James Gustave Speth (2008). “The Bridge at the Edge of the World: Capitalism, the Environment, and Crossing from Crisis to Sustainability”, p.1, Yale University Press
Jean Baptiste Say, Charles Robert Prinsep (1857). “A Treatise on Political Economy; Or, The Production, Distribution, and Consumption of Wealth”, p.433
Jean Baptiste Say, Charles Robert Prinsep (1857). “A Treatise on Political Economy; Or, The Production, Distribution, and Consumption of Wealth”, p.487