Jus cogens is the ultimate source of international law. Differently from other forms of customary laws, jus cogens underpins non-rebuttable norms overriding all contrary customary (not peremptory) and contractual law. What is special about jus cogens? The paper argues that jus cogens norms are characterized by a moral dimension that is not subject to the will of states; yet, as law it often (even if not always) requires states’ positive endorsements. There is not a single specific source of jus cogens legitimation, as among those listed by art.38 of the Statute of the ICJ, nor jus cogens is a separate source of law. The common trait of different processes of law-creation (customary, contractual etc.) stands in the higher moral status of the defended value. Why state entities have to presuppose a “moral hypothesis” for making sense of the validity of the law? Does jus cogens fill-in such moral presupposition?