When people of good faith and sound mind disagree deeply about moral, religious, and other philosophical matters, how can we justify political institutions to all of them? The idea of public reason--of a shared public standard, despite disagreement--arose in the seventeenth and eighteenth centuries in the work of Hobbes, Locke, Rousseau, and Kant. At a time when John Rawls' influential theory of public reason has come under fire but its core idea remains attractive to many, it is important not to lose sight of earlier philosophers' answers to the problem of private conflict through public...