Abstract
Legal positivism maintains a distinction between law as it is and law as it ought to be. In other words, for positivists, a law can be legally valid even if it is immoral. H. L. A. Hart hoped to defend legal positivism against natural law. This paper analyses Hart’s criticism of Gustav Radbruch, a natural lawyer, before suggesting that Hart’s account of legal positivism gives rise to a logical problem. It is concluded that this problem leaves logical space for a theory of natural law based on moral authority rather than legal validity.