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 structural similarities in normative and positive political theory. Specifically, we consider the analytical parallels between Rawls’ account of political liberalism and a rational choice model of legal order recently proposed by Hadfield & Weingast (2011). The positive model proposes that a shared system of reasoning—a common logic—plays a key role in coordinating a stable equilibrium when legal rules depend on decentralized collective enforcement efforts by individual agents. The common logic enables individuals to predict how others will behave in the face of wrongful conduct and incentivizes participation in costly collective punishments by reassuring agents that their personal concerns will be taken into account in the resulting equilibrium. Rawls’s theory of political liberalism, we argue, is based on a comparable recognition that citizens in a pluralistic society face a practical as well as a moral problem in sustaining a stable political conception of justice. How can individual citizens have confidence that others will reciprocate their commitment to support fair and reasonable governing principles that depart from their own ideal conceptions of truth and value? Citizens face a practical problem of mutual assurance that public reason helps them solve by making individual ongoing commitments to a political conception of justice a matter of common knowledge. The solution, on both views, requires citizens’ reciprocal commitment to basing law on a system of shared public reasons. Both views thus place public reason at the core of liberal democratic politics in conditions of diversity, and for quite similar reasons. Our argument illustrates the (often) complementary roles of positive and values-based analysis in constitutional (in the broadest sense) design.