Chapter 117 — That nothing can be essentially or supremely evil, but evil is the corruption of some good

Knowing that evil is the privation of a due perfection, now we can easily see how evil corrupts good; it does so inasmuch as it is the privation of good, as, for example, blindness is said to corrupt sight because it is the privation of sight. However, evil does not completely corrupt good, because, as we remarked above, not only form, but also potency to form, is good; and potency is the subject of privation as well as of form. Therefore, the subject of evil must be good, not in the sense that it is the opposite of evil, but in the sense that it is a potency to the opposite.

This also brings out the fact that not every good can be the subject of evil, but only such a good as is in potency with respect to some perfection of which it can be deprived. Hence in beings which are exclusively act, or in which act cannot be separated from potency, there can, to this extent, be no evil.

And this makes it clear that nothing can be essentially evil, since evil must always have as its foundation some subject that is good. And so there cannot be a supreme evil in the way that there is a supreme good which is essentially good.

The same consideration also makes it clear that evil cannot be desired, and that it can only do something in virtue of the good connected with it. For only perfection and end are desirable; and the principle of action is form. However, since a particular perfection or form involves the privation of some other perfection or form, it can happen accidentally that privation or evil is desired and is the principle of some action—not insofar as it is evil, but because of the good connected with it, like a musician who constructs a house not insofar as he is a musician, but insofar as he is a builder.

From this we may also infer that evil cannot be a first principle, for an accidental principle is subsequent to a principle that is such essentially.