What Is Property? is a foundational work of anarchist philosophy that explores the concept of property and its relationship to individual liberty and social justice. Proudhon argues that the common conception of property conflates two distinct components: possession, which is the legitimate result of an individual's labor and occupation, and property, which is the illegitimate accumulation of wealth through exploitation and coercion. He argues that interest on loans and rent are forms of theft, as they allow individuals to profit from the labor of others without contributing anything themselves. Proudhon's ideas have been influential in the development of anarchist thought and continue to be debated today.