What constraints should be imposed on individual liberty for the sake of protecting our collective security? A helpful approach to answering this question is offered by a theory that grounds political obligation and authority in a moral requirement of fair contribution to mutually beneficial cooperative schemes. This approach encourages us to split the opening question into two—a question of correctness and a question of legitimacy—and generates a detailed set of answers to both subsidiary questions, with a nuanced and plausible set (...) of implications. The plausibility of its treatment of the issues surrounding liberty and security, I argue, helps to confer credibility on the fairness-based theory that carries these implications. (shrink)