 Public reason requires that the moral or political rules that regulate our common life be, in some sense, justifiable or acceptable to all those persons over whom the rules purport to have authority. It is an idea with roots in the work of Hobbes, Kant, and Rousseau, and has become increasingly influential in contemporary moral and political philosophy as a result of its development in the work of John Rawls, Jürgen Habermas, and Gerald Gaus, among others. Proponents of public reason often present the idea as an implication of a particular conception of persons as free and equal. Each of us is free in the sense of not being naturally subject to any other person’s moral or political authority, and we are equally situated with respect to this freedom from the natural authority of others. How, then, can some moral or political rules be rightly imposed on all of us, particularly if we assume deep and permanent disagreement amongst persons about matters of value, morality, religion, and the good life? The answer, for proponents of public reason, is that such rules can rightly be imposed on persons when the rules can be justified by appeal to ideas or arguments that those persons, at some level of idealization, endorse or accept. But public reason is not only a standard by which moral or political rules can be assessed: it can also provide standards for individual behavior. Because we make moral and political demands of each other, if we are to comply with the ideal of public reason, we must refrain from advocating or supporting rules that cannot be justified to those on whom the rules would be imposed. We should instead, some insist, only support those rules we sincerely believe can be justified by appeal to suitably shared or public considerations—for example, widely endorsed political values such as freedom and equality—and abstain from appealing to religious arguments, or other controversial views over which reasonable people are assumed to disagree. In this way, public reason can be presented as a standard for assessing rules, laws, institutions, and the behavior of individual citizens and public officials.