Non-​aggression Principle

The non-​aggression axiom is an ethical principle often appealed to as a basis for libertarian rights theory. The principle forbids “aggression,” which is understood to be any and all forcible interference with any individual’s person or property except in response to the initiation (including, for most proponents of the principle, the threatening of initiation) of similar forcible interference on the part of that individual.

The axiom has various formulations, but two especially influential 20th-​century formulations are those of Ayn Rand and Murray Rothbard, who appear to have originated the term. Ayn Rand maintained that “no man may initiate the use of physical force against others.… Men have the right to use physical force only in retaliation and only against those who initiate its use.” This quote is similar to Murray Rothbard’s thesis that “no man or group of men may aggress against the person or property of anyone else.”

Some libertarians use the term coercion as synonymous with aggression, whereas others use coercion more broadly to designate all use of force, including legitimate defensive use. Hence, under the former formulation, but not the latter, the non-​aggression axiom would prohibit all coercion. Although the ordinary sense of coercion arguably involves getting somebody to do something—so that simply assaulting somebody would not count as coercion—conformity with this usage is the exception, rather than the rule, in libertarian theory.

The axiom is often regarded as virtually equivalent to Herbert Spencer’s law of equal freedom (“Every man has freedom to do all that he wills, provided he infringes not the equal freedom of any other man”), or to the principle of self-​ownership, or both inasmuch as all three principles specify protected boundaries around each individual, ordinarily understood to include not just the individual’s mind and body, but also legitimately acquired external property. Within these boundaries, people are to be allowed complete liberty from forcible interference by others, the extent of one individual’s boundary being limited only by the similar boundaries of others.

Actions that might otherwise ordinarily count as aggression against an individual become permissible if the individual consents, although libertarian theorists disagree among themselves as to whether and under what circumstances such consent can be irrevocable (e.g., is a contract to alienate one’s rights over oneself legitimate?). To say that an action is permissible under the axiom, it should be noted, is simply to say that the action is not a rights violation and so may not legitimately be obstructed by force. The non-​aggression axiom does not rule out such an action’s possible moral wrongness on other grounds or the possible appropriateness of attempting to combat it by peaceful means. The non-​aggression axiom is intended as a rule specifically for actions involving force, not as a guide to the whole of moral conduct.

This axiom is intended, however, to govern the actions not only of private citizens, but of government officials. Hence, the enforcement of laws or regulations requiring anything more from individuals than their bare abstention from aggression counts as aggression and so is prohibited under the principle. The entire range of libertarian rights to personal and economic liberty is thus taken to follow from the non-​aggression axiom.

The non-​aggression axiom is not to be confused with a call to minimize the total amount of aggression—first, because the axiom is purely prohibitory and does not call for positive action of any kind; and, second, because the axiom does not countenance, as a minimization requirement might, the inflicting of a small amount of aggression to prevent a greater amount (e.g., conscripting citizens to deter foreign invasion). The prohibition on aggression thus counts, in Robert Nozick’s terminology, as a side constraint to be respected, rather than a goal to be promoted. To be sure, adherents of the non-​aggression axiom unsurprisingly tend to favor the overall reduction of aggression in society—so long as such reduction can be accomplished without violating the axiom—but the non-​aggression axiom per se calls for no such commitment.

Some formulations of the non-​aggression axiom make specific reference to external property while others do not, but it is widely agreed that the application of the axiom requires some additional principle to determine when the use of a person’s possessions without permission counts as aggression. Unless such use does count as aggression, forcibly preventing unpermitted use will violate the axiom. How, then, must one be related to an external object so that another’s appropriation of that object constitutes an illegitimate appropriation of the possessor? Many attempts to answer this question draw on or develop John Locke’s theory that one acquires just ownership by either mixing one’s labor with previously unclaimed resources or acquiring resources by consent from already legitimate possessors. Samuel Wheeler, for example, argues that external property is an artificial extension of one’s body and so entitled to the same protection as bodily integrity, whereas Nozick maintained that seizing the products of another person’s labor is tantamount to forcing that person to labor for one’s own benefit. Thus, theft is condemned as an indirect form of force, whereas fraud is typically condemned as an indirect form of theft (inasmuch as a transfer of property to which consent is obtained under false pretenses is tantamount to taking property without consent).

Although the non-​aggression axiom prohibits initiatory force, it does not specify what forms of retaliatory force, if any, are permissible, and so is in principle compatible with a variety of conclusions on this issue. These conclusions include Rothbard’s view that the victim may inflict on the aggressor an amount of force proportionate to that which the aggressor had inflicted; Randy Barnett’s milder view that aggressors may be coerced only insofar as it is necessary to restrain their aggression and secure restitution to the victim; and Robert LeFevre’s belief that all force, whether initiatory or retaliatory, is morally impermissible. Thus, the non-​aggression axiom by itself does not specify whether its own enforcement is permissible, although it does specify that no other principle could be permissibly enforced because to enforce anything other than non-​aggression is a form of aggression.

Proponents of the principle differ as to the basis of its justification, and a variety of defenses have been offered. For example, Nozick upholds non-​aggression as an application of the duty to treat persons as ends in themselves, rather than as mere means. Rothbard argued that to aggress against another person is to treat that person as one’s property, thus introducing an asymmetry of rights inconsistent with the requirement that ethical norms be universalizable. Jan Narveson has maintained that a mutual rejection of non-​aggression would be endorsed by rationally self-​interested contractors. Ayn Rand condemned aggression as a form of parasitism inconsistent with the independent mindset needed for an individual’s successful living. Utilitarian libertarians argue, often on economic grounds, that a general commitment to non-​aggression will tend to maximize social welfare. Douglas Den Uyl and Douglas Rasmussen regard the prohibition of aggression as part of a “metanormative” framework to protect the conditions within which individuals can pursue their own Aristotelian flourishing. Hans-​Hermann Hoppe holds that inasmuch as the justification of any proposition presupposes a context of uncoerced interpersonal dialogue, no assertion of the right to aggress can be justified without self-​contradiction. Less theoretically, the non-​aggression axiom is often held to be simply a consistent application of the commonsense norms that govern ordinary personal morality; that we usually deal with our neighbors through persuasion rather than compulsion.

