Many libertarians, though, expect much more out of the NAP. They want it to serve as the fundamental axiom of justice, from which (along with the corollary principles of Lockean homesteading and free exchange) can be derived the blueprints for the ideal polity.

Alas, life is much more complicated than that. First of all, the NAP leaves unanswered a whole host of vitally important questions about how the rules of society should be structured. Meanwhile, the implications of strict fidelity to the NAP are sometimes unacceptable even where they are unambiguous.

Matt Zwolinski has covered much of this territory already (see here, here, and here), but let me offer my own take. Much of the indeterminacy of the NAP results from the fact that what counts as aggression depends on how property rights are defined. For example, A enters a house and B, already in the house, shoots and kills A. Who is the aggressor here looks very different depending on whether A or B owns the house. And any elucidation of property rights sufficient to guide behavior and resolve disputes–in short, to establish a working legal order–will regularly confront ambiguities, gray areas, and lacunae that must be resolved by recourse to some additional principle or principles.

How far below the surface should property rights in land extend? How high into the sky? To what extent can an owner of land bind future owners and restrict their ability to sell? Specifically, is fee tail an enforceable type of ownership? Is the rule against perpetuities a restriction on the rights of ownership or a protector of those rights’ integrity? Can ownership of land be lost by abandonment, or is the rule of adverse possession an impingement on property rights? If possession of land was acquired by force, how much time must pass and how many times must the land change hands before the rights of the original owners and their descendants are extinguished and the current possessors are deemed the legal owners? When do duties to indemnify the dispossessed for their losses expire?

What about pollution or other nuisances? If any physical invasion of another’s property constitutes an unlawful trespass, then things look grim for industrial civilization: a single molecule of effluent blown from my factory smokestack across your property constitutes a violation of your property rights. Such an interpretation leads to absurd outcomes, as even photons from my indoor lighting may constitute trespasses as they cross your yard. But if we forsake a bright‐​line rule, where do we draw the line? The common law of nuisance regards as actionable only “unreasonable” interference with quiet enjoyment of one’s property. How to give content to that weasel word?

And what about actions that create the risk of harm or evince the intention to cause harm? If you are trying to build a nuclear bomb in your garage next door, do I have a right to complain? If you shoot at me in the street but miss, have you committed an offense against me or is it no harm, no foul? Is planning a crime itself a crime? If so, how far must the planning proceed before an infraction has been committed?

What about unintentional harms? If some action of yours leads to harm, are you culpable no matter what? Or do you have to be negligent for your actions to violate the rights of the person you harmed? How does one define negligence? Here the common law punts again with its weaselly standard of “lack of reasonable care.”

What about children? Negative rights of security against aggression aren’t enough for kids; they need someone else to care for them. What is their legal status? What are the obligations of their parents? What happens if their parents fail to meet those obligations?

Finally, what about remedies? What social institutions will enforce rights and protect people against aggression? Here the indeterminacy of the NAP is glaringly apparent, as libertarians who claim it as their pole star are divided over whether a government with a monopoly over enforcing laws is necessary. Minarchists claim that, without government, the NAP is a dead letter; anarchists retort that a government is unnecessary and its establishment runs directly counter to the NAP. Minarchists think that anarchy is utterly impractical; anarchists think limited government violates the NAP and is impractical itself to boot as government won’t stay limited. Whatever the merits of their respective cases, the NAP won’t settle the matter.

Moving beyond this basic point, how rights are enforced–by whatever governmental or private agency–has a huge bearing on the real‐​world significance of those rights, yet the NAP tells us nothing about how to enforce the rights that give it content. Can ongoing harmful activity be enjoined, or are money damages sufficient? How are damages calculated? Must all ensuing damages, or only “foreseeable” ones, be compensated? Should there be a distinction between “public” crimes and “private” wrongs? Is imprisonment a valid form of enforcement? Is it imposed in lieu of or in addition to restitution? What about other forms of punishment? How is guilt determined in criminal cases? What are the standards of evidence and burdens of proof? What are the concrete institutional arrangements (like the writ of habeas corpus, or the right to a court‐​appointed attorney) that flesh out how crimes are adjudicated? What are the arrangements for resolving civil disputes?

