Proudhon's most famous declaration that "property is theft" comes from this, his most famous work, published in French in 1840; the English translation dates from 1890. According to Proudhon, only that which is being used is real property. Land must be lived on or farmed to be property, and goods must have been made by one's own labor to be owned. These new definitions challenge the very basis of capitalist systems, and Proudhon used them as the foundation for his writings in support of anarchy. Activists, historians, and philosophers will find themselves pondering his arguments long after they have finished reading.