Matt Bruenig has been discussing quite a bit about the bizarre ways in which people argue for the concept of property. In this entry, he points out that the argument that property is not a form of aggression relies entirely on a partisan definition of entitlement. If you already believe that people are entitled to something, then no amount of force used to enforce it will matter, despite your commitment against force.
Under this four-prong definition of theft, it is in fact entitlement that is doing all of the argumentative work. The initial anger at “force” and “violence” and “coercion” drops out entirely. People who adhere to this definition are not opposed to those things. In fact, they are quite happy threaten and use force constantly and endlessly so long as doing so does not run afoul of the Entitlement Prong. What started as an opposition to force, coercion, aggression, and so on turns out to really be an opposition to arrangements that do not afford to each what they are entitled to.
But here’s the problem: people disagree about what everyone is entitled to. There are different philosophical theories about what belongs to each person. This means that you have to actually make an independent case for why your own theory of entitlement is correct, not just assume it in the background and thereby call all forceful actions contrary to it theft.