Libertarians typically claim that the non-aggression principle includes property and freedom of contract as a part of self-ownership, and that any interference is equivalent with aggression.
The basis for this extension of self-ownership to one's property is John Locke's argument (also called the homestead principle) that mixing of labor with an unowned resource makes that resource part of one's self. Subsequent exchange of such property (e.g. sale, rental) simply transfers this right. Hence, the argument goes, to aggress against, or in more neutral terms interfere with, someone's property is the exact equivalent to aggressing against, or interfering with, the individual's physical body.
As for freedom of contract, the right of self-ownership is held to imply freedom of action in the absence of aggression (e.g. in the absence of false or duress contracts, and the absence of contracts stipulating aggression against third-parties).
The principle has been derived by various philosophical approaches, including:
Argumentation ethics. Hans-Hermann Hoppe has argued that rights are presumed by the very act of arguing with another person;
Natural law. Some derive non-aggression from self-ownership or sovereignty of the individual, such as Josiah Warren and Lysander Spooner in the 19th century, and Murray Rothbard in modern times;
Contractarianism as described by Jan Narveson and David Friedman;
Objectivism. Ayn Rand has made efforts in deriving the principle from the right to life.
Universally preferable behavior. Stefan Molyneux of Freedomain Radio has argued that non-aggression is valid because it is the only rule that meets the logical requirement for moral universality."http://common-law.net/nap.html
"To paraphrase, the Non-Aggression Principle (NAP) is usually stated as "do not initiate force or fraud", or "if it harms none, do what you will", or "treat others as you'd like to be treated", or "live and let live". In more detail, “Do not initiate force or fraud against anyone else’s person or property." In other words, except for self-defense, don’t harm others, don’t harm or steal their property, don’t break your word, don’t try to coerce anyone by threatening to do any of these things, and don’t delegate or encourage anyone to do any of these things.
The Equal Rights Principle (ERP) states that everyone has equal inherent rights, there should be no special privileged class, no "divine right of kings". This also implies that a group of people, no matter how many, can't have more rights than any individual. You cannot delegate a right to another individual or group if you do not have that right in the first place.
The Individual Sovereignty Principle (ISP) is that we, as individual sentient human beings with free will, each have the right to do anything we want as long as we do not violate NAP or ERP; and that we create organizations (including governments) and we grant specific, limited, enumerated privileges to them, not the other way around; they have no inherent rights of their own
(this latter point is further discussed here). These 3 principles (ISP/NAP/ERP) form the tripod upon which any viable and just civilization must be founded, but for simplicity, we shall hereinafter refer to them collectively as NAP.
The term "Common Law" has several meanings or derivations. In one sense, it means the informal body of law (in effect) consisting of customary behaviors and practices of civilizations over millenia. In America, that tends to mean English Common Law. In another sense, it means what are the most common or universal laws all over the world despite the different laws in different countries or the different laws and rules that different religions impose on their followers. This is why Common Law must be secular to be truly neutral, universal, and common.
These can all be considered imperfect examples of trying to figure out what are the minimum universal common principles that people must live by in order to have a functioning civilization, without the extra laws and customs that are specific to particular countries or religions or cultures. If you've read this entire web page so far, it should be quite obvious that the answer is NAP. Thus, throughout the rest of this web site, the phrase "Common Law" (or more specifically "Universal Common Law") will be considered to mean NAP. This is what Common Law should really mean, and the world would be much better off if was truly the common (and only) law of the whole world." http://www.lewrockwell.com/block/block26.html
"The non-aggression axiom is the lynchpin of the philosophy of libertarianism. It states, simply, that it shall be legal for anyone to do anything he wants, provided only that he not initiate (or threaten) violence against the person or legitimately owned property of another. That is, in the free society, one has the right to manufacture, buy or sell any good or service at any mutually agreeable terms. Thus, there would be no victimless crime prohibitions, price controls, government regulation of the economy, etc.
If the non-aggression axiom is the basic building block of libertarianism, private property rights based on (Lockean and Rothbardian) homesteading principles are the foundation. For if A reaches into B's pocket, pulls out his wallet and runs away with it, we cannot know that A is the aggressor and B the victim. It may be that A is merely repossessing his own wallet, the one B stole from him yesterday. But given a correct grounding in property rights, the non-aggression axiom is a very powerful tool in the war of ideas. For most individuals believe, and fervently so, that it is wrong to invade other people or their property. Who, after all, favors theft, murder or rape? With this as an entering wedge, libertarians are free to apply this axiom to all of human action, including, radically, to unions, taxes, and even government itself.
Second case. You are lost in the woods, freezing, with no food. You will die without shelter and a meal. Fortunately, you come upon a warm cabin stocked with staples. You intend to eat, stay the night, leave your business card, and pay double any reasonable price that could be asked. Unfortunately, the cabin has a sign posted on the door: "Warning. Private Property. No Trespassing." Do you tamely go off into the woods and die?
There are several grave problems with these critiques of the non-aggression axiom.
1. They misunderstand the nature of libertarianism. These arguments implicitly assume that libertarianism is a moral philosophy, a guide to proper behavior, as it were. Should the flagpole hanger let go? Should the hiker go off and die? But libertarianism is a theory concerned with the justified use of aggression, or violence, based on property rights, not morality.
Therefore, the only proper questions which can be addressed in this philosophy are of the sort, if the flagpole hanger attempts to come in to the apartment, and the occupant shoots him for trespassing, Would the forces of law and order punish the home owner? Or, if the owner of the cabin in the woods sets up a booby trap, such that when someone forces his way into his property he gets a face full of buckshot, Would he be guilty of a law violation? When put in this way, the answer is clear. The owner in each case is in the right, and the trespasser in the wrong. If force is used to protect property rights, even deadly force, the owner is not guilty of the violation of any licit law.
2. These examples purposefully try to place us in the mind of the criminal perpetrator of the crime of trespass. We are invited, that is, to empathize with the flag pole hanger, and the hiker, not the respective property owners. But let us reverse this perspective. Suppose the owner of the apartment on the 15th floor has recently been victimized by a rape, perpetrated upon her by a member of the same ethnic or racial group as the person now hand walking his way down her flag pole, soon to uninvitedly enter her apartment. May she not shoot him in self-defense before he enters her premises? Or, suppose that the owner of the cabin in the woods has been victimized by several break-ins in the past few months, and has finally decided to do something in defense of his property. Or, suppose that the owner, himself, views his cabin as his own life preserver. Then, may he not take steps to safeguard his property? To ask these questions is to answer them, at least for the consistent libertarian.
3. The criticisms of libertarian property rights theory base their views on the philosophy of emergencies. The non-aggression axiom is all well and good in ordinary circumstances, but when there are life boat situations, all bets are off. The problem, however, with violating libertarian law for special exigencies is that these occurrences are more commonplace than supposed. Right now, there are numerous people dying of starvation in poor parts of the world. Some are suffering from illnesses which could be cured cheaply, e.g., by penicillin. We have all read those advertisements placed by aid agencies: "Here is little Maria. You can save her, and her entire village, by sending us some modest amount of money each month."