The Non-Aggression Principle, or NAP, is a rational principle of ethics. Many self-identified libertarians and individualists base their views of ethics and liberty on the Non-Aggression Principle. A common formulation of that principle is:
Aggression is inherently illegitimate.
Perhaps another definition is in order to help set things straight:
aggression (n.): initiation of a coercive relationship
Initiation should be understood in much the same terms as used in defensive complaints by children in schoolyards -- evidence that there is a strong instinctual understanding of the basic ethical position of the Non-Aggression Principle: "He started it." While the Non-Aggression Principle does not have anything direct to say about the legitimacy of retaliation, it very clearly prohibits initiating a coercive relationship.
To coerce is the intentional or negligent use of some action -- generally violent, threatening, or deceptive -- to manipulate a person's condition or decisions contrary to that person's intent. Some draw the line at violence itself, while the vast majority of adherents to the Non-Aggression Principle at least include explicit (and often implicit) threats of violence in their definition. Malicious dishonesty is a logically consistent, and often included, form of manipulation, and the combination of violent, threatening, and deceptive forms of coercion is sometimes referred to as "force or fraud". Many consider lethal violence to be the most extreme example of a coercive act, but still only the pinnacle of a tall mountain of possibilities.
Once a person makes a coercive overture toward another person, a relationship of a coercive character has been established. That relationship may be quickly annulled, or it may be perpetuated, depending on the nature and unavoidable consequences of the initiating act as well as the subsequent actions of the individuals involved.
What that means, in short, is that any initiation of coercive action (that is, any aggressive act) is ethically wrong. In common parlance, people often use the term "aggressive" to mean "assertive", though in the context of ethical theory they should be understood as distinct terms that mean very different (though occasionally overlapping) things.
Several practical characteristics of the Non-Aggression Principle differentiate it from most other moral and ethical concepts:
It is possible to construct a rational argument, from broadly agreeable premises, that leads directly to the NAP.
It is possible to use the NAP as the guiding principle of personal ethics within the world as it exists right now and thereby live an ethical, practical, consistent, and successful life.
It is possible to build a complete, workable system of jurisprudence from the NAP, that many consider superior to any presently in practice.
It is quite simple, such that any person capable of useful abstract reasoning should be able to fully grasp the NAP.
It is practical, focused on the real world, and says nothing in and of itself one way or another about metaphysical belief systems like Christianity, Taoism, Islam, Neopaganism, Materialist Atheism, and so on, nor about any moralities rational believers may infer as necessary consequences of these belief systems.
