“The libertarian creed rests upon one central axiom: that no man or group of men may aggress against the person or property of anyone else. This may be called the “nonaggression axiom.” “Aggression” is defined as the initiation of the use or threat of physical violence against the person or property of anyone else. Aggression is therefore synonymous with invasion.”
http://mises.org/rothbard/newlibertywhole.asp
Also look:
http://mises.org/daily/3660
https://mises.org/daily/4641
