The law persists because people have reasons to comply with its rules. What characterizes those reasons is their interdependence: each of us only has a reason to comply because he or she expects the others to comply for the same reasons. The rules may help us to solve coordination problems, but the interaction patterns regulated by them also include Prisoner's Dilemma games, Division problems and Assurance problems. In these "games" the rules can only persist if people can be expected to be moved by considerations of fidelity and fairness, not only of prudence. This book takes a fresh look at the perennial problems of legal philosophy - the source of obligation to obey the law, the nature of authority, the relationship between law and morality, and the nature of legal argument - from the perspective of this conventionalist understanding of social rules. It argues that, since the resilience of such rules depends on cooperative dispositions, conventionalism, properly understood, does not imply positivism.