Zwolinski continues that the NAP “holds that aggression against the person or property of others is always wrong.…” Except that, as we have seen, very often two parties cannot help impinging on the liberties of each other (for instance, whether pollution is allowed or prohibited: one side or the other side must suffer an interference/​constraint/​cost). And in such cases “aggression” may not seem to be exactly the right word, though it will do. And what is inevitable is not obviously “wrong”.

Zwolinski writes that “aggression is defined narrowly in terms of the use or threat of physical violence.” I had rather say, somewhat less imprecisely, “aggression” is proactively interfering with another’s person or property (when these are not themselves the result of any proactive interference). Is it true that “From this principle, many libertarians believe, the rest of libertarianism can be deduced as a matter of mere logic”? Most libertarians appear to have supplementary or additional principles. However, my own view is that only a pre‐​propertarian conception of libertarian liberty can fully allow that “the rest of libertarianism can be deduced as a matter of mere logic.” But that is, indeed, a single principle and one that I wish to explain and defend.

As an implicit criticism, Zwolinski observes that “The libertarian armed with the NAP has little need for the close study of history, sociology, or empirical economics.” That is surely a great virtue in a practical principle for everyone. Moreover, this appears to overlook that, because study is bound to be finite, no study can support a universal theory such as the NAP—although it can test it and possibly refute it. He continues that “With a little logic and a lot of faith in this basic axiom of morality, virtually any political problem can be neatly solved from the armchair.” And such simplicity is clearly highly desirable. Strictly speaking, no faith is required or possible: we do not choose what to believe. However, any—necessarily conjectural—solutions can be derived. And they are then ready for criticism and testing.

What is the philosophical significance of the fact that “On its face, the NAP’s prohibition of aggression falls nicely in line with common sense”? Common sense is a fallacious criterion of truth or morality. So it is similarly irrelevant to say that “it is far from common sense to think that its badness is absolute.” But it is relevant to present “any other possible consideration of justice or political morality” as a criticism of the NAP conjecture. It might seem that “There is a vast difference between a strong but defeasible presumption against the justice of aggression, and an absolute, universal prohibition.” But in practice our, necessarily conjectural, theories are always open to potentially refuting criticism no matter how “absolute” we might think them to be. Zwolinski approves of Brian Caplan’s view that “if you can’t think of counterexamples to the latter, you’re not trying hard enough.” But counterexamples that are merely logical possibilities and unlikely scenarios are beside the point. Real systematic refutations of the practical morality of the NAP/MAP do not appear to exist, as far as I can tell.

We then move on to the “six reasons why libertarians should reject the NAP.” And we ought to note immediately that to refute one, dubious, interpretation of the NAP is not to refute every interpretation of it.

1. Prohibits All Pollution

Zwolinski asserts that “industrial pollution violates the NAP and must therefore be prohibited” moreover, even “personal pollution produced by driving, burning wood in one’s fireplace, smoking, etc., runs afoul of NAP.” As I have explained, prohibiting pollution (for instance, coercively preventing someone from lighting his fire for needed warmth and cooking) also violates the absolute NAP. Hence the MAP comes into play.

2. Prohibits Small Harms for Large Benefits

Zwolinski asks us to “suppose, to borrow a thought from Hume, that I could prevent the destruction of the whole world by lightly scratching your finger?” And here I would reply that the NAP is about the real world rather than about every logically possible world and thought experiment. He goes on to “suppose that by imposing a very, very small tax on billionaires, I could provide life‐​saving vaccination for tens of thousands of desperately poor children.” This is slightly less implausible but it is still not realistic. We don’t need to tax anyone to develop new vaccines. And the institution of any taxation would disrupt productivity immediately and then do cumulative damage as the economy has its growth slowed. Moreover, that growth would probably have included new advances in vaccines sooner or later. Zwolinski concludes by asking “is it really so obvious that the relatively minor aggression involved in these examples is wrong, given the tremendous benefit it produces?” And the obvious answer appears to be that implausible assumptions do not refute a practical principle.

3. All‐​or‐​Nothing Attitude toward Risk

Zwolinski asks, “what if I merely run the risk of shooting you by putting one bullet in a six‐​shot revolver, spinning the cylinder, aiming it at your head, and squeezing the trigger?” And the answer is that it is an aggressive act to take such a serious risk at someone else’s expense. In monetary terms, the degree of the aggression is something like the amount of money that the victim would have to be paid to accept such a risk (I don’t mean to imply that everything can be reduced to money, of course). Without such an agreement, you are using someone else’s property—his head—without his permission for your dangerous game. Imposed risks are aggressions; actual damage is not necessary. Otherwise, by analogy, you may as well say that coercing someone to do something at gunpoint only becomes an aggression if you actually shoot them when they fail to comply.

Zwolinski then observes that “almost everything we do imposes some risk of harm on innocent persons” and that “Most of us think that some of these risks are justifiable, while others are not” but our reasonable explanations “carry zero weight in the NAP’s absolute prohibition on aggression.” And, again, this overlooks that there is aggression whether such risks are allowed or coercively prohibited. But there is no insuperable problem with applying the MAP, as long as we have a reasonable account of what policy best deals with the clash in an unbiased way (it need not be perfect or admit of cardinal accounting).

4. No Prohibition of Fraud

Zwolinski asserts that “Libertarians usually say that violence may legitimately be used to prevent either force or fraud.” Do libertarians “usually” use the word “violence”? “Coercion” seems more likely and more appropriate. He continues that “according to NAP, the only legitimate use of force is to prevent or punish the initiatory use of physical violence by others. And fraud is not physical violence.” This is easily answered. A fraud is an aggression because it violates the property rights that the relevant agreement establishes. All this talk about “violence” is merely confused.

