"Besides therefore, you might know that even the deepest being of souls in captivity to the worst are ignorant of neither the good nor the foul, but rather they are negligent concerning these. Everyone hides their sins, and even where a man enjoys the fruit of his sins when they bring him benefit, he still hides them. While a good conscience wants to speak up and be noticed, those of an immoral conscience fear even the darkness."
-Seneca's Moral Epistle XCVII
Even people in bondage to terrible sin, even those indeed who make their living by such evil deeds, have a sense of shame concerning these evil deeds. There is simply some part of a man that knows the difference between good and evil, even when a man would rather he didn't. This inner knowledge of good and evil, and of the moral imperative to do the good and refrain from the evil - even when one is above temporal punishment - is what drives men to cover up, justify, hide, and disguise their evil deeds.
This may seem like common sense, but it is the most hated brand of common sense. It is that most deeply felt sense of moral awareness and obligation that drives men to work like mad to hide their sins.
We live in a society that tries to make everything into a disease, and therefore to make everyone into a victim. Even in morality (perhaps especially in morality), people use all manner of sophistry to arrive at this position: we cannot know what is good or evil, or more positively - we must each decide for ourself what is good or evil. Seneca, far from being a Christian, as a rational pagan puts an immediate stop to such self-deceit. "It is not ignorance of the good that plagues you, but negligence of the good." It isn't that a person doesn't KNOW what is good, but that a person does not particularly WANT TO DO what is good, especially when there are benefits to reap from evil deeds.
Should Christians, then, seeing that it is a cheap parlor-trick of moral sophistry for a person to claim ignorance of the good, waste time trying to argue that good and evil CAN be known? Perhaps Seneca sounds a call for us to simply address the true problem: unwillingness to do the very well-known good.
Negatively, the accusation is as obvious to Seneca as it is to common-sense "everyman" lawyers like Perry Mason or Matlock: "If you didn't know that it was wrong, then why did you try to cover it up." Many a murderer's psychological appeal to the court has been foiled by the same's attempts to cover up their crime. [In my own experience, an acquaintance's husband killed a man while intoxicated, but was given the maximum sentence because he then tried to dispose of the body.] Obviously, a jury of our peers can see the truth of Seneca's accusation. It isn't such a bad argument, after all: If you think that a man can't or doesn't know good and evil, then why the fear of exposure?
Positively, Seneca makes an invitation to those who would live in freedom, both socially and morally: if you do good, then you will not share the fear of exposure with the evildoers.
In the end, it doesn't take social mores or even divine revelation to argue for moral behavior. A simple appeal to the nature of every individual's conscience works quite nicely, and such an appeal may be the only recourse one has in a radically secular and individualistic society.
"Take no part in the unfruitful works of darkness, but instead expose them. For it is shameful even to speak of the things that they do in secret."
-Saint Paul to the Ephesians (5:11-12, ESV)