One opposite is known through the other, as darkness is known through light. Hence also what evil is must be known from the nature of good (ST I Q48 A1).By all means feel free to go to the source for this subject; otherwise, you can take a look at my modest attempt to understand what he says about it here.
Since (as St. Thomas says) "goodness and being are really the same," it follows that evil cannot be a being, since this would make good and evil the same thing - which is irrational. Hence we must conclude that evil is really privation, or "the absence of good:"
Now, we have said above that good is everything appetible; and thus, since every nature desires its own being and its own perfection, it must be said also that the being and the perfection of any nature is good. Hence it cannot be that evil signifies being, or any form or nature. Therefore it must be that by the name of evil is signified the absence of good. And this is what is meant by saying that "evil is neither a being nor a good." For since being, as such, is good, the absence of one implies the absence of the other (ST I Q48 A1; emphasis added).Aquinas goes on to explains this a bit more in the same article, and I think what he has to say is instructive for understanding his philosophy on this point. With respect to good and evil in morality, he says:
Good and evil are not constitutive differences except in morals, which receive their species from the end, which is the object of the will, the source of all morality. And because good has the nature of an end, therefore good and evil are specific differences in moral things; good in itself, but evil as the absence of the due end. Yet neither does the absence of the due end by itself constitute a moral species, except as it is joined to the undue end; just as we do not find the privation of the substantial form in natural things, unless it is joined to another form. Thus, therefore, the evil which is a constitutive difference in morals is a certain good joined to the privation of another good; as the end proposed by the intemperate man is not the privation of the good of reason, but the delight of sense without the order of reason. Hence evil is not a constitutive difference as such, but by reason of the good that is annexed (emphasis added).I'm not sure I know a way to put this into other words that will make sense, explain it, and accurately reflect what he says here, but if I understand him correctly the point is that evil in morality isn't simply defined by the absence of the due end, but rather by that deprivation in association with some other "good" in place of the due end: as when we prefer some pleasure or other in the place of the good that reason rightly identifies. The evil in stealing isn't simply that I steal, but rather that I put my own craving for another man's goods ahead of the other man himself, irrationally supposing that I "deserve" or "have a right" to his things, or (even more irrationally) ignoring such questions entirely and just taking his stuff "because I want it" - as though it is good to be ruled by our desires rather than by the reason that God has given us. In the latter case especially, it seems pretty easy to see that I would have put another thing that I perceive as "good" - the irrational, immoderate satisfaction of my desires - ahead of the real goods of contentment with my own possessions and respect for my neighbor's goods as his. The privations I can see here are my discontent with my lot, my lack of respect for my fellow man's right to private property, and a lack of self control with respect to my desires for his things.
It might be worth pointing out here one implication of this doctrine: it is not possible to will to do evil. Because evil is a privation of good or being, it makes no sense to think of doing evil as something that we want to do. Rather, what we do is substitute some other thing we perceive as good for the good we ought to want to do.
A thing is said to act in a threefold sense...[I]t is said in the sense of the final cause, as the end is said to effect by moving the efficient cause. But ... evil does not effect anything of itself, that is, as a privation, but by virtue of the good annexed to it. For every action comes from some form; and everything which is desired as an end, is a perfection. And therefore, as Dionysius says (Div. Nom. iv): "Evil does not act, nor is it desired, except by virtue of some good joined to it: while of itself it is nothing definite, and beside the scope of our will and intention."What we will to do is something that we desire. Our desires may be disordered, so that we identify as a good thing that which is not really, but we don't really say, "This is evil, and I want to do it." Even Satan himself does not do so: what he has done is decide that fulfillment of his own desires is more important than obeying God, and so he refuses to do what God commands. Now we may, if we step back and consider things from a rational perspective, identify these things as evil; but to the man who is sinning, what he is actually doing is not choosing to do evil per se but rather to pursue something that he has decided is good to pursue - even better, in his own eyes, than the good he ought to be pursuing. The privation is the lack of the due end in what we have chosen to do.
Lastly for this post (I hope to have more to say about St. Thomas' views on evil), it's interesting to me that the Reformed Protestant Westminster Shorter Catechism has echoes of this formulation of evil in its answer to the question, "What is sin?"
Sin is any want of conformity unto, or transgression of, the law of God (WSC Question 14; emphasis added).Sin here is defined (at least partly) as a failure to conform to God's law: in other words, as a privation. The Reformers owed more to Aquinas and the Scholastics than they would probably ever have been willing to concede. :-)
It's easy (and in some contexts reasonable) to think about good and evil as contraries or opposites. But St. Thomas shows us that we need to be more cautious in our thinking about this. Evil isn't a contrary per se, because that would suggest that it has some existence; but as we have seen, good and being are really the same (says St. Thomas), and so evil really doesn't have any existence.