I argue that social-contract theory cannot succeed because reasonable people may always disagree, and that social-contract theory is irrelevant to the problem of the legitimacy of a form of government or of a system of moral rules. I note the weakness of the appeal to implicit agreement, the conflation of legitimacy with stability, the ... undesirability of `public justification' and the apparent blindness to the evolutionary critical-rationalist approach of Hayek and Popper. I employ that approach to sketch answers to the theoretical, historical and practical questions about the legitimacy of government or of systems of moral rules.