The principle has been derived through various philosophical approaches, including:
Libertarians, propertarians who are pro-life and pro-choice both justify their position on NAP grounds. One question to determine whether or not abortion is consistent with the NAP is at what stage of development a fertilized human egg cell can be considered a human being with the status and rights attributed to personhood. Some supporters of the NAP argue this occurs at the moment of conception while others argue that since the fetus lacks sentience until a certain stage of development, it does not qualify as a human being and may be considered property of the mother. On the other hand, opponents of abortion state that sentience is not a qualifying factor. They refer to the animal rights discussion and point out the argument from marginal cases that concludes the NAP also applies to non-sentient (i.e. mentally handicapped) humans.
Another question is whether an unwelcome fetus should be considered to be an unauthorized trespasser in its mother's body. The non-aggression principle does not protect trespassers from the owners of the property on which they are trespassing.
The NAP is applicable to any unauthorized actions towards a person's physical property. Supporters of the NAP disagree on whether it should apply to intellectual property rights as well as physical property rights. Some argue that because intellectual concepts are non-rivalrous, intellectual property rights are unnecessary while others argue that intellectual property rights are as valid and important as physical ones.
Although the NAP is meant to guarantee an individual's sovereignty, libertarians, propertarians greatly differ on the conditions under which the NAP applies. Especially unsolicited intervention by others, either to prevent society from being harmed by the individual's actions or to prevent an incompetent individual from being harmed by his own actions or inactions, is an important issue.[to whom?] The debate centers on topics such as the age of consent for children, intervention counseling (i.e. for addicted persons, or in case of domestic violence), involuntary commitment and involuntary treatment with regards to mental illness, medical assistance (i.e. prolonged life support vs euthanasia in general and for the senile or comatose in particular), human organ trade, state paternalism (including economic interventionism) and foreign intervention by states. Other discussion topics on whether intervention is in line with the NAP include nuclear weapons proliferation, human trafficking and immigration.
Supporters of the NAP often appeal to it in order to argue for the immorality of theft, vandalism, sexual assault, assault, and fraud. Compared to nonviolence, the non-aggression principle does not preclude violence used in self-defense or defense of others. Many supporters argue that NAP opposes such policies as victimless crime laws, taxation, and military drafts. NAP is the foundation of propertarian and libertarian philosophy.
NAP faces two kinds of criticism: the first holds that the principle is immoral, and the second argues that it is impossible to apply consistently in practice; respectively, consequentialist or deontological criticisms, and inconsistency criticisms. Libertarian, propertarian academic philosophers have noted the implausible results consistently applying the principle yields: for example, Professor Matt Zwolinski notes that, because pollution necessarily violates the NAP by encroaching (even if slightly) on other people's property, consistently applying the NAP would prohibit driving, starting a fire, and other activities necessary to the maintenance of industrial society.
The NAP also faces definitional issues regarding what is understood as forceful interference and property, and under which conditions it applies. The NAP has been criticized as circular reasoning and a rhetorical obfuscation of the coercive nature of propertarian/right-libertarian property law enforcement because the principle redefines aggression in their own terms.
Brennan argued that the non-aggression principle is a simple moral slogan and therefore difficult to persuade people. He pointed out that this principle can be interpreted in various ways depending on how the concepts of ownership and rights implied in it are defined, and that it is actually used in various ways among people in reality. Therefore, he argued that it is difficult to function as a clear guideline or implementation strategy for realizing an ideal liberty society.
Critics argue that the non-aggression principle is not ethical because it opposes the initiation of force even when they would consider the results of such initiation to be morally superior to the alternatives that they have identified. In arguing against the NAP, philosopher Matt Zwolinski has proposed the following scenario: "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. Even if we grant that taxation is aggression, and that aggression is generally wrong, is it really so obvious that the relatively minor aggression involved in these examples is wrong, given the tremendous benefit it produces?"
