Calvin comments of some words of another writer, Themistius, an Aristotlean philosopher, on the ability of men and women to judge what is evil and what is good. It takes a couple of reads to get the gist:
Themistius more correctly teaches that the intellect is very rarely deceived in general definition or in the essence of the thing; but that it is illusory when it goes farther, that is, applies the principle to particular cases.In reply to the general question, every man will affirm that murder is evil. But he who is plotting the death of an enemy contemplates murder as something good. The adulterer will condemn adultery in general, but will privately flatter himself in his own adultery. Herin is man’s ignorance: when he comes to a particular case, he forgets the general principle that he has just laid down.
(Calvin’s Institutes II.ii.23, Battles’ translation)
Calvin himself actually goes further than Themistius. He writes that in addition to the above, the conscience sometimes kicks in so that although the general principle is known, a particular act can also be known to be wrong.
Interesting insights into how the sinful human mind works.