  What about the basic norm, is efficacy a condition of its validity? One might have thought that Kelsen would have opted for a negative answer here. After all, the basic norm is a presupposition that is logically required to render the validity of law intelligible. This would seem to be the whole point of an anti-reductionist explanation of legal validity: since we cannot derive an “ought” from an “is”, some “ought” must be presupposed in the background that would enable us to interpret certain acts or events as having legal significance. Kelsen, however, quite explicitly admits that efficacy is a condition of the validity of the basic norm: A basic norm is legally valid if and only if it is actually followed in a given population. In fact, as we shall see below, Kelsen had no choice here. And this is precisely why at least one crucial aspect of his anti-reductionism becomes questionable.