Zwolinski and Smith have already discovered that much of their disagreement is verbal, rather than substantive. But there are some real differences here.

In an earlier post, Matt wrote:

The NAP clearly implies that it’s wrong for me to shoot you in the head. But, to borrow an example from David Friedman, what if I merely run therisk of shooting you by putting one bullet in a six-shot revolver, spinning the cylinder, aiming it at your head, and squeezing the trigger? What if it is not one bullet but five? Of course, almost everything we do imposes some risk of harm on innocent persons. We run this risk when we drive on the highway (what if we suffer a heart attack, or become distracted), or when we fly airplanes over populated areas. Most of us think that some of these risks are justifiable, while others are not, and that the difference between them has something to do with the size and likelihood of the risked harm, the importance of the risky activity, and the availability and cost of less risky activities. But considerations like this carry zero weight in the NAP’s absolute prohibition on aggression. That principle seems compatible with only two possible rules: either all risks are permissible (because they are not really aggression until they actually result in a harm), or none are (because they are). And neither of these seems sensible.

Smith replies:

Unless you have consented to play Russian Roulette, this situation involves far more than a risk; it is first and foremost a threat, and you have the right to defend yourself against legitimate threats…

Here Zwolinski raises the possibility of accidents, but accidents are in an altogether different category than deliberatethreats…

And if you don’t want to run the risk of an airplane crashing into your house, then move to a safer location. (You don’t own the airspace used by planes, after all.)

So, Smith is right that deliberatively threatening people is not the same thing as imposing mere risk upon them, without their consent. Still, Matt is right that the NAP cannot be a foundational principle of justice. After all, as Matt’s example shows, we need a theory about what counts as acceptable risk. The most cartoony version of NAP implies that any risk I impose upon you without your consent is wrong and violates your rights.

Now, one might try to get around this by arguing that in fact people consent to certain risks. Is this Smith’s strategy? (It seems like it is, given the quotations above and the rest of the essay.) I’m not sure if that’s what Smith is up to. If it is, that doesn’t seem promising to me. Do you consent to the risk of being struck by a car when you walk alongside the road? Do you consent to the risk of being hit by an airplane when you live anywhere in which a plane might fly overhead? Do you consent to the risk of being struck by a falling satellite because you sometimes choose to be above ground? Etc. We can certainly choose to live with more or less risk here and there, and most of us make such tradeoffs when choosing where to live, how to get to work, etc. But it’s not plausible to say that I consented to all or even most of the risks to which I am exposed, because I don’t have any real alternatives. If the risk gets imposed upon me no matter what I do (short of killing myself), then I haven’t consented to it. There’s more to be said about this, but it doesn’t seem promising off hand. I’d be surprised if a theorist could show that most of the things we are inclined to call acceptable risks turn out to be things we consent to.

Matt also wrote,

Even if the NAP is correct, it cannot serve as a fundamental principle of libertarian ethics, because its meaning and normative force are entirely parasitic on an underlying theory of property.

Smith vigorously agrees and wonders why Matt thinks this is a point worth making:

As before, Zwolinski seems unaware of how Rothbard dealt with the topic at hand. In “Property and Criminality” (Chapter 9 of The Ethics of Liberty), Rothbard clarified “the basic rule of the libertarian society” by stating “that no one has the right to aggress against the legitimate orjust property of another.” As Rothbard put it in the second chapter ofFor a New Liberty, “the central axiom of the libertarian creed is nonaggression against anyone’s person and property.” Again, in the first paragraph under “The Nonaggression Axiom,” Rothbard incorporates the notion of property rights into his formulation of the NAP.

The libertarian creed rests upon one central axiom: that no man or group of men may aggress against the person or property of anyone else. This may be called the “nonaggression axiom.” “Aggression” is defined as the initiation of the use or threat of physical violence against the person or property of anyone else. Aggression is therefore synonymous with invasion [of property rights].

Given these and many similar statements by Rothbard, it is very odd to complain, as Zwolinski does, that the “meaning and normative force [of the NAP] are entirely parasitic on an underlying theory of property.” Rothbard made precisely that point many times and in considerable detail by incorporating a theory of property rights into his conception of aggression. In other words, it is only by specifying the relevant property rights that we can identify aggression in the first place.

I’d like to invite Matt to say more about why he thinks is a point worth making. I suspect it’s because it shows that many libertarian responses to arguments from social justice or arguments about consequences turn out to be question-begging. Rawls advocates the difference principle, which might in principle require redistribution. Many cartoon libertarians respond that this involves unjust aggression against innocent people and their property. But as the exchange above shows, this won’t do as a response. The NAP is parasitic on a theory of property rights. Rawls and others (including Arizona-school liberals like Schmidtz, Gaus, and Zwolinski) think that for a system of property rights to be legitimate, it has to tend to produce certain kinds of results. Maybe we Arizona school people wrong and Rothbard is right. (I doubt it. I really really doubt it.) But libertarians can’t use the NAP against Rawlsians or Marxists or others who advocate social justice. Instead, the dispute is over the nature and grounding of property rights. “The difference principle violates the non-aggression principle” presupposes rather than shows that Rawls is wrong about property.