On the Principles of Political Economy and Taxation

Ricardo introduced the principle of comparative advantage, demonstrating that nations can benefit from trade even when one country is more efficient at producing every good. The key is opportunity cost: a country should specialize in goods where it sacrifices the least of other productions. This idea, first set out in the chapter on foreign trade, remains the foundation of modern trade theory and is considered one of the few propositions in social science that is both true and non-trivial[citation:15][citation:3].

Ricardo argued that rent is not a component of price but a consequence of price. It arises only when land of different fertility is cultivated, with rent representing the difference between the produce of more fertile and marginal land. Smith had treated rent as one of three sources of value; Ricardo insisted that rent is a surplus paid to landlords because of the scarcity of fertile soil, not a cost of production. Understanding rent correctly, he believed, was essential for tracing the effects of wealth on profits and wages[citation:8][citation:10].

Ricardo wanted a measure of value that would not itself change when wages or profits shifted, so that he could determine which commodities rose or fell in real value. He initially used embodied labor as this standard, but in later editions and unfinished manuscripts he acknowledged that differences in the ratio of fixed to circulating capital across industries meant relative prices could change even when labor inputs remained constant. He never resolved this problem, and it was left to later economists—and Marx—to address the transformation of values into prices[citation:5][citation:13].

Because comparative advantage depends on relative, not absolute, costs. If England can produce cloth with relatively less sacrifice than wine, and Portugal can produce wine with relatively less sacrifice than cloth, both gain by specializing and trading—even if Portugal could produce both goods more cheaply than England. Ricardo's numerical example of England and Portugal exchanging cloth for wine illustrates that the total output of both commodities increases, allowing both countries to consume more than they could in isolation[citation:3][citation:15].

Ricardo's labor theory of value proved vulnerable to the problem of capital: when production takes time and involves fixed capital, relative prices depend on the rate of profit, not just labor inputs. Ricardo recognized this difficulty and tried to minimize its practical importance, suggesting deviations were small. Later economists showed that with unequal time intensities across industries, relative prices cannot be predicted from labor coefficients alone. This critique, developed by Jevons and later formalized by Sraffa and others, contributed to the decline of the labor theory of value in mainstream economics[citation:13][citation:5].