Me, a sophisticated libertarian:
The justness of any human behaviour can be tested by referring to the Non Aggression Principle, or NAP.
You, a statist rube:
Does that mean it’s unjust to commit acts of aggression against others?
Me: Yes! Who could disagree with that, right? Oh, and by aggression I mean anything that violates property rights.
You: Hold on, property rights according to what theory?
Me: I’m thinking of Rothbardian or neo-Lockean property rights specifically.
You: Just a minute. On that view isn’t it a rights violation for a starving person to steal an apple from the orchard of a wealthy owner?
Me: Yes, it’s definitely a violation of property rights, and therefore aggression. The apple theft is unjust, but might still be a morally permissible action. You see, the non-aggression …
You: Wait. You think that a starving person stealing an apple is aggression. But you think that the angry owner, yelling at the thief and threatening to shoot him, isn’t aggression?
Me: Right, the owner isn’t violating property rights.
You: The principle you want to sell is called non-aggression. That sounds great to begin with. But it turns out that you have a really weird way of defining aggression. The way you use the word has nothing to do with what it means to me. If this is libertarianism you can keep it.
Me: Enjoy your chains! [scoff]
* I advocate ditching the NAP as a rhetorical tool, because of the problem that the above exchange illustrates. Libertarians are better off, I believe, talking directly about property rights, and violations of those rights. We can jettison this non-standard definition of aggression, we don’t need it.