The non-aggression principle typically includes property as a part of the owner; to aggress against someone's property is to aggress against the individual. Thus, the principle leads to the rejection of theft, vandalism, murder and fraud. When applied to governments, it has been taken to prohibit many policies including taxation and the military draft. When taken to the logical conclusion, individualist anarchists argue that it calls for abolition of the state itself and protecting individuals from aggression through voluntary payments rather than taxation.
The United States Libertarian Party and others view it as an essential tenet of all libertarian thought, though not all libertarians agree. The principle has been derived by various philosophical approaches, including natural law, utilitarianism, contractarianism, egoism, and Objectivism. Murray Rothbard derived the principle from self-ownership. Ayn Rand derived it from the right to life. However, some espouse the principle as a simple matter of personal preference.
Historically, the non-aggression principle supersedes an earlier formulation by Herbert Spencer, called the law of equal freedom: "Every man is free to do that which he wills, provided he infringes not the equal freedom of any other man." This notion of equal freedom goes back to earlier liberal thought. For example, Mary Wollstonecraft, in "Vindication of the Rights of Men" (1790), wrote, "The birthright of man ... is such a degree of liberty, civil and religious, as is compatible with the liberty of every other individual with whom he is united in a social compact, and the continued existence of that compact."
Nevertheless, there are precursors of the NAP. In On the Duty of Man and Citizen (1682) Samuel von Pufendorf wrote, "Among the absolute duties, i.e., of anybody to anybody, the first place belongs to this one: let no one injure another. For this is the broadest of all duties, embracing all men as such." The Islamic theologians Abu Mansur Al Maturidi, Ibn Qayyim Al-Jawziyya and the philosopher Averroes wrote that man could rationally know that man had a right to life and property. Natural law theorist Murray Rothbard traces the non-aggression principle to natural law theorist St. Thomas Aquinas and the early Thomist scholastics of the Salamanca school In Second Treatise on Government John Locke is the most well known Enlightenment classical liberal to exposit the non-aggression principle. Locke, according to some modern scholars, was influenced by Islamic philosopher Ibn Tufayl who in Hayy ibn Yaqzan discussed the life story of a baby living alone without prior knowledge who discovered natural law, and natural rights, which obliged man not to coerce against another's life or property. Ibn Tufayl influenced Locke's notion of Tabula Rasa. Locke wrote, "Being all equal and independent, no one ought to harm another in his life, health, liberty, or possessions." Thomas Jefferson wrote, "Rightful liberty is unobstructed action according to our will within limits drawn around us by the equal rights of others. I do not add ‘within the limits of the law’, because law is often but the tyrant’s will, and always so when it violates the rights of the individual." and "No man has a natural right to commit aggression on the equal rights of another, and this is all from which the laws ought to restrain him." Another formulation pertaining to government (as opposed to individual conduct) was John Stuart Mill's harm principle, which states 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".
Early formulations that use terms such as "harm" or "injury," such as those of Epicurus and Mill above, are today generally considered imprecise. "Harm" and "injury" are too subjective; one man's harm may be another man's benefit. For example, a squatter may make "improvements" that the owner considers detrimental. Modern formulations avoid such subjectivity by formulating the NAP in terms of individual rights or observable conduct (initiation of force/violence).
Some consequentialists libertarians promote the non-aggression principle by basing its advocacy on forms consequentialism such as rule utilitarianism and rule egoism. These utilitarians do not believe that it is immoral to engage in aggression, but because they believe situations where aggression would lead to the best consequences are rare, they promote the non-aggression principle with the justification that if others accept it as a rule it would lead to better consequences than if they did not accept it as a rule. They believe the consequences of advocating the rule are superior to advocating that other individuals attempt to calculate each of their own actions to determine whether aggression or non-aggression would lead to better consequences. Other consequentialist libertarians do not promote the non-aggression principle at all; they simply believe that allowing a very large scope of political and economic liberty results in the maximum well-being or efficiency for a society, even if securing this liberty involves some governmental actions that would be considered violations of the non-aggression principle. It just so happens that these actions are limited in the free society they envision. This type of libertarianism is associated with Milton Friedman, Ludwig von Mises, and Friedrich Hayek.
