The weak natural law thesis and the common good

The weak natural law thesis asserts that any instance of law is either a rational standard for conduct or defective. At first glance, the thesis seems compatible with the proposition that the validity of a law within a legal system depends upon its sources rather than its merits. Mark C. Murphy has nonetheless argued that the weak natural law thesis can challenge this core commitment of legal positivism via an appeal to law’s function and defectiveness conditions. My contention in the current paper is that in order to make good on the challenge, the defender of the weak natural law thesis should appeal explicitly to the common good, understood as the principal normative reason in the political domain. In section I I outline the main implications of the weak natural law thesis and clarify a common misunderstanding regarding its explanatory role. Section II then argues for the indispensability of the common good to the natural law jurisprudential thesis on the grounds that it has an essential role to play in a natural law account of law’s defectiveness conditions and the presumptive moral obligatoriness of legal norms. Finally, in section III I examine the compatibility of a strengthened version of the weak natural law thesis with legal positivism in light of the centrality of the common good to the natural law jurisprudential position.