Zwolinski also notes that the NAP is incompatible with any practice that produces any pollution, because pollution encroaches on the property rights of others. Therefore, the NAP prohibits both driving and starting fires. Citing David D. Friedman, Zwolinski notes that the NAP is unable to place a sensible limitation on risk-creating behavior, arguing:
Some supporters argue that no one initiates force if their only option for self-defense is to use force against a greater number of people as long as they were not responsible for being in the position they are in. Murray Rothbard's and Walter Block's formulations of NAP avoid these objections by either specifying that the NAP applies only to a civilized context (and not "lifeboat situations") or that it applies only to legal rights (as opposed to general morality). Thus a starving man may, in consonance with general morality, break into a hunting cabin and steal food, but nevertheless he is aggressing, i.e., violating the NAP, and (by most rectification theories) should pay compensation. Critics argue that the legal rights approach might allow people who can afford to pay a sufficiently large amount of compensation to get away with murder. They point out that local law may vary from proportional compensation to capital punishment to no compensation at all.
Other critics state that the NAP is unethical because it does not provide for the violent prohibition of, and thereby supposedly legitimizes, several forms of aggression that do not involve intrusion on property rights such as verbal sexual harassment, defamation, boycotting, noninvasive striking etc. If a victim thus provoked would turn to physical violence, they would be labeled an aggressor according to the NAP. However, supporters of the NAP state that boycotting and defamation both constitute freedoms of speech and that boycotting, noninvasive striking and noninvasive discrimination all constitute freedoms of association and that both freedoms of association and of speech are nonaggressive. Supporters also point out that prohibiting physical retaliation against an action is not itself condonement of said action, and that generally there are other, nonphysical means by which one can combat social ills (e.g., discrimination) that do not violate the NAP. Some supporters also state that while most of the time individuals choose voluntarily to engage in situations that may cause some degree of mental battering, this mental battering begins to constitute unauthorized physical overload of the senses (i.e., eardrum and retina) when it cannot be avoided and that the NAP at that point does apply.
Many supporters consider verbal and written threats of imminent physical violence sufficient justification for a defensive response in a physical manner. Those threats would then constitute a legitimate limit to permissible speech. Because freedom of association entails the right of owners to choose who is permitted to enter or remain on their premises, legitimate property owners may also impose limitation on speech. The owner of a theatre wishing to avoid a stampede may prohibit those on her property from calling 'fire!' without just cause. However, the owner of a bank may not prohibit anyone from urging the general public to a bank run, except insofar as this occurs on the property of said owner.
Supporters also consider physical threats of imminent physical violence (e.g. pointing a firearm at innocent people, or stocking up nuclear weapons that cannot be used discriminately against specific individual aggressors) sufficient justification for a defensive response in a physical manner. Those threats would then constitute a legitimate limit to permissible action.
Also called the non-aggression axiom, the non-coercion principle, the non-initiation of force and the zero-aggression principle.
