Samuel Mountifort Longfield (1802-1884) was an Irish lawyer and economist, and the first holder of the Whately Chair in Political Economy at Trinity College, Dublin. A member of the Oxford-Dublin school of proto- Marginalists, he promoted the idea of supply and demand as the joint determinants of value. Longfield was among the first to recognize that the principle of diminishing marginal productivity applied generally to factors of production. He also expanded the principle of comparative advantage in international trade to more than the typical "two-commodity" case. His works include: Four Lectures on Poor Laws (1834), Lectures on Political Economy (1834) and Three Lectures on Commerce and One on Absenteeism (1835).