Other controversies over the axiom include what exceptions, if any, may be made to the axiom in emergencies, and whether the axiom permits the use of force against innocent shields. This question arises when collateral damage to bystanders cannot be avoided in the course of self-​defense against an aggressor. A further question concerns so-​called innocent threats; that is, the actions of those who aggress through no fault of their own, and whether these threats are to be considered aggression and, therefore, illegitimate.

The non-​aggression axiom has “a long past but a short history.” In some form, a prohibition on aggression recurs frequently throughout human history—as one might expect if it is indeed a generalization of commonsense moral norms. For example, the principle of ahimsa (nonviolence, noninjury) is central to Hinduism, Buddhism, and Jainism, while the notion of justice as a mutual non-​aggression pact is put forward by the Greek philosophers Lycophron and Epicurus, as well as by the character Glaucon in Plato’s Republic. The Institutes of the Byzantine Emperor Justinian define the essence of legal obligation as “to live honestly, to injure no one, and to give every man his due,” whereas in China, Kao-​tsu, the founder of the Han dynasty, announced that the only valid laws were those against murder, theft, and personal injury. But in practice the actual content of legislation generally far outstripped these suggested limits; more broadly, the invocation of a non-​aggression principle was seldom applied consistently, having usually been coupled with the endorsement of institutions and practices (e.g., slavery) that seem strikingly inconsistent with it.

It is in the 17th century that the prohibition on aggression began to bear radical political fruit. Precursors of the non-​aggression axiom were employed in support of revolutionary liberalism by writers like Richard Overton, who wrote that “every man by nature [is] a king, priest and prophet in his own natural circuit and compass, whereof no second may partake but by deputation, commission, and free consent from him whose natural right and freedom it is.” John Locke wrote similarly that “being all equal and independent, no one ought to harm another in his life, health, liberty or possessions.” With the classical liberals and individualist anarchists of the 19th century, the axiom became the foundation of a thoroughgoing libertarian political program; American anarchist Benjamin R. Tucker, for example, described his fundamental political principle as “the greatest amount of liberty compatible with equality of liberty; or, in other words, as the belief in every liberty except the liberty to invade.”

There are a small group of libertarians who do not accept the non-​aggression axiom. Its critics, including some libertarians, charge that it offers too simplistic an approach to the complexities of social life and ignores context; that it is illegitimately absolutistic, disallowing uses of force that might bring beneficial consequences; or that it cannot be unambiguously applied without appeal to additional ethical principles. Not all proponents of the axiom regard this last comment as an objection.

Another objection focuses on the term axiom, which is sometimes taken to imply that the prohibition of aggression enjoys a special epistemic status analogous to that of the law of noncontradiction (e.g., that it is self-​evident or knowable a priori, or a presupposition of all knowledge, or that it cannot be denied without self-​contradiction). Although some proponents of the prohibition do indeed claim such a status for it, many do not. Accordingly, it is sometimes suggested that non-​aggression principle or zero aggression principle is a more accurate label than non-​aggression axiom.

Nevertheless, an axiom also can denote a foundational presupposition of a given system of thought even if it rests on some deeper justification outside that system. For example, Isaac Newton described his fundamental laws of motion as axioms within his deductive system of mechanics, yet regarded them as grounded empirically. In this sense, non-​aggression might legitimately be regarded as an axiom of libertarian rights theory, regardless of what one takes its ultimate justification to be.

The non-​aggression principle must be distinguished from a number of popular moral principles easily confused with it. The golden rule (“Do unto others as you would have them do unto you”), unlike the non-​aggression axiom, does not distinguish between negative and positive obligations. Again unlike the axiom, it does not clearly rule out paternalistic legislation because paternalists might sincerely prefer that they be coerced should they, in the future, stray from what they presently regard as the true path. The non-​aggression axiom also should not be confused with John Stuart Mill’s “harm principle,” which specifies that “the only purpose for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others.” Despite their similarity, the two principles are arguably not equivalent. First, harm seems to be a broader concept than aggression: outcompeting an economic or romantic rival is not aggression, but might count as harm. Second, Mill’s principle does not specify that the person to be coerced in order to prevent harm must be the author of the harm to be prevented. The non-​aggression axiom also should be distinguished from Immanuel Kant’s categorical imperative that persons are to be treated as ends in themselves and never as mere means. Kant’s requirement is broader because it forbids all forms of manipulative and degrading treatment (even when those so treated consent) and not aggression alone. Finally, the non-​aggression axiom is distinct from John Rawls’s principle that each person is to have an equal right to the most extensive basic liberty compatible with similar liberty for others because Rawls explicitly excludes from his notion of “basic liberty” the freedom to do as one likes with one’s property.

Further Readings

Hoppe, Hans-​Hermann. The Economics and Ethics of Private Property. 2nd ed. Auburn, AL: Ludwig von Mises Institute, 2006.

Nozick, Robert. Anarchy, State, and Utopia. New York: Basic Books, 1974.

Rand, Ayn. The Virtue of Selfishness: A New Concept of Egoism. New York: Signet, 1964.

Rothbard, Murray N. The Ethics of Liberty. New York: New York University Press, 2003.

———. For a New Liberty: The Libertarian Manifesto. Rev. ed. New York: Collier, 1978.