"Property is Theft" is a contradiction - and it must be.

Proudhon, from The System of Economic Contradictions, responding to the critics who said "Property is Theft is a contradiction:

Property is, in fact and in right, essentially contradictory and it is for this very
reason that it is anything at all. In fact,

• Property is the right of occupancy; and at the same time the right of exclusion.
• Property is labor’s reward; and the denial of labor.
• Property is society’s spontaneous work; and society’s dissolution.
• Property is an institution of justice; and property is theft.

From all this it results that one day property transformed will be a positive idea,
complete, social and true; a property that will abolish the older one and will become for
all equally effective and beneficent. And what proves this is once again the fact that
property is a contradiction.
From this moment property started being recognized, its intimate nature was
unveiled, its future predicted. And yet, it could be said that the critic had not realized
even half of its task, because, to definitely constitute property, to take away its exclusion
characteristics and grant its synthetic form, it was not sufficient to have analyzed it in
itself, it was also necessary to find the order of the things, of which property was not
more than a particular moment, the series that ended it, outside of which it would be impossible either to comprehend or to initiate property.

 Proudhon actually never finished his analysis of property, because he couldn't solve these problems. But it's a fascinating look at the problems of property.