  The natural law tradition has undergone a considerable refinement in the 20th century, mainly because its classical, popular version faced an obvious objection about its core insight: it is just difficult to maintain that morally bad law is not law. The idea that law must pass, as it were, a kind of moral filter in order to count as law strikes most jurists as incompatible with the legal world as we know it. Therefore, contemporary natural lawyers have suggested different and more subtle interpretations of the main tenets of natural law. For example, John Finnis (1980) views natural law (in its Thomist version) not as a constraint on the legal validity of positive laws, but mainly as an elucidation of an ideal of law in its fullest, or highest sense, concentrating on the ways in which law necessarily promotes the common good. As we have noted earlier, however, it is not clear that such a view about the necessary moral content of law is at odds with the main tenets of legal positivism. To the extent that there is a debate here, it is a metaphysical one about what is essential or necessary to law, and about whether the essential features of law must be elucidated in teleological terms or not. Legal positivists do not tend to seek deep teleological accounts of law, along the lines articulated by Finnis, but whether they need to deny such metaphysical projects is far from clear.