Do citizens of any modern state have a general duty to acknowledge its authority to determine for them, for action guiding purposes, whether certain kinds of conduct are morally permissible, required, or forbidden? That is, is there a duty to obey the law? Moral Contractualism, I contend, entails that citizens of a liberal democratic state have such a duty. Treating others morally often requires agents to act collectively, but even agents who accept the moral necessity of collective action will sometimes disagree over the specification of the ends to be achieved, and the means for doing so. I argue that a liberal democratic state (and only such a state) can justifiably claim the authority to resolve such disagreements, which it does mainl...