Abstract This article considers natural law perspectives on the nature of law. Natural law theories are united by what Mark Murphy calls the natural law thesis : law is necessarily a rational standard for conduct. The natural law position comes in strong and weak versions: the strong view holds that a rational defect in a norm renders it legally invalid, while the weak view holds that a rational defect in a legal norm renders it legally defective. The article explores the motivations for the natural law position, before considering three lines of natural law argument found in the literature. I conclude by examining the arguments offered by John Finnis and Murphy in support of the weak natural law view. I suggest that these arguments fail to impugn the strong natural law thesis. Indeed, the functional argument outlined by Murphy provides a plausible route to a hybrid natural law view that incorporates both weak and strong claims.