  This last point brings us to another observation that is central to Kelsen’s theory, about the relations between legal validity and, what he called, “efficacy”. The latter is a term of art in Kelsen’s writings: A norm is efficacious if it is actually (generally) followed by the relevant population. Thus, “a norm is considered to be legally valid”, Kelsen wrote, “on the condition that it belongs to a system of norms, to an order which, on the whole, is efficacious” (GT, 42). So the relationship here is this: efficacy is not a condition of legal validity of individual norms. Any given norm can be legally valid even if nobody follows it. (e.g. think about a new law, just enacted; it is legally valid even if nobody has yet had an opportunity to comply with it.) However, a norm can only be legally valid if it belongs to a system, a legal order, that is by and large actually practiced by a certain population. And thus the idea of legal validity, as Kelsen admits, is closely tied to this reality of a social practice; a legal system exists, as it were, only as a social reality, a reality that consists in the fact that people actually follow certain norms.