Among individualists, it's common to cite the Non-Aggression Principle (NAP) as a near-axiom. The NAP states that force or threat of force should only be used to defend people from the aggression of others.

Oh wait, what's that clamor I hear over my right shoulder? Ah, okay. Let's try that again.

The NAP states that force or threat of force should only be used to defend people and their property from the aggression of others.

Why the little addition? The incorporation of property into the definition of the NAP is necessary because otherwise property itself would be a violation of the NAP, and people who like to cite the NAP tend to really like the idea of property.

What is property, anyway? What constitutes ownership? Physical possession? Exercising some kind of monopoly over its use? What kinds of things can be owned? Can ownership of some kinds of property impose on others' use of their own property? Who decided who owned what in the first place?

All kinds of different approaches and opinions exist on how to answer these questions, but there is one thing that can be said for sure. Property is something that only exists because it is enforced through force or threat of force, either by the state or something indistinguishable from it; that is its defining characteristic.

In other words, property is violence. That's why propertarians make special note of property as something which must be "defended". Because it is not aggression that they reject, but what they would consider its misapplication.

The property-excepting NAP is not a statement of ethical imperative, but is instead an arbitrary call for violence to be committed against those who would oppose the violent enforcement of the particular property system being espoused.