The nonaggression axiom is an ethical principle often appealed to as a basis for libertarian rights theory. The principle forbids “aggression,” which is understood to be any and all forcible interference with any individual's person or property except in response to the initiation (including, for most proponents of the principle, the threatening of initiation) of similar forcible interference on the part of that individual.

The axiom has various formulations, but two especially influential 20th-century formulations are those of Ayn Rand and Murray Rothbard, who appear to have originated the term. Ayn Rand maintained that “no man may initiate the use of physical force against others…. Men have the right to use physical force only in retaliation and only against those who initiate its use.” This quote ...