Marmor considers the separation thesis, which he understands as saying that whether a given norm is legally valid depends on its sources, not its merits; and this means that he is concerned with the separation thesis conceived as a thesis about legal status, not as a thesis about the content of legal statements. Observing that the distinction between sources and merits is very close to the distinction between is and ought, he considers the objection that the separation thesis cannot be upheld because one cannot clearly distinguish between sources and merits, between is and ought. He responds to this objection, however, that the separation thesis can be upheld if it is seen as an answer to the question ‘What counts as law?’ rather than to the question ‘What is law?’, and that this response is in keeping with a common wish on the part of legal positivists to provide a reductive explanation of legal validity, that is, an explanation of legal validity exclusively in terms of social facts.

Baum Levenbook explores another criticism made against legal positivism. As she explains, Mark Greenberg objects to legal positivism, first, that it is a mistake to hold that legal facts are determined solely by social facts and, second, that the content of authoritative pronouncements, such as statutes, is determined by their linguistic content. But, she points out, Greenberg’s first objection is premised on the mistaken assumption that the nature of law requires the connection between legal facts and the determinants of legal facts to be, as Greenberg puts it, ‘rationally intelligible’, and the second objection is based on the equally unwarranted assumption that it is part of the nature of law to operate in such a way as to ensure that legal obligations are genuinely binding.