Peter Kooijmans' inquiry into the doctrine of the legal equality of states and, with it, the foundations of international law, reflects a peculiar brand of cosmopolitan thought, namely innate cosmopolitanism. Though under-recognized, innate cosmopolitanism is an argument for re-conceiving the modern international legal order according to a deep unity underlying the whole of human relations. As such, innate cosmopolitanism is distinct from better-recognized examples of cosmopolitan thinking, including liberal cosmopolitanism and cosmopolitan constitutional theory. Rejecting the normative individualism of liberal cosmopolitanism, and eschewing the formal orientation of constitutional theory, innate cosmopolitanism envisions the world as a viable collectivity that is perceived to exist, irrespective of formal recognition, as a matter of historical fact. But while innate cosmopolitanism operates according to a top-down model of collectivity, it nonetheless recognizes and incorporates smaller units of collectivity. As such, it holds especial relevance at a time when international legal doctrine looks beyond the preeminence of states, but continues to be bound to them in practice. Following innate cosmopolitanism, the international system incorporates all members equally when it takes into account the different material position of each member vis-à-vis the collective whole. The rights and responsibilities enjoyed by each, and their political situation within the community, will vary accordingly. This article will explore the innate cosmopolitan contribution to international law by reference to two current discourses, concerning ethical legitimacy and constitutional theory, as they grapple with justifications for and the doctrinal viability of an expanding public order globally. Additionally, examples drawn from the activation of the crime of aggression in the Rome Statute, and the Kadi case, will be considered for the critical light they throw on innate cosmopolitan theory.