Property is theft! (French: La propriété, c'est le vol !) is a slogan coined by French anarchist Pierre-Joseph Proudhon in his 1840 book What is Property? Or, an Inquiry into the Principle of Right and of Government.
If I were asked to answer the following question: What is slavery? and I should answer in one word, It is murder!, my meaning would be understood at once. No extended argument would be required to show that the power to remove a man's mind, will, and personality, is the power of life and death, and that it makes a man a slave. It is murder. Why, then, to this other question: What is property? may I not likewise answer, It is robbery!, without the certainty of being misunderstood; the second proposition being no other than a transformation of the first?
By "property", Proudhon referred to a concept regarding land property that originated in Roman law: the sovereign right of property, the right of the proprietor to do with his property as he pleases, "to use and abuse," so long as in the end he submits to state-sanctioned title, and he contrasted the supposed right of property with the rights (which he considered valid) of liberty, equality, and security. Proudhon was clear that his opposition to property did not extend to exclusive possession of labor-made wealth.
In the Confessions d'un revolutionnaire Proudhon further explained his use of this phrase:
In my first memorandum, in a frontal assault upon the established order, I said things like, Property is theft! The intention was to lodge a protest, to highlight, so to speak, the inanity of our institutions. At the time, that was my sole concern. Also, in the memorandum in which I demonstrated that startling proposition using simple arithmetic, I took care to speak out against any communist conclusion.
In the System of Economic Contradictions, having recalled and confirmed my initial formula, I added another quite contrary one rooted in considerations of quite another order—a formula that could neither destroy the first proposition nor be demolished by it: Property is freedom. [...] In respect of property, as of all economic factors, harm and abuse cannot be dissevered from the good, any more than debit can from asset in double-entry book-keeping. The one necessarily spawns the other. To seek to do away with the abuses of property, is to destroy the thing itself; just as the striking of a debit from an account is tantamount to striking it from the credit record.