Property is defined in this context as both personal possessions and private property. /wiki/Private_property
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Murray N. Rothbard, "Freedom of Speech," §1 of "Personal Liberty," ch. 6, in "Propertarian Applications to Current Problems," pt. 2 of For a New Liberty: The Libertarian Manifesto (Auburn, AL: Ludwig von Mises Institute, 2006; orig. 1973, 1978), pp. 116–17. /wiki/Murray_N._Rothbard
Murray N. Rothbard, "Knowledge, True and False," ch. 16, in "A Theory of Liberty," pt. 2 of The Ethics of Liberty (New York, N. Y.: New York University Press, 1998; orig. 1982), pp. 121–122, 126–28. /wiki/Murray_N._Rothbard
Murray N. Rothbard, "Freedom of Speech," §1 of "Personal Liberty," ch. 6, in "Propertarian Applications to Current Problems," pt. 2 of For a New Liberty: The Libertarian Manifesto (Auburn, AL: Ludwig von Mises Institute, 2006; orig. 1973, 1978), pp. 117–18. /wiki/Murray_N._Rothbard
Murray N. Rothbard, "The Boycott," ch. 18, in "A Theory of Liberty," pt. 2 of The Ethics of Liberty (New York, N. Y.: New York University Press, 1998; orig. 1982), pp. 131–32. Cf., pp. 77, 79, 240. /wiki/Murray_N._Rothbard
Murray N. Rothbard, "The Boycott," ch. 18, in "A Theory of Liberty," pt. 2 of The Ethics of Liberty (New York, N. Y.: New York University Press, 1998; orig. 1982), pp. 131–32. Cf., pp. 77, 79, 240. /wiki/Murray_N._Rothbard
Murray N. Rothbard, "Anti-Strike Laws," §4 of "Involuntary Servitude," ch. 5, in "Propertarian Applications to Current Problems," pt. 2 of For a New Liberty: The Libertarian Manifesto (Auburn, AL: Ludwig von Mises Institute, 2006; orig. 1973, 1978), pp. 102–04. Cf., pp. 93, 118. Cf., The Ethics of Liberty, pp. 77, 132. /wiki/Murray_N._Rothbard
Murray N. Rothbard, "Street Rules," §2 of "The Public Sector, II: Streets and Roads," ch. 11, in "Libertarian Applications to Current Problems," pt. 2 of For a New Liberty: The Libertarian Manifesto (Auburn, AL: Ludwig von Mises Institute, 2006; orig. 1973, 1978), pp. 255–256. Cf., pp. 103, 128. /wiki/Murray_N._Rothbard
Murray N. Rothbard, in "Natural Law and Natural Rights," ch. 4, in "Introduction: Natural Law," pt. 1 of The Ethics of Liberty (New York, N. Y.: New York University Press, 1998; orig. 1982), p. 24. "Right" has cogently and trenchantly been defined by Professor Sadowsky: When we say that one has the right to do certain things we mean this and only this, that it would be immoral for another, alone or in combination, to stop him from doing this by the use of physical force or the threat thereof. We do not mean that any use a man makes of his property within the limits set forth is necessarily a moral use?
Sadowsky's definition highlights the crucial distinction we shall make throughout this work between a man's right and the morality or immorality of his exercise of that right. We will contend that it is a man's right to do whatever he wishes with his person; it is his right not to be molested or interfered with by violence from exercising that right. But what may be the moral or immoral ways of exercising that right is a question of personal ethics rather than of political philosophy—which is concerned solely with matters of right, and of the proper or improper exercise of physical violence in human relations. The importance of this crucial distinction cannot be overemphasized. Or, as Elisha Hurlbut concisely put it: "The exercise of a faculty by an individual is its only use. The manner of its exercise is one thing; that involves a question of morals. The right to its exercise is another thing." Cf., pp. 25, 77, 79, 98 (note 2), 100–101, 107, 121, 124 (note 2), 127, 131–133, 136, 138, 142, 146, 151–153, 173–174, 220, 222. /wiki/Murray_N._Rothbard
Murray N. Rothbard, "The Boycott," ch. 18, in "A Theory of Liberty," pt. 2 of The Ethics of Liberty (New York, N. Y.: New York University Press, 1998; orig. 1982), pp. 131–32. Cf., pp. 77, 79, 240. /wiki/Murray_N._Rothbard
