If I were compelled to summarize the libertarian philosophy's distinguishing feature while standing on one foot, I'd say the following: Every person owes it to all other persons not to aggress them. This is known as the nonaggression principle, or NAP.

What is the nature of this obligation?

The first thing to notice is that it is unchosen. I never agreed not to aggress against others. Others never agreed not to aggress against me. So if I struck you and you objected, you would not accept as my defense, "I never agreed not to strike you."

Even an explicit agreement rests on an unchosen obligation. Let's say you lent me five dollars, I refused to repay the loan, and when you demanded repayment, I said, "Why am I obligated to repay the money?" You would probably reply, "Because you agreed to repay me." If I replied, "True, but when did I agree to abide by my agreements?," what would you say? If you said that failure to repay constituted aggression, and I replied that I never agreed not to aggress against you, we'd be back where we started.

Of course this would point the way to absurdity — an infinite regress of agreements to keep my agreements. We would get nowhere. There has to be a starting point.

If I were to ask, "Why do we owe it to others not to aggress against them," what would you say? I presume some answer rooted in facts would be offered because the alternative would be to say this principle has no basis whatsoever, that it's just a free-floating principle, like an iceberg. That would amount to saying the principle has no binding force. It's just a whim, which might not be shared by others.

In other words, if a nonlibertarian demands to know why he is bound by the unchosen NAP, libertarians will have answers. Their answers will differ—some will be more robust than others—but they will have answers. At least I hope so. Read the rest here: A Libertarian Society Owes Itself More Than Non-Aggression