This is where you should expect to find elaborations on various definitions, justifications, and forms of Non-Aggression Principle. Other matters of theory subordinate to theories of the NAP itself may also be collected here. Skip to the bottom of the page for links to essays concerned with NAP theory.
While most of us refer to "The NAP", or "The Non-Aggression Principle", the truth is that there are many non-aggression principles. They have in common that they all depend on some definitions of what constitutes "aggression", as mentioned on the homepage for this site, and ethically prohibit any acts that conform to such definitions. Any definitions that do not involve aggression being confined to cases of initiating whatever coercive acts qualify for "aggression" are generally outside the realm of qualification for the NAP as it is understood philosophically and politically, though some decidedly heterodox definitions may require absolute pacifism.
The following NAP formulations are simplified to get at the gist of their meanings and applications. Some details may be misrepresented in the interests of brevity, but not with any intent to mislead. These are distinct from justifications, which may come from a variety of philosophical sources such as rule utilitarianism, argumentation ethics (see Hoppe), natural rights theory (see Rothbard), various social contract mechanisms, and moral caution, among others.
Murray Rothbard, whose NAP theory is often held up as the canonical example, proposed the idea that self-ownership -- and by extension all property right -- is a fundamental condition of our existence. The violation of this constitutes an intiation of force; that initiation of force is aggression. Central to this is the idea that all private property is a matter of fundamental right, and any violation of such a right is aggression.
Hans-Hermann Hoppe's NAP theory is something akin to a behaviorist's reformulation of Rothbard's, predicated not on a natural rights theory of self-ownership but on a theory of demonstrated ethical intent by the fact people are willing to debate the matter of rights at all. Central to this is the idea that only voluntary exchange is legitimate, with any violation of voluntary exchange constituting aggression against the established right of peaceful resolution of differences, which then implies a private proprietary right.
Ayn Rand's NAP theory is not as formalized as some others, but she explicitly stated that acts initiating violence against others are wrong, by virtue of the denial of the fundamental principles of life affirmation and self-interest that lie at the center of her Objectivist philosophy. Central to this is the idea that both private property (as the consequence of human achievement) and liberty in one's person are ethically inviolable.
A variety of market left-libertarians have espoused a form of NAP theory whereby a distinction is made between personal property and private property, where the former is that set of things necessary to human life and dignity while the latter is that set of other proprietary claims a person might make. Only the former is legitimate, and that is usually only as a matter of extension from the underlying right to be free from assaults on life and dignity by others. Central to this is the idea that people require practical supports to the maintenance of their lives and dignities, and any interference with such supports, whether personal or systematic, constitute aggression. Such interference notably includes systems of private property wherein exclusive possession of profiteering resources is at the expense of others who could make use of those resources for the maintenance of life and dignity. This is (by most standards) a rather unothodox NAP formulation.
Various forms of Christian NAP theory draw from New Testament teachings to produce a largely agorist political philosophy, such as the golden rule, the admonition to respect no illegitimate claims of authority, and statements amounting to instructions to never resort to violence except in self defense.
The etymological NAP theory defines aggression as the initiation of a coercive relationship, and coercion is the use of some action, that may be violent, threatening, or deceptive, to manipulate others. This formulation of a NAP theory of ethics is often described as a prohibition against "force or fraud". Central to this is the idea that a person subject to coercion may be a proximate aggressor, but not the ultimate aggressor, so that the ultimate aggressor who initiated coercion against him or her holds at least a far greater share of the ethical culpability for resulting aggression, if not the full weight of such culpability.
The notion of a "victimless crime", or of a crime against oneself or against some collective entity (such as the state), is foreign to NAP theories in general, and attempts to formulate a non-aggression principle that incorporates such notions typically create internal ethical conflicts that render such formulations self-defeating. Some NAP theories are consistent with orthodox anarchocapitalism; others are consistent with certain market anarchist models of mutualism; still others are consistent with various forms of libertarian minarchism. Some fail to fit into such neat categories of political persuasion, but are not necessarily any less valid. Most of them imply an unethical character to intellectual property law, though some explicitly set out to support or repudiate it.
The range of possible forms and justifications of non-aggression principle is significant, but they all share something in common: they agree that, given some reasonable set of definitions, aggression is the central determinant of whether an act is unethical.