What a World Following the Non-Aggression Principle Looks Like
As we learned before, libertarians are huge fans of initiating force against people. The core move in libertarianism is to permit violently attacking people when they do not follow the libertarian's preferred economic regulations regarding resource use. Just like almost every other theory, libertarians support initiating violence when doing so is consistent with their theory of resource entitlement and oppose initiating violence when doing so is inconsistent with it.
When I make this obvious point, it is clear some read me as saying that it is impossible to follow the non-aggression principle, i.e. the principle that you should not initiate force against others. But that's not what I am saying. There is actually a system that follows the non-aggression principle; it is just a system nobody advocates.
The Grab-What-You-Can World
The world which follows the non-aggression principle is the one Roderick Long calls the "grab-what-you-can world." He describes it thusly:
Imagine a world in which people freely expropriate other people’s possessions; nobody initiates force directly against another person’s body, but subject to that constraint, people regularly grab any external resource they can get their hands on, regardless of who has made or been using the resource.
The phrase "other people's possessions" is a bit question-begging if it is understood to connote ownership or entitlement. Other than that, this quote clearly describes the only world that follows the non-aggression principle. Why? It's simple: 1) grabbing pieces of the world does not, by itself, involve initiating force against other people (if it did, then all resource use would be considered aggression), and 2) attacking someone for grabbing up a piece of the world does involve initiating force against other people. Since grabbing pieces of the world is permissible and violently preventing grabbing is not, the grab-what-you-can world satisifes the non-aggression principle and no other world does.
Arguments Against Are Not About Aggression
There are many arguments you can make against the grab-what-you-can world, but none of them have anything to do with aggression.
You can argue that this kind of world would sap all incentive and thereby lead to all sorts of misery and poverty. But that is a welfarist consequentialist argument for why we should use violent aggression to create a world other than the grab-what-you-can world.
You can argue that this kind of world goes against the principle that people should be entitled to what they produce. But that is a just deserts argument for why we should use violent aggression to keep people from grabbing what they can.
Finally, you can combine welfarist-consequentialist and desert theory intuitions into a vague argument that says grabbing stuff is force even though it clearly is not. This is Roderick Long's move. He writes: "If I starve to death because someone raids my food stores every time I turn my back, it does not seem implausible to describe the situation as one in which I am forced into starvation." Long is transparently equivocating when he uses the word "forced." The scenario he described involves no force (strictly defined) because no body has been attacked. This involves no more force than someone starving because they are poor, something libertarians like Long do not call force. Maybe grabbing the food stores runs against desert and maybe it has bad welfare consequences, but vaguely combining these two considerations and wishing real hard does not turn non-forceful grabbing into force.
When to Initiate Force
To the extent that almost everyone opposes following the non-aggression principle—as it requires the grab-what-you-can world—what folks are actually debating about is when it is alright to initiate force against others even though they have not attacked you.
If you abstract out, the way these debates actually unfold is to first establish a theory of entitlement (some positive theory of what ought to belong to who) and then declare that it is permissible to use aggressive violence to non-consensually impose the rules of that theory of entitlement on others. Libertarians have a particular theory of entitlement (usually driven by desert or utilitarian considerations) that causes them to want to violently impose certain laissez-faire property rules. Leftists, on the other hand, have a different theory of entitlement (usually driven by utilitarian or egalitarian considerations) that causes them to want to violently impose certain welfare capitalist, social democratic, or socialist property rules.
Ultimately, the non-aggression principle is not logically impossible to follow. The grab-what-you-can world is the world that follows the non-aggression principle. But as nobody really advocates that world, non-aggression has nothing to do with any actual debates people are having (despite libertarian delusions to the contrary). In practice, the real debate is always and anywhere between competing theories of positive entitlement, which are then used to justify the use of aggression.