In this formulation, aggression means “initiating physical violence” in violation of another person’s rights to person and property. If Jack hits Jill, Jack aggresses against Jill and Jill is the victim. But if Jack hits Jill because Jill is coming at him with a knife, then Jack may simply be acting in self-defense—that is, in defense of his rights. Jill is the aggressor. (The classical-liberal legal theorist Richard Epstein addresses scenarios like this in this important essay on strict liability.)
Now, some libertarians argue that any kind of taxation by the state constitutes wrongful aggression because it threatens violence (e.g., arresting and imprisoning) against a person if she merely tries to protect her rights to her property by refusing to pay. Others argue the state does not aggress when it uses or threatens physical violence in the course of its legitimate duties (e.g., taxing to finance national defense), but does violate the NAP when it goes beyond those duties (e.g., taxing to finance a war of aggression). But people often strongly disagree about what those duties are or how they are defined: What exactly constitutes national defense?
Determining what constitutes aggression can get very complicated even if you grant that the state has a right to tax or take private property. (Epstein provides some clarity on this issue as well.) Here are a couple more complications.
Property as a Bundle of Rights
Richard Pipes offers a useful definition of property:
Property refers to the right of the owner or owners, formally acknowledged by public authority, both to exploit assets to the exclusion of everyone else and to dispose of them by sale or otherwise. (Emphasis original)
In this sense, property offers a person a “sphere of autonomy” that no one else may enter without permission.
But a right to property is not monolithic. Some characterize a property right as a “bundle of rights” over a range of uses, which can increase or decrease.
Let’s say that Fred owns an apartment and he agrees to let Wilma occupy the apartment for one year. There may be many complicating factors, but let’s put those to one side to keep the situation as simple as possible. So that means Fred has given up the right to use the apartment for that year, even as he retains overall ownership of the property. He has agreed to give up some of his usage rights in exchange for some sort of compensation. If Fred enters the apartment during that period without Wilma’s permission, he is in the wrong, and she has some limited right to use force against him if he refuses to leave. In this case, because Fred doesn’t have Wilma’s permission, he is the aggressor.
If, however, Fred enters the apartment because Wilma has stayed beyond the period they agreed to and still refuses to leave, he is not the aggressor, Wilma is. He may legitimately use limited violence to force her to leave.
Thus, the same behavior—Fred entering the apartment—may be either an act of aggression or justified self-defense, depending on who has the legitimate claim under a particular regime of property rights.
It All Depends on the Property Rights
What I just said pertains to a particular regime of property rights that specifies more or less clearly who has rights to what, and when and where they apply. What I would like to emphasize here, however, is that it can get really hard even to identify aggression—and therefore really hard to usefully apply the NAP—when people disagree fundamentally about what the prevailing property-rights regime is.
Again, for simplicity, suppose there is a regime of maximal private-property rights, as under free markets in a free society, versus a regime of maximal state-ownership of property rights, as under collectivist central planning.
Suppose Fred thinks he has a right to enter an apartment on a given day under property-rights regime X, which entitles him to Richard-Pipes-like exclusive use and disposal of the apartment. Wilma also thinks she has a claim to the apartment under property-rights regime Y, which says when she has the right to use the apartment as shelter against dangerous elements as long as she causes no damage. (See the “doctrine of private necessity.”) Here there is a conflict between property-rights regimes, not just a disagreement over who has a claim to use the apartment under a given property-rights regime.
If Fred forcibly removes Wilma under regime X, he is acting to defend his property as he understands it. If he does that under regime Y, he is the aggressor, not Wilma.
To take an extreme example, the communist dictator of North Korea, Kim Jong-un, could truthfully say he believes in the non-aggression principle, within the framework of collectivist central planning. So if one of his subjects refuses to work in a labor camp, which is that person’s obligation under the law, to Kim that is an act of aggression. If Kim uses force here, as objectionable as that would be to a libertarian, he would only be defending his property right (to that person’s labor).
Not as Simple as It May Seem
There are complications and subtleties in these cases that I won’t even try to analyze here. But my point is precisely that such complications and subtleties do exist. Unfortunately, I often hear libertarians wielding the non-aggression principle as if it were a shut-up argument, not realizing that without first establishing a common understanding of what the underlying (legitimate) property-rights regime is, the NAP won’t sway anyone. The NAP alone is not enough.