A few assumptions are required for this argument:
- Moral values and duties are real.
- People are not responsible for events outside their control.
- There is sufficient human freedom to make moral decisions.
Imagine for a moment that two men are independently planning to mug someone. One set his alarm clock for 8:00AM and the other for 8:05AM. The early riser of the two men sets out to find a victim, but discovers it is just too early in the morning to find a victim. However, at 8:02AM, an innocent bystander strolls out of his apartment where he finds himself between the two would-be muggers. Since the first mugger has passed, he does not notice the innocent man. The second mugger, though, proceeds to strike the innocent man and steal his wallet.
On moral realism, we would likely agree that the successful mugger has commited a serious wrong in injuring and robbing the innocent man. We would also likely agree that the first man who intended to assault and rob an innocent individual has committed a wrong, although perhaps not as serious. His intent to violate moral norms is itself a violation of those norms. We recognize this widely in legal systems today as “conspiracy” to commit a crime is itself a crime.
Now, imagine that the second mugger (the one who successfully commited the crime) had lucked upon a winning lottery ticket on the ground with a cash value $2500. This amount of money was sufficient for him to not consider mugging anyone, despite having done nothing to earn the money. Here is the pivotal question. Is the first attempted mugger, whose motivation was financial, worse than the second successful mugger given this new scenario, even though all that has changed is the circumstances and nothing about the persons themselves? We know that had he not stumbled upon the $2500, he would have intended to and followed through with mugging. Are we innocent when the only thing that has prevented us from contemplating and completing some immoral act are circumstances beyond our control?
I think not. While certainly the man who found the lottery ticket has not committed any wrong, he has within him the potential to commit wrong. All that separates him from commiting a moral wrong is out of his control.
If the implications of this thought experiment are true, it follows that if there is any possible state of affairs in which you would be willing to commit a wrong, then you have committed a wrong. Thus, unless there is a person born for whom there are no possible states of affairs in which that person would be willing to commit a wrong, then all persons have some form of intrinsic sinfulness.
Thus, regardless of whether one believes in original sin, moral realism seems to entail some form of intrinsic sinfulness.