You’ve heard libertarians talking about it. You’ve seen the dank memes. But what exactly is the non-aggression principle? What does it do? And why does it get talked about so much?

In this post, I’ll try to explain.

There are many historical antecedents to the NAP, but libertarians usually trace its current formulation to Murray Rothbard, who put it as follows:

The fundamental axiom of libertarian theory is that no one may threaten or commit violence (“aggress”) against another man’s person or property. Violence may be employed only against the man who commits such violence; that is, only defensively against the aggressive violence of another.

In short, no violence may be employed against a nonaggressor. Here is the fundamental rule from which can be deduced the entire corpus of libertarian theory.

The nonaggression principle is sometimes (and confusingly) called an axiom, a practice which Rothbard also began. If we use the standard meaning of the word “axiom,” the NAP is no such thing: an axiom is a statement that is self-evidently true or that cannot reasonably be denied. An example of a philosophical axiom might be something like “I am aware of phenomena” or “modus ponens is a valid form of reasoning.”

The NAP requires more argument than these. Although it may be foundational to libertarian theory – and thus axiomatic in a weaker sense of the term — the NAP clearly does not prove itself; just as clearly, reasonable people can and do deny it. Crucially, the NAP depends on the existence of a definition of aggression; if this definition of aggression is to encompass assaults not committed directly on the body, then the NAP also depends on a valid theory of property ownership. Neither of these is self-evident. Both are contentious topics in political theory about which libertarians offer a specific set of answers, but not everyone will agree with us.

Note that we can’t use the NAP to establish that property may be justly held. Nor can we use it to establish the validity of a particular pattern of property ownership among many — that would be circular: ownership rights cannot acquire the condition of justice simply by asserting that their violation would be unjust.

And yet property claims must derive from something; they seem all but inescapable. Claims about property are found even among animals. The earliest known forms of writing are tallies that were apparently used to keep track of possessions. Contrary to what some on the left may say, no human society appears ever to have been entirely without property.

Indeed, even a wholly communist society would run on the assertion that the whole of the people is the collective owner of all property. By no means does communism lack property claims: on the contrary, its claims in this area are almost impossibly rigid and ubiquitous. How well such a society could instantiate these claims (and what results may come of trying) are different questions entirely. What matters is that even communist societies make claims about property constantly.

If property claims are an inevitable feature of human society, as seems likely, then we cannot escape the question of what status these claims will have, whether collectively or in particular. We must ask not so much whether property is justified, but rather what its extent should be, which objects should be subject to property claims, and which entities within society should be the rightful possessors of what goods, and for what reasons.

John Locke’s theory of property, which has frequently been invoked by classical liberals, holds that property began as a grant of the entire world, from God, to all of humanity in common. Property became private, Locke held, because property existed from the beginning to satisfy human needs, and because private property was apt to satisfy those needs more effectively. Individuals improve private property, a step which they tend not to take with a commons, and thus private property is more apt to the purpose for which property exists in any form.

For those not satisfied by the Lockean account — myself included — David Hume offered a justification for private property that rests on its effects upon human beings:

Who sees not, for instance, that whatever is produced or improved by a man’s art or industry ought, for ever, to be secured to him, in order to give encouragement to such useful habits and accomplishments? That the property ought also to descend to children and relations, for the same useful purpose? That it may be alienated by consent, in order to beget that commerce and intercourse, which is so beneficial to human society? And that all contracts and promises ought carefully to be fulfilled, in order to secure mutual trust and confidence, by which the general interest of mankind is so much promoted?

Examine the writers on the laws of nature; and you will always find, that, whatever principles they set out with, they are sure to terminate here at last, and to assign, as the ultimate reason for every rule which they establish, the convenience and necessities of mankind.

Emphasis added. Societies in which property is privately held will cultivate useful habits and accomplishments in their members: property conduces to virtue. (Hume’s argument here is sometimes taken for a rejection of natural law altogether, but I do not agree. Although it relies on no supernatural justifications, observations about the nature of mankind may indeed form the basis for a type of natural law theory.)

We might add to Hume the further observation that where property is held in common, individuals will often endeavor to live by the labor of others, using the common property as a means to their own ends. The efforts expended in pursuing this strategy, however, are not productive; they do not add to the stock of goods that humanity has at its disposal. In this sense, they represent wasted effort, and the waste is encouraged by the system of common property itself. Similarly, when property is not held in common, but when its tenure is doubtful or insecure, individuals will not exercise the industry needed to improve it for the long term, and this too impoverishes humanity in general.

None of these considerations are likely to be terribly problematic to someone who has grown up in a society where private property predominates. We are used to the usefulness of property.

But there is something about the NAP that is nonetheless politically important, because it serves as an indictment of much government action that is otherwise held to be morally acceptable. The NAP reminds us that theories of property in many of their most common and seemingly inoffensive formulations stand deeply at odds with the justifications for government action that are held by (perhaps) the vast majority of citizens in the modern world. That this vast majority simultaneously holds to something like a Lockean or a Humean conception of private property ought to trouble them enormously: such a conception may call into question the propriety of the state itself.

As Rothbard put it, “The problem is not so much in arriving at [the NAP] as in fearlessly and consistently pursuing its numerous and often astounding implications.” This task has always been the work of the libertarian movement, and it has indeed brought us to some astounding implications, including the idea that taxation is tantamount to theft.

Almost everyone has some theory of property, even if it’s a badly considered one. And almost everyone has a theory of what government ought to do. Pointing out that these theories are usually in conflict with one another is an important move, above all when government is apt to justify itself by arguing that it preserves property rights. Thus, the NAP’s importance is not that it founds a theory of property, but rather that it points out a conflict: considered as classes, theories of property and theories of government usually don’t get along too well. Actions that deprive individuals of property without their consent stand as exceptions to the rule of private property, a rule which most of us generally endorse. And yet “actions that deprive individuals of private property without their consent” are precisely what make governments function.

Forcing people to confront this conflict in their intuitions isn’t trivial work by any means. Resolutions to the conflict may vary, but libertarians can almost be defined as those who refuse to grant special exemptions to the government when private property is at stake. It may be that particular government actions can be justified, but doing so will require a careful revision of our deeper ideas about private property. This sort of revision is almost never actually undertaken by the proponents of state action, and when it is undertaken, it is seldom to the satisfaction of libertarians. Even without fully adopting the libertarian program, others may do well to consider more carefully these conflicting intuitions.