In this article, I want to consider Alf Ross’s intriguing 1946 critique of the dualism of reality and validity and the implications of this critique for so-called dual-nature theories of law, such as the theory defended by Robert Alexy. The meaning of this dualism, which Ross considers to be at the foundation of traditional legal thinking, is that both the very concept of law and certain other fundamental legal concepts – such as the concept of the sources of law and the concept of subjective law (which encompasses the concept of a legal right and the concept of a legal duty) – consist of two parts, one factual and empirical and one normative and metaphysical. Ross objects, however, that this dualism gives rise to certain very troublesome antinomies both in the concept of law and in the other fundamental legal concepts, and that dualists cannot avoid these antinomies by eliminating the concept of validity from their theories, since doing so would leave the resulting (monist) theory unable to even ¿nd its study object, but must instead substitute a non-cognitivist account of claims of legal validity for the non-naturalist (intuitionist) account characteristic of such dualism. I am, however, going to argue that Ross’s discussion of the dualism of reality and validity and the ensuing antinomies, although very interesting, is ultimately not persuasive, that the solution proposed by Ross to the problem of the antinomies would likely have been successful if there really had been a problem that needed a solution, and that in any case Ross’s critique of the dualism of reality and validity does not apply to Robert Alexy’s dual-nature theory of law.