Libertarians usually respond by stating that such pragmatic issues are well dealt with already in today's legal systems. For instance, an example of a dissipative game is a situation where a person starts insulting another in a bar, the other person responds by shoving, the other throws a punch, and the situation dissipates into a fight. In such cases, both parties are simply guilty of aggressing on the other because the aggression clearly went beyond self defense.
A version of this second criticism is often upheld by libertarian socialists and others, who point out that almost every patch of land on Earth was stolen (i.e. obtained through initiation of force) at some point in its history. The stolen land was later inherited or sold until it reached its present owners. Thus, property over land and natural resources is based on the initiation of force. Among those who make this argument, some (such as followers of Henry George) claim that private property over natural resources is unique in being based on the initiation of force, while others hold that, by extension, private property over all goods derives from violence, because natural resources are required in the production of all goods.
Libertarians often reply to the "who started it?" problem by contending that the difficulty in determining who is the transgressor should not dissuade us from engaging in that process.
Furthermore, libertarians often reply to either variant of this last point with the "water under the bridge" argument: that transgressions of the past cannot all be rectified, and that an act of theft which happened very long ago can reasonably be ignored - since it is irrelevant to people living today. This, however, can generate more charges of inconsistency, because it implies that peaceful possession of property in the present legitimizes theft and/or trespass in the past the principle known in some contexts as "squatters' rights" and in other contexts as "the right of adverse possession". This requires a "cutoff" point: a point in time when illegitimate property becomes legitimate property. Opponents argue that any such point is arbitrary.
One prominent libertarian (and also anarcho-capitalist theorist), Murray Rothbard, argued that much private property in the U.S. southwest ought to be taken from its present private owners, and given to the lawful heirs of those who had claim to it under Mexican law before the Mexican–American War, although he also regarded this case as historically unique.
Another reply can be made on the basis of the existence of proof of possession and individuality of owners. That is: if current heirs of long dead property owners can identify the property that has been stolen and prove that the owner has passed down to him or her his initial right of property - then those persons must be recognized as the rightful owners. This is similar to the "innocent until proven guilty" proposition of modern law, that owners should be regarded as legitimate until proven differently.
Some libertarians, however, concede that minarchism stemming from the non-aggression principle is logically inconsistent. Because of this, some readily choose to become anarcho-capitalists, since anarcho-capitalism takes the non-aggression principle to its rational and logical end. Others feel that minarchism from a consequentialist libertarian perspective is more cogent and a better alternative to rights-libertarian minarchism.
Inspired by such considerations, some minarchists have suggested establishing social contracts (and taxes or "fees") only at local or regional levels, the smaller the better, since leaving a neighborhood, for example, is less burdensome and is much better at preserving freedom of choice, than having to leave an entire country. Such a system also introduces an element of competition among the various taxing entities to which a monopolistic central government does not have to answer. On the other hand, this system also makes it difficult to undertake any sort of large-scale public project, since an agreement between different regional governments would be needed. The additional challenges would limit the implementation of large-scale public projects to only those with widespread support, perhaps consolidated through an umbrella organization of smaller governments (e.g., the United Nations, European Union, North American Free Trade Agreement, North Atlantic Treaty Organization). Furthermore, lacking a centralized point of taxation control, this system also allows for the creation of "tax havens": if a certain region has little or no local taxes, many of the rich people from neighboring regions may move to it, thus depriving their original home regions of tax revenue. Whether this is good or bad depends on one's political views.
Of course, the existence of a social contract, much like the non-aggression principle, is itself a matter of dispute among the advocates of different political ideologies and views. Many libertarians argue that no contract can exist without conscious and voluntary assent by all participants. American individualist anarchist Lysander Spooner is a noted proponent of this view, while espousing the non-aggression principle as natural law. Spooner holds that the fact that governments hold the threat of force over the heads of those who would not pay taxes, any supposed social contract to pay taxes would not be legitimate since legitimate contracts can only be made in the absence of coercion (i.e., they must be voluntary).
Others argue that a social contract can indeed exist, but that it is an implicit contract between individuals to adhere to the non-aggression principle; these argue that taxation is therefore a violation of the social contract. For example, anarchist Pierre-Joseph Proudhon maintains that rather than being a contract between man and government, the "social contract is an agreement of man with man; an agreement from which must result what we call society" to "abdicate all pretension to govern each other."