Libertarian adherents of the NAP’s axiomatic status sometimes argue that common‐​law dispute settlement and prevailing conventions can fill in all the blanks of the legal order. Yes, they can, but there’s no guarantee that the results will look anything remotely like what contemporary libertarians have in mind when they envision a free society. Perhaps much of the land will be locked up in giant feudal estates. Or maybe quiet enjoyment of one’s property is protected with sufficient vigor to make any kind of industrial activity impossible because it constitutes an enjoinable nuisance. And if nuisance law alone weren’t enough to maintain bucolic purity (or, in other words, grinding poverty), strict liability for all calculable damages resulting directly or indirectly from unintentional harms could lend its support. It’s worth recalling that the English common law, for all its impressive features, nonetheless recognized fee tail, maintained rules against buying goods and reselling them for a profit (under the common law offenses of engrossing, forestalling, and regrating), and provided for the subjugation of married women through the doctrine of coverture (a woman loses her separate legal identity upon marriage).

The idea of the NAP as a fundamental axiom amounts to a dream of life without politics: there exists one neutral, value‐​free basic principle that can generate an entire neutral, value‐​free system of rights and obligations. The legitimacy of the legal order, in this view, comes from nature: the order commands our assent because it recognizes and vindicates our pre‐​existing natural rights. All disputes are logical disputes over what specific rules needed to cover a given situation are implicit in the NAP. Value‐​laden disputes over rival conceptions of the good are irrelevant and out of bounds.

It’s a dream of understandable appeal, given the inevitability of moral squalor and distinct possibility of outright horror that come with politics. But it is a delusion all the same, for the truth is that politics is unavoidable. There is no possibility of constructing a legal order without taking sides repeatedly in value‐​laden controversies: some visions of the good will win, and others will lose. And in the large and complex social orders of modernity, value pluralism is inevitable, so political disputes between groups with rival conceptions of the good are inextinguishable. Politics just is the process whereby some people impose their values on others, and there’s no getting around it. The legitimacy of a political order therefore hinges, not just on the content of the rules, but on the process whereby the inevitable value conflicts over rules are resolved. Political decision‐​making must be structured so that the losers will respect the results because the process was sufficiently inclusive and fair.

The NAP’s problems are not limited to indeterminacy. In addition, strict fidelity to its unambiguous implications leads in numerous instances to results that almost everybody, including most libertarians, would reject out of hand. In the spirit of charity, I’ve treated the NAP’s positions on pollution, risk, and children as ambiguous rather than absurd, but a commitment to hew to bright‐​line rules in literal conformity with the NAP can certainly be imagined. In that case, virtually all industrial activity would violate somebody’s rights, but you are free to stockpile explosive materials in your place of business even if an accident would wipe out the whole block. You’d have done nothing wrong if you conspired to commit a crime or attempted a crime but failed. And psychopathic parents could starve their kids for fun.

In other cases, there is no escape hatch of ambiguity to rescue the NAP from absurd or repugnant implications. It seems quite clear that there could no legal enforcement of contracts in the way we are familiar with: restitution could be ordered when actual money has changed hands, but in a mere exchange of promises there could be no NAP‐​consistent remedy in the event of a breach. Harboring a fugitive would not be a crime. Neither would blackmail. Neither would even the most heinous cruelty to animals. There could be no bankruptcy law that extinguishes debts. Tort liability for corporations would either be unlimited or nonexistent (in which case only the individual employees responsible would be liable). There could be no compulsory vaccines during epidemics. And of course, the NAP proscribes taxation as well as any government spending to alleviate poverty, promote education, or do anything else besides protecting persons and property. Although radical libertarians may be willing to bite these last bullets, nobody else is.

For all the reasons provided above, I conclude that NAP fundamentalism is utterly unworkable as a basis for libertarian thought. What then do I suggest as an alternative? Rather than searching vainly for some value‐​free intellectual foundation, I begin with an explicit commitment to liberal values: a belief in the moral worth of the individual, and a vision of social progress in which more and more people are flourishing in lives of their own choosing. And rather than grounding my political views in speculations about some hypothetical (and, to my mind, chimerical) utopia, I look to history and the social sciences for guidance as to how best to advance those liberal values in the specific circumstances that confront us today. In my view, the case for a libertarian approach to politics rests on its combination of moral seriousness and empirical soundness: its principled commitment to individual autonomy, as well as its insightful recognition of both the pitfalls of centralized control and the immense fertility of decentralized experimentation, competition, and coordination.

Yes, the approach I suggest is messy and fuzzy–not too many bright‐​line rules, lots of slippery slopes and difficult tradeoffs. But whoever said life was going to be simple?