John Rawls's A Theory of Justice, originally published in 1971 and revised by Rawls in 1999, is the most influential work of political philosophy in English of the twentieth century. Its central thought experiment asks: what principles for the basic structure of society would rational people choose if they did not know in advance who they would be in that society—their race, sex, talents, wealth, religion, or conception of the good life? Rawls argues that, from behind this 'veil of ignorance,' people would choose two principles: equal basic liberties for all, and arrangements of social and economic inequality that benefit the least advantaged members of society (the famous 'difference principle'). The book develops this argument through several hundred pages of careful philosophical construction, engaging with utilitarianism, classical liberalism, and his contemporaries' alternatives. Rawls, who taught at Harvard for most of his career and lived an unusually private intellectual life, recast political philosophy by reviving the social contract tradition of Locke, Rousseau, and Kant. The book provoked a vast secondary literature, from Robert Nozick's libertarian rejoinder to Michael Sandel's communitarian critique to G. A. Cohen's egalitarian one, and is essentially required reading for anyone serious about contemporary debates over equality, distributive justice, or liberal political theory.