This chapter focuses on four aspects of a critical philosophy of international law. First, there is a paradoxical relationship between international law and philosophy, at the same time natural and a bit tense, if not conflictual. Second, the assumptions at the heart of international law are comprised of notions/values and distinctions: universal/particular, hierarchy/equality, inclusion/exclusion, self/other, and public/private. Third, these assumptions and their interactions have three major characteristics: they have a structuring power that plays a crucial role in the determination of issues of legitimacy; the assumptions are presented as true, but this quality of truth is more posited than demonstrated; the assumptions at the core of international law are not only descriptive but also prescriptive. Fourth, all of this has an impact in terms of the legitimacy of international law. The assumptions/distinctions influence the nature, organization, and practice of the building blocks of international law and its sense of legitimacy.