Murray N. Rothbard, "Street Rules," §2 of "The Public Sector, II: Streets and Roads," ch. 11, in "Libertarian Applications to Current Problems," pt. 2 of For a New Liberty: The Libertarian Manifesto (Auburn, AL: Ludwig von Mises Institute, 2006; orig. 1973, 1978), pp. 255–256. Cf., pp. 103, 128. /wiki/Murray_N._Rothbard
Dutton, Donald G. (2006). Rethinking Domestic Violence. Vancouver, BC, Canada: UBC Press. ISBN 978-1282741072. 978-1282741072
Murray N. Rothbard, "Self-Defense," ch. 12, in "A Theory of Liberty," pt. 2 of The Ethics of Liberty (New York, N. Y.: New York University Press, 1998; orig. 1982), pp. 77–78, 80. Cf., For a New Liberty, p. 27. /wiki/Murray_N._Rothbard
Linda & Morris Tannehill, The Market for Liberty (San Francisco: Fox & Wilkes, October 1993; orig. March 1970), pp. 4, 10. Cf., pp. 77, 80. /wiki/Linda_%26_Morris_Tannehill
Murray N. Rothbard, "Property Rights and "Human Rights"," §5 of "Property and Exchange," ch. 2, in "The Libertarian Creed," pt. 1 of For a New Liberty: The Libertarian Manifesto (Auburn, AL: Ludwig von Mises Institute, 2006; orig. 1973, 1978), pp. 52–53. Cf., pp. 85–86, 115. /wiki/Murray_N._Rothbard
Murray N. Rothbard, "Freedom of Speech," §1 of "Personal Liberty," ch. 6, in "Propertarian Applications to Current Problems," pt. 2 of For a New Liberty: The Libertarian Manifesto (Auburn, AL: Ludwig von Mises Institute, 2006; orig. 1973, 1978), pp. 117–18. /wiki/Murray_N._Rothbard
Ben's Blog. "Fred Phelps, Freedom of Speech and "the Last Limits of the Endurable" (Ben's Blog on Infinite Monkeys, Fri, 10/08/2010)". Archived from the original on 2012-01-11. Retrieved 2011-11-26. https://web.archive.org/web/20120111041922/http://blog.infinitemonkeysblog.com/?q=node%2F7319
Murray N. Rothbard, "Self-Defense," ch. 12, in "A Theory of Liberty," pt. 2 of The Ethics of Liberty (New York, N. Y.: New York University Press, 1998; orig. 1982), pp. 77–78, 80. Cf., pp. 81–82, 189–191, 194. /wiki/Murray_N._Rothbard
Murray N. Rothbard, "The Nonaggression Axiom," §1 of "Property and Exchange," ch. 2, in "The Libertarian Creed," pt. 1 of For a New Liberty: The Libertarian Manifesto (Auburn, AL: Ludwig von Mises Institute, 2006; orig. 1973, 1978), p. 27. Cf., p. 335. /wiki/Murray_N._Rothbard
Linda & Morris Tannehill, The Market for Liberty (San Francisco: Fox & Wilkes, October 1993; orig. March 1970), pp. 4, 10. Cf., pp. 77, 80. /wiki/Linda_%26_Morris_Tannehill
Murray N. Rothbard. "Law, Property Rights, and Air Pollution (Cato Journal, Vol. 2, No. 1, Spring 1982, pp. 55–99)" (PDF). Archived from the original (PDF) on 2011-09-11. Retrieved 2011-11-24. https://web.archive.org/web/20110911075448/http://www.cato.org/pubs/journal/cj2n1/cj2n1-2.pdf
Graham Dawson. "Free Markets, Property Rights and Climate Change: How To Privatize Climate Policy (Libertarian Papers, Vol. 3, No. 10, 2011, pp. 1–29)" (PDF). Retrieved 2011-12-04. http://libertarianpapers.org/articles/2011/lp-3-10.pdf
Hillel Steiner (24 April 2012). "Left-Libertarianism and the Ownership of Natural Resources (Bleeding Heart Libertarians, April 24, 2012)". Retrieved 2012-07-07. http://bleedingheartlibertarians.com/2012/04/left-libertarianism-and-the-ownership-of-natural-resources/
Eric Mack (23 April 2012). "Natural Rights and Natural Stuff (Bleeding Heart Libertarians, April 23, 2012)". Retrieved 2012-07-07. http://bleedingheartlibertarians.com/2012/04/natural-rights-and-natural-stuff/
Roderick Long (26 April 2012). "Self-Ownership and External Property (Bleeding Heart Libertarians, April 25, 2012)". Retrieved 2012-07-07. http://bleedingheartlibertarians.com/2012/04/self-ownership-and-external-property/
David D. Friedman. "Problems (The Machinery of Freedom, 1989, Chapter 41)". Retrieved 2013-01-03. http://www.daviddfriedman.com/Libertarian/Machinery_of_Freedom/MofF_Chapter_41.html