Why is it important for people to agree on and articulate shared reasons for just laws, rather than whatever reasons they personally find compelling? What, if any, practical role does public reason play in liberal democratic politics? We argue that the practical role of public reason can be better appreciated by examining the confluence of normative and positive political theory; the former represented here by liberal social contract theory of John Rawls and others, and the latter by rational choice or (...) game theory. Citizens in a diverse society face a practical as well as a moral problem. How can they have confidence that others will reciprocate their commitment to supporting governing principles that depart from their own ideal conceptions of truth and value in order to be reasonable to all? Citizens face a practical problem of mutual assurance that public reason helps them solve, and solve as a matter of common knowledge. The solution, on both views, requires citizens’ reciprocal commitment to basing law on a system of shared reasons. Both views place public reason at the core of liberal democratic politics in conditions of diversity, and for quite similar reasons. Our argument illustrates the complementary roles of positive and values-based analysis in constitutional design. (shrink)