The most rigorous of the classical political economists — a self-made stockbroker who retired at 42, taught himself economics, and produced a theoretical framework that shaped 19th-century economic argument from John Stuart Mill to Karl Marx. Where Adam Smith wrote in broad, morally-textured paragraphs, Ricardo thought in abstract models.
Two ideas carried farthest. His theory of comparative advantage (1817) showed that even a country less productive at everything still benefits from specializing in what it makes relatively best and trading for the rest — the single most durable argument for free trade ever made, and still the core of international economics. His labor theory of value held that the exchange value of goods is determined, under competitive conditions, by the labor required to produce them. Ricardo deployed this descriptively; Karl Marx would later turn the same apparatus into a moral indictment of capitalism.
His Principles of Political Economy and Taxation (1817) also developed a grim account of distribution. As population grew, Ricardo argued, ever-poorer land would have to be cultivated, rents would rise to landlords, wages would drift toward subsistence, and profits would be squeezed — a secular crisis he thought only free trade in grain could postpone. This analysis armed Richard Cobden's campaign against the Corn Laws, which were repealed two decades after Ricardo's death.