What Is Property?: Or, an Inquiry Into the Principle of Right and of Government

What Is Property?: or, An Inquiry into the Principle of Right and of Government is a work of political philosophy on the concept of property and its relation to anarchist philosophy by the French anarchist and mutualist Pierre-Joseph Proudhon, first published in 1840. In the book, Proudhon most famously declared that "property is theft".

Pierre-Joseph Proudhon was a contemporary of Marx and one of the most acute, influential, and subversive critics of modern French and European society. What is Property? (1840) has become a classic of political thought through its wide-ranging and deep-reaching critique of private property as at once the essential institution of Western culture and the root cause of greed, corruption, political tyranny, social division, and violation of natural law.