Where does the authority of law ultimately come from? The Stoic philosophers of Greece and Rome proposed a powerful and enduring answer: it comes from nature itself. They argued that the cosmos is governed by a divine and rational principle—the logos—and that true justice is found when human laws align with this universal, natural law. This episode explores the revolutionary concept of natural law and its profound influence on Roman legal thought. We examine how thinkers like Cicero embraced the Stoic idea that there is a higher, eternal law that stands above the statutes of any particular state. The great Roman jurists later refined this idea into the ius naturale, or "natural law," which they believed was the source of the fundamental legal principles common to all humanity. The Stoic concept of natural law is one of the most important intellectual legacies of the classical world. It provided the philosophical foundation for the later development of international law and the modern theories of universal human rights. By grounding justice in nature rather than convention, the Stoics transformed law from a local custom into a universal aspiration.