Political contractualism is important in societies characterized by substantial moral and political disagreement and diversity. The very disagreement that makes the social contract necessary, however, also makes agreement difficult. Call this the paradox of diversity, which is the result of a tension between two necessary conditions of political contractualism: existence and stability. The first involves showing the possibility of some agreement, while the second involves showing that the agreement can persist. To solve both of these problems, I develop a multilevel contract theory that I call the “open society” model of political contractualism that incorporates diversity into the contractual model at different levels solving the existence...