Property is theft

The phrase “property is theft” comes from P.J. Proudhon’s magnum opus What Is Property?

simplified version of Proudhon’s argument

in proudhon’s period, the animating assumption of essentially all works on property was that god created the earth and gave it to mankind in common to use. this was the long-standing christian view, which was notably reflected in thomist thought and even received a ringing endorsement from john locke. the christianist idea of the common ownership of all of the earth is proudhon’s starting point.

from there, a question arises: if god initially gave the entire earth to mankind to own in common, then how can you ever have individual property? or, to borrow a line from locke, since the earth is given to mankind in common, “it seems to some a very great difficulty, how any one should ever come to have a property in any thing.” the correct answer to this question, reached by proudhon but not locke, is that the only way to move from universal common ownership to individual private ownership is through theft. when an individual appropriates pieces of the earth out of the commons and into private ownership, that individual steals from everyone else. everyone else’s ownership share in that piece of the earth is taken from them, violently and without their consent.

you don’t need initial common ownership

at this point, someone might try to get out of this outcome by saying that they don’t believe in initial common ownership. but that’s not really crucial to the argument.

even under the hypothetical stories libertarians tell about how property can originate, the fact is that at the initial point in time, everyone can access and use every single piece of the earth at their will. there are no restrictions. you can move about the world freely. nobody can stop you. you truly have negative liberty in the sense that it would be wrong to interfere with your bodily movements.
but then something curious happens. somehow (regardless of how its justified), individuals are permitted to appropriate pieces of the world privately. the upshot of such appropriation is that everyone else’s previously-existing ability to access and use the appropriated piece of the world is stolen from them without their consent. those who do not go along with having their access and use stolen from them are met with violence. this is theft. access and use, both valuable things, are taken from people at the barrel of a gun.