In his note 'Levels of Rules and Hart's Concept of Law' (Mind, vol. lxxxi, January, I972) Professor D. Gerber argues against Hart's view that in the union of primary and secondary rules can be found the key to the science of jurisprudence. Gerber asks us to imagine that all of the rules of adjudication in a community are changed in 'non-systematic' ways-that is, in ways not provided for by the rules of change of the system (e.g. by mistake in transcription), and that these changes are accepted by the officials and citizens of the community. In such a case, Gerber maintains, Hart has a dilemma: he must hold either (i) that the new rule is a rule of the system and hence that a legal system exists, which is false; or (2) that the new rule is prevented from being a rule of the system by the existence of some tertiary rule, and hence that there is a hierarchy of higher levels of rules whose existence must be inferred or assumed, which Hart denies. In either case, Gerber concludes, 'Hart is wrong in thinking that he can account for the existence of a legal system in terms of primary and secondary rules alone'