peace is only possible under absolute sovereign rule.[2] Nevertheless, we can and should embrace one of Hobbes’s central insights: that problems of assurance are of fundamental importance for questions of social justice, even, or especially, justice questions of global scale. In general, agents face normatively significant problems of assurance because they have imperfect knowledge about the conduct of others and must therefore weigh consequent risks of action. The basic human device for their resolution, practically speaking, is for agents to form “agreements”—promises, conventions, social practices, or institutions—that reduce uncertainty and thus “assure” the parties involved. None of this necessarily bears on basic principles of morality or justice, at least not without further argument.[3] Hobbes’s dramatic assurance problem—the state of nature—makes this further step. It shows vividly how agreement-making may be not simply a useful device but a condition for the applicability of basic principles. In the absence of an agreed upon common power to assure compliance, Hobbes explains, basic principles of conduct—including considerations of justice and injustice—are simply out of place. The resulting uncertainty about what others will do in the name of self-preservation gives us sweeping liberty to defend ourselves. Contemporary political philosophy is concerned with substantive political morality and “ideal theory,” so it may seem that Hobbes’s problem of assurance—a matter of amoral self-preservation—can be simply set aside. At best, it may be said, assurance and other coordination problems bear, not on the fundamental nature of justice, but its practical implementation. This is to underestimate the depth of Hobbes’s insight. As I will explain, assurance problems can take specifically moral forms, arising even among morally motivated agents, in a way which bears on the very applicability of fundamental moral principles. In central cases of normative political philosophy, justification of basic principles, even in “ideal theory,” must be tailored to the circumstances that give rise to assurance problems and the available human means for their resolution...