As far as I know Proudhon introduced the concept that property is theft while possession is ok. But what is the difference between those two?
Having a possession: physically maintaining a hold on something.
Having a property: socially/legally maintaining an entitlement on something.
He [Proudhon] argued that the result of an individual's labor which is currently occupied or used is a legitimate form of property [which he called "possession"]. Thus, he opposed unused land being regarded as property, believing that land can only be rightfully possessed by use or occupation (which he called "possession").