Initiation of force

The non-aggression principle (or NAP; also called the non-aggression axiom, the anti-coercion, zero aggression principle or non-initiation of force) is an ethical stance which asserts that "aggression" is inherently illegitimate. "Aggression", for the purposes of NAP, is defined as initiating or threatening the use of any and all forcible interference with an individual or individual's property. In contrast to pacifism, the non-aggression principle does not preclude violent self-defence. The NAP is considered to be a defining principle of natural-rights libertarianism.

The non-aggression principle has existed in various forms. Although the principle has been traced back as far as antiquity, it was first formally described by this name by the Objectivist philosopher Ayn Rand, and then further popularized by libertarian thinkers.

A number of authors have created their own formulation of the non-aggression principle, as shown in the table below.

The principle has been derived by various philosophical approaches, including:

Many supporters and opponents of abortion rights justify their position on NAP grounds. One important question to determine whether or not abortion is consistent with 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 NAP argue this occurs at the moment of conception. Others argue that since they believe the fetus lacks sentience until a certain stage of development, it does not qualify as a human being, and as such may be considered property of the mother. Opponents of abortion, on the other hand, state sentience is not a qualifying factor. They refer to the animal rights discussion and point out the Argument from marginal cases that concludes NAP also applies to non-sentient (i.e. mentally handicapped) humans.

Another important 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. It can also be argued that unwelcome fetuses are themselves committing aggression against their mothers, by taking materials (oxygen, water, nutrients) from her bloodstream, by injecting toxic metabolic end-products (carbon dioxide and creatinine) into her bloodstream, and by preparing to subject her to major medical/surgical trauma in the form of full-term labor and delivery.

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