Having rejected the idea that man becomes evil because of something external to him, St Augustine pursues in On Free Choice of the Will Book I, chapter iii what he admitted (in chapter ii) is a difficult question: namely, why we do evil. On his way to an answer, he begins by asking what it is that makes a given act to be evil.
With adultery as an example, he rejects the idea that it is evil because it is forbidden by law. Rather, it is forbidden because it is evil. But that mere declaration is insufficient:
I cry out that all people and nations ought to believe that adultery is evil. But now we are struggling to know with our understanding and to establish most firmly what we have already accepted by faith. Therefore, consider as best you can and tell me the reason why you know adultery to be wrong (p. 7, emphasis added).
He affirms, as a faithful Catholic, that adultery is evil. This is something that at least we can know by faith, because the Church teaches it to be so. But there are truths that are not taught in Scripture, and there are truths that are taught in Scripture alone, and there are likewise truths taught in Scripture and also discoverable by reason. What he is seeking here is to know what we may say about the source of evil on terms of reason.
One thing that he rejects immediately is the appeal to the Golden Rule as a sufficient basis for identifying what is evil. Such an argument would allow the euthanizer to justify his so-called “mercy killings” on the grounds that he’d like the same done to him someday. Obviously then this is an insufficient basis for objectively identifying that which is evil. Similarly he rejects the appeal of statists everywhere when he denies that being condemned for an act by human courts is what constitutes the deed as evil. This is one reason that free societies have trials by jury, and that those juries necessarily have authority to judge not merely the facts of the case but also the law: so as to prevent evil rulers from punishing men for “violations” of unjust laws. But I digress :-)
At the end of the chapter, St Augustine proposes that we identify lust (or, more generally speaking, evil desire) as “the evil element.”
To help you understand that the evil element in adultery is lust, consider the case of a man who does not have the opportunity to lie with another’s wife; but nevertheless, if it is somehow obvious that he would like to do so and would do so had he the opportunity, he is no less guilty than the man taken in the very act.
Evodius. Nothing is more obvious, and I now see that there is no need for a long discussion to show me how homicide and sacrilege and all the other sins are evil. Now it is clear that lust [i.e., evil desires – F.] is dominant in every kind of evildoing (p. 8).
More will have to be said than this, of course, but this is as far as Augustine gets in chapter iii. Obviously if a man decides for himself what is evil, he may not think that his desires are evil at all, and so Augustine’s argument would die a death of subjectivism. This is not the end of the story, however.