"What is Property? An Inquiry into the Principle of Right and of Government" is a seminal work by French philosopher and socialist Pierre-Joseph Proudhon, first published in 1840. In this book, Proudhon famously declares that "property is theft," arguing against the concept of property rights as conventionally understood and critiquing the economic and social injustices they perpetuate. Proudhon examines the nature of property and its relation to freedom and justice, challenging contemporary views on ownership and the accumulation of wealth. This work is foundational in the development of anarchist thought, significantly influencing the anarchist movement and debates about property, justice, and social organization.