv) that "evil is distant from existence, and even more distant from non-existence." Therefore evil is not at all found in things.
Objection 2. Further, "being" and "thing" are convertible. If therefore evil is a being in things, it follows that evil is a thing, which is contrary to what has been said (1).
Objection 3. Further, "the white unmixed with black is the most white," as the Philosopher says (Topic. iii, 4). Therefore also the good unmixed with evil is the greater good. But God makes always what is best, much more than nature does. Therefore in things made by God there is no evil.
On the contrary, On the above assumptions, all prohibitions and penalties would cease, for they exist only for evils.
I answer that, As was said above (47, 1,2), the perfection of the universe requires that there should be inequality in things, so that every grade of goodness may be realized. Now, one grade of goodness is that of the good which cannot fail. Another grade of goodness is that of the good which can fail in goodness, and this grade is to be found in existence itself; for some things there are which cannot lose their existence as incorruptible things, while some there are which can lose it, as things corruptible.
As, therefore, the perfection of the universe requires that there should be not only beings incorruptible, but also corruptible beings; so the perfection of the universe requires that there should be some which can fail in goodness, and thence it follows that sometimes they do fail. Now it is in this that evil consists, namely, in the fact that a thing fails in goodness. Hence it is clear that evil is found in things, as corruption also is found; for corruption is itself an evil.
Reply to Objection 1. Evil is distant both from simple being and from simple "not-being," because it is neither a habit nor a pure negation, but a privation.
Reply to Objection 2. As the Philosopher says (Metaph. v, text 14), being is twofold. In one way it is considered as signifying the entity of a thing, as divisible by the ten "predicaments"; and in that sense it is convertible with thing, and thus no privation is a being, and neither therefore is evil a being. In another sense being conveys the truth of a proposition which unites together subject and attribute by a copula, notified by this word "is"; and in this sense being is what answers to the question, "Does it exist?" and thus we speak of blindness as being in the eye; or of any other privation. In this way even evil can be called a being. Through ignorance of this distinction some, considering that things may be evil, or that evil is said to be in things, believed that evil was a positive thing in itself.
Reply to Objection 3. God and nature and any other agent make what is best in the whole, but not what is best in every single part, except in order to the whole, as was said above (Question 47, Article 2). And the whole itself, which is the universe of creatures, is all the better and more perfect if some things in it can fail in goodness, and do sometimes fail, God not preventing this. This happens, firstly, because "it belongs to Providence not to destroy, but to save nature," as Dionysius says (Div. Nom. iv); but it belongs to nature that what may fail should sometimes fail; secondly, because, as Augustine says (Enchir. 11), "God is so powerful that He can even make good out of evil." Hence many good things would be taken away if God permitted no evil to exist; for fire would not be generated if air was not corrupted, nor would the life of a lion be preserved unless the ass were killed. Neither would avenging justice nor the patience of a sufferer be praised if there were no injustice.
Source: Summa Theologica (New Advent)