There is no evidence for any Roman jurist writing a treatise entitled On Natural Law, or similar. Ius naturale had a very limited place in Roman jurisprudence, and when Roman jurists want to reason about law, they pretty much always began from the standpoint of the Roman ius civile and worked outwards. There is a fundamental difference between this concentric way of reasoning about natural law, and the way in which the medieval natural lawyers influenced by Thomas Aquinas, as well as later 17th and 18th century thinkers, reason about it. In this lecture, Caroline Humfress examines this tension.