We have a very easy week, a short chapter defining how evil is in a good thing, and what that means. This is a mopping up chapter, collecting the threads of the weeks before. We’re not done with evil, by any stretch. Note that St Thomas is being very careful to define all this terms, which are all technical. This is especially necessary for us moderns, who use evil in loose, inconsistent and confusing ways. In other words, Aquinas is under obligation to use words in the way we moderns use them, but we are obliged to understand precisely what he means.
1 It can also be shown from the preceding considerations that every evil is based on some good.
2 Indeed, evil cannot exist by itself, since it has no essence, as we have demonstrated. Therefore, evil must be in some subject. Now, every subject, because it is some sort of substance, is a good of some kind, as is clear from the foregoing. So, every evil is in a good thing.
3 Again, evil is a certain privation, as is evident from the foregoing. Now, privation and the form that is deprived are in the same subject. But the subject of form is being in potency to form, and such being is good, because potency and act belong in the same genus. Therefore, the privation which is evil is present in a good thing, as in a subject.
4 Besides, something is called evil due to the fact that it causes injury. But this is only so because it injures the good, for to injure the evil is a good thing, since the corruption of evil is good. Now, formally speaking, it would not injure the good unless it were in the good; thus, blindness injures a man to the extent that it is in him. So, evil must be in the good.
5 Moreover, evil is not caused, except by the good, and then only accidentally. But everything that occurs accidentally is reducible to that which is by itself. So, with a caused evil which is the accidental effect of the good, there must always be some good which is the direct effect of the good as such, and thus this good effect is the foundation of the evil. For what exists accidentally is based on that which exists by itself.
6 However, since good and evil are contraries, one of these contraries cannot be the subject for the other; rather, it excludes the other. It will seem to someone, at first glance, that it is improper to say that good is the subject of evil.
7 Yet it is not improper, provided the truth be investigated to its limit. Good is spoken of in just as general a way as being, since every being, as such, is good, as we have proved.
Now, it is not improper for non-being to be present in being, as in a subject. Indeed, any instance of privation is a non-being, yet its subject is a substance which is a being. However, non-being is not present in a being contrary to it, as in a subject. For blindness is not universal non-being, but, rather, this particular non-being whereby sight is taken away. So, it is not present in the power of sight as its subject, but, rather, in the animal.
Likewise, evil is not present in a good contrary to it, as in its subject; rather, this contrary good is taken away by the evil. For instance, moral evil is present in a natural good, while a natural evil, which is a privation of form, is present in matter which is a good, in the sense of a being in potency.