Here republished, by permission, is the first 40 percent of David K. Lewis’s Convention: A Philosophical Study, originally published in 1969. In this portion Lewis develops his final definition of convention, “along the lines of Hume,” using such ideas as coordination equilibrium, coordination problem, and common knowledge. Lewis’s work is useful for understanding law, political theory, economic theory, social theory, and moral theory. It is also useful for understanding David Hume and his influence.