 Law does not necessarily satisfy the conditions by which it is appropriately assessed (Lyons 1984: 63; Hart 1961 [2012: 185–186)]. Law should be just, but it may not be; it should promote the common good, but sometimes it doesn’t; it should protect moral rights, but it may fail miserably. This we may call the moral fallibility thesis. The thesis is correct, but it is not the exclusive property of positivism. Aquinas accepts it, Fuller accepts it, Finnis accepts it, and Dworkin accepts it. Only a crude misunderstanding of ideas like Aquinas’s claim that “an unjust law seems to be no law at all” might suggest the contrary. Law may have an essentially moral character and yet be morally deficient. Even if every law always does one kind of justice (formal justice; justice according to law), this does not entail that it does every kind of justice. Even if every law has a prima facie claim to be applied or obeyed, it does not follow that it has such a claim all things considered. The gap between these partial and conclusive judgments is all a natural law theory needs to accommodate the fallibility thesis. It is sometimes said that positivism gives a more secure grasp on the fallibility of law, for once we see that it is a social construction we will be less likely to accord it inappropriate deference and better prepared to engage in a clear-headed moral appraisal of the law. This claim appealed to several positivists, including Bentham and Hart. But while this might follow from the truth of positivism, it cannot provide an independent argument for it. If law has an essentially moral character then it is obfuscating, not clarifying, to describe it as a source-based structure of governance.