Thomas Aquinas argues that ‘an unjust law is no law at all’, or at least “that which is not just seems to be no law at all”. (Aquinas in Dimock, ed., 2002, p. 19) (Aquinas credits Augustine for the quoted words.) But he also seems aware that a human law-giver might promulgate a command that has the form of law, and is enforced like law, but yet is unjust. How does he try to reconcile these two seemingly inconsistent claims? What does he mean when he says that ‘an unjust law is no law at all’? Is his defense of this claim plausible (believable, well-supported by his arguments)?