5. Parasitic on a Theory of Property

We are told that “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.” In fact, it need not be “parasitic on an underlying theory of property.” It is true that some NAP advocates argue along the following lines: “aggression” is the violation of legitimate property, and legitimate property is only established using assumptions that libertarians independently argue to be legitimate (self‐​ownership, labor‐​mingling ownership, etc.). That is because they don’t have an abstract theory of liberty from which to derive property. However, if we say that libertarian ‘liberty’ is ‘the absence of aggression’, then we can interpret this in a pre‐​propertarian way. Property comes into existence in a libertarian manner when that property does not aggress on (i.e., proactively constrain or interfere with) other people. For instance, I make and claim this spear, hut, rabbit stew, at no cost or loss to you: you are not worse off as a result. And if there is some vestigial cost or loss to others (for instance, you cannot now use the very same natural resources that I did), then we again resort to the MAP. I hope the gist of this view is clear enough (I have written at length in other places to deal with myriad details, but some readers become lost in their own inaccurate paraphrases of the details without first showing that they have grasped the basic problem or the basic idea of the solution). In this way, respecting liberty—as the absence of aggression—can indeed be the “fundamental principle of libertarian ethics.”

By way of illustration, Zwolinski asks us to “Suppose A is walking across an empty field, when B jumps out of the bushes and clubs A on the head … If it’s B’s field, and A was crossing it without B’s consent, then A was the one who was actually aggressing against B.” It seems worth noting that a disproportionately large retaliation itself becomes a new act of aggression. I won’t give a theory of proportionality here, but it is derivable from the NAP/MAP.

Thus we can readily agree with Zwolinski that “‘aggression,’ on the libertarian view, doesn’t really mean physical violence at all.” And we can even agree that “It means ‘violation of property rights’”—as a rule of thumb. But property rights themselves can be derived from whatever control of resources does not aggress, i.e., proactively constrain or interfere with others (or, in the event of a clash, what minimizes such constraints or interferences). Hence, it is false to say that “It is the enforcement of property rights, not the prohibition of aggression, that is fundamental to libertarianism.” As we have now seen, it is liberty itself—interpreted as the absence of interpersonal aggression—that is “fundamental to libertarianism.” That conclusion should not be completely astounding.

6. What About the Children???

Zwolinski then tells us “the NAP implies that there is nothing wrong with allowing your three year‐​old son to starve to death, so long as you do not forcibly prevent him from obtaining food on his own.” An analogy might help to answer this point. A child will not swim in the pool without a lifeguard. You volunteer to be the lifeguard, and as a consequence he gets into the pool. Then to allow the child to drown flouts the claim to your protection that you have previously given him: it is thereby an aggression against the child (positive actions are not always necessary to aggress against the claims we cede to people). In a relevantly similar way, a parent has assumed a duty of care for the vulnerable person that he has brought into existence. Negligently to allow one’s own child to starve to death is to flout that duty and thereby commit an aggression against that child. Therefore, one has a libertarian obligation either to feed him or to discharge the parental duty by finding someone who is willing to take it on. (I have some reservations about this position, but I won’t discuss them here.)

Consequently, it is incorrect to say that the NAP “implies that it would be wrong for others to, say, trespass on your property in order to give the child you’re deliberately starving a piece of bread.” As the starving child is having his given claims aggressed against, anyone has a right to come to his aid in his defense. Any duties that we create by our behavior, including but not limited to explicit contracts, may be coercively enforced if that is what is necessary to minimize any overall aggression.

Zwolinski then sums up his position with a few observations. He first notes that “There’s more to be said about each of these, of course. Libertarians haven’t written much about the issue of pollution.” Is that correct? For what it’s worth, when I typed “pollution” into cato​.org I saw “465 results.” Then he observes that libertarians “can think up a host of ways to tweak, tinker, and contextualize the NAP in a way that makes some progress in dealing with the problems I have raised in this essay.” And, indeed, the Rothbardians have already done this with their interpretation of the NAP. But Zwolinski concludes that “There comes a point where what you need is not another refinement to the definition of ‘aggression’ but a radical paradigm shift in which we put aside the idea that non‐​aggression is the sole, immovable center of the moral universe.” However, this overlooks a third possibility: one can have a paradigm shift within the interpretation of what constitutes “non‐​aggression” (or ‘liberty’). And this is what I claim to provide.

Zwolinski’s concluding sentence is that “Libertarianism needs its own Copernican Revolution.” The analogy is more apposite than he realizes (although, of course, Aristarchus of Samos long antedated Copernicus). For the “Copernican Revolution” that we can have here is to stop trying to theorize “non‐​aggression” (or liberty) ultimately in terms of legitimate property and do the reverse: to theorize legitimate property ultimately in terms of non‐​aggression (or liberty). And with this approach all six given reasons to reject the non‐​aggression principle can be comprehensively refuted.

Yet I fear that this ‘revolution’ is seen as ‘heretical’ by some libertarians—where it has been noticed at all—and this is compounded by my ‘incomprehensible’ rejection of all supposed justifications in favor of the critical‐​rationalist epistemology that I apply (see my earlier reply on this matter). And so I should just like to emphasize that this position is not a criticism of libertarianism or any kind of compromise with non‐​libertarian principles. Rather, it is intended to clarify and unify much currently diverse libertarian theory behind a single principle of liberty itself. With that aim, at least, real libertarians ought to have some sympathy.