Among those who champion human liberty – the sovereignty of every adult when it comes to managing one's own life – some hold that if one could know what good conduct amounts to, one would be authorized to impose it on others. Among those who thought this was the world famous classical liberal economist, Milton Friedman. Friedman, who was an avid champion of human liberty, denied that we can "really know what sin is". By this he meant that what for people is the wrong thing to do is not something anyone can know. And he also held that one could not reasonably champion human liberty, the sort that one enjoys when others must abstain from imposing their ideas of how one ought to act, "if you could be absolutely certain that you had the revealed truth." By the "revealed truth" he merely meant whatever it is that others ought or ought not do. If one knew such things, one "could not let another man sin."
Right away there is a problem here because the argument advanced is actually supposed to show how one should act, namely that no one ought to impose himself or herself on another adult human being. So others ought to be left free. And that is, of course, based on the moral knowledge that it is wrong to make people act in ways they don't choose to. The only exception is when they choose to impose their idea on others. But that is self-defense, not any kind of imposition.
This point actually makes it evident that in some cases we do know what amounts to sinning or doing the wrong thing – for example, when one coerces peaceful others to act as one believes they should. But that is not all. We can pretty well know that when someone wastes away his or her life, say by becoming a junkie or a bum, this isn't something the person should do. The details may vary but it doesn't require rocket science to know that people who waste away their lives are normally misbehaving and ought to change. What is crucial here, however, is that it is they who must do the changing, not someone else. So imposing hard work or prudence on them simply cannot improve matters.
Some argue that if you impose worthwhile conduct on others and they later realize that this is indeed worthwhile and they should henceforth conduct themselves accordingly, your imposition is justified. This line of reasoning is advanced precisely because it is widely realized that to have moral significance one's conduct must be freely chosen. So those who want urgently to make other people moral – for example, Professor Robert P. George in his book Making Men Moral (Oxford University Press, 1993) – reject the moral right to act immorally, a right to do what is wrong (provided no one is being victimized). Yet, since morally significant conduct does have to be chosen, be it right or wrong, adult human beings do have such a right. It is not a moral but a political or natural right, however.
The contrary doctrine, namely, libertarian paternalism or nudging, applies only to children. It does not to adults. One is of course welcome to attempt to persuade people to do the morally right thing, maybe even implore or exert peer pressure to encourage another to do what is right. However, in the end an adult must make the choice and not be coerced to do so. That's part of what it means to respect human dignity. In nearly all major religions this is fully acknowledged. One must choose to accept Christ, for example. Even Stalinists believed that good communists had to voluntarily admit their flaws or ideological crimes before they could be punished meaningfully.
The only serious challenge to the idea that morally significant conduct must be voluntary comes from those who consider all of morality bogus, meaningless. In every known era of human history, including ours, there are serious moral skeptics – a recent issue of the magazine Philosophy Now features five philosophers arguing this position. Today the basis of the case rests mainly with neuroscience which supposedly shows that people are never free to choose to take actions, so in effect nothing they do is really their own doing! (This is what many defense attorneys set out to show about their clients and there are major institutes at universities embarking on research that promises to make the case for them!) But this brings up a whole bunch of other issues for which there is no room in this short essay.