Pierre-Joseph Proudhon's seminal work, 'What Is Property? or, An Inquiry into the Principle of Right and of Government,' is a foundational text in anarchist philosophy. Published in 1840, this treatise famously declares 'Property is theft,' challenging conventional notions of ownership and advocating for a society based on justice and equality. Proudhon critically examines various forms of property, dissecting their origins, implications, and their role in creating social inequality. He proposes a system of 'possession' based on use and labor, rather than absentee ownership, laying the groundwork for mutualist anarchism. This book remains a potent critique of capitalist property relations and a call for radical social transformation.