John Rawls’s A Theory of Justice is generally recognized as the greatest work of political philosophy in the twentieth century; his Political Liberalism may well be the most revolutionary.1 PL has set the agenda for much of political philosophy in the twenty-first century. As we shall emphasize, Rawls’s political liberalism was not simply a new approach to political theory, but part of a wider rethinking of the relation of moral and social theory, and of the place of diversity in moral thinking. Rawls developed PL in response to his increasing appreciation of the depth of disagreement among reasonable people and the complexities of formulating a moral theory appropriate for societies with such diversity.