When liberty is attacked by its critics and enemies, often defenders pull out the skeptical ploy: "No one can know right from wrong, so no one may force others to comply with any standards of right versus wrong. Who can tell how we ought to act? And if no one can, as surely no one can, then no one may force anyone to do the right thing. It would be shooting in the dark."
The famous American classical liberal/libertarian, the late Milton Friedman, put it this way in an interview he gave in 1975 in Reason Magazine: "I think that the crucial question that anybody who believes in freedom has to ask himself is whether to let another man be free to sin. If you really know what sin is, if you could be absolutely certain that you had the revealed truth, then you could not let another man sin. You have to stop him." He also wrote, in his famous book, Capitalism and Freedom: "The liberal conceives of men as imperfect beings. He regards the problem of social organization to be as much a negative problem of preventing 'bad' people from doing harm as of enabling 'good' people to do good; and, of course, 'bad' and 'good' people may be the same people, depending on who is judging them."
It is well known that Friedman was a great champion of human liberty. He supported his position, however, by claiming that no one can "really know what sin is." And he argued that "if you could be absolutely certain that you had the revealed truth, [then] you could not let another man sin."
This is a pivotal matter and doesn't really help support human liberty very much, although it has some notable champions, such as Professor Richard Epstein of the University of Chicago. In his book Skepticism and Freedom: A Modern Case for Classical Liberalism Epstein lays out in great detail this line of support for the free society. Don't interfere with people so as to promote some valuable goal because, well, you cannot know what is valuable.
There is something seriously amiss with this way of defending human liberty and the free society. It can be made evident without too much difficulty. The bottom line is that very few people actually believe that one cannot know what is right versus wrong. Our criminal law certainly assumes the opposite, so many defendants are sent to prison for doing the criminally wrong thing (which is often supported by our supposedly knowledge of what is morally wrong). Parents, surely, profess to know right versus wrong when they rear their children. And as far as our conduct is concerned, it is totally unrealistic to hold that when we try to do what is right and refrain from doing what is wrong, we could accept that we are always in the dark.
But not only is moral skepticism not widely accepted by people throughout the world, it is illogical to maintain the position. To start with a very plain case, even to say that "one should not interfere with others" is to commit oneself to a position on what is right versus wrong. This is the moral imperative requiring respect for others' liberty. You are saying, clearly, that people ought to refrain from intrusive conduct toward their fellows, which is a moral or normative judgment if anything is.
But what about Milton Friedman's claim that if one knows what sin is – what doing wrong is – one must stop it? Well, it is wrong. When people are required to do the right thing or avoid doing what is wrong, they must do this of their own free will. Otherwise their conduct has no moral significance. Forcing others to be good is an oxymoron. Doing what is right or not doing what is wrong has to be a matter of choice to be morally worthwhile. The only time one may intrude on others who do the wrong thing is if what they do amounts to intruding on their fellows, as in murder, theft, assault, rape, etc. When their wrongdoing is peaceful, no interference is justified.
That is what lies at the heart of human freedom: it is absolutely necessary for the morally significant life (although it is also very useful). That is why the nanny state, authoritarianism, paternalism, and totalitarianism are all very bad ideas – they promote treating people without regard to their moral agency, their responsibility to lead a moral life of their own free choices.
Knowing someone else is doing wrong, sinning or being vicious, doesn't justify interference. One may advocate that such people improve themselves but not force them to do so, not unless self-defense of the defense of innocent victims is involved. Everything else that is right needs to be done voluntarily. Otherwise the very humanity of people is being denied.