The paper analyzes the philosophical underpinnings and the methodological status of C. Wolff's doctrine of the civitas maxima, of the State of States. Two important points are made. The first is that Wolff understands the civitas maxima as a fictitiously presumed rational consensus of all human beings and of all states. The civitas maxima is the state that state-actors would create if they were to establish among themselves a structure that corresponds to the rational principles of law. Secondly, it is shown that Wolff applies a new philosophical concept of the social contract that he conceives as a contrafactual, normative hypothesis. Wolff anticipates Kant's understanding of the contract and sets contractarian theory on a track that leads all the way up to John Rawls. Wolff applies this concept to international relations and thereby brings about an important supranational turn into the modern philosophy of international law. The last part of the paper is dedicated to a theoretical evaluation of Wolff's theory, mainly in the light of Hans Kelsen's remarks on Wolff.