In spite of the shortage in Rawls’s work of references to Smith’s later and even more famous book, the ideas and arguments of An Inquiry into the Nature and Causes of the Wealth of Nations are central to Rawls’s theory of justice. This article intends to show that without the ideas Smith proposed in The Wealth of Nations, Rawls would not have been able to write A Theory of Justice. Smith’s ideas in The Wealth of Nations supply Rawls with the central question he attempts to answer in his theory of justice. They also supply him with a key component of his answer to that question, a component without which Rawls’s answer to the question would have looked sharply different. Smith’s contributions to the set of ideas on which Rawls drew to formulate his theory of justice are as important to that theory as Kant’s contributions and are more important to Rawls’s theory than the contributions of any thinker other than Kant (with the possible exception of Sidgwick).