Libertarians tend to agree on a wide array of policies and principles. Nonetheless, it is not easy to find consensus on what libertarianism's defining characteristic is, or on what distinguishes it from other political theories and systems.
Various formulations abound. It is said that libertarianism is about individual rights, property rights,1 the free market, capitalism, justice, or the nonaggression principle. Not just any of these will do, however. Capitalism and the free market describe the catallactic conditions that arise or are permitted in a libertarian society, but do not encompass other aspects of libertarianism. And individual rights, justice, and aggression collapse into property rights. As Murray Rothbard explained, individual rights are property rights.2 And justice is just giving someone his due, which depends on what his rights are.3
The nonaggression principle is also dependent on property rights, since what aggression is depends on what our (property) rights are. If you hit me, it is aggression because I have a property right in my body. If I take from you the apple you possess, this is trespass — aggression — only because you own the apple. One cannot identify an act of aggression without implicitly assigning a corresponding property right to the victim.
