   Has Kelsen just violated his own adherence to Hume’s injunction against deriving “ought” from an “is” here? One gets the clear impression that Kelsen was aware of a serious difficulty in his position. In both editions of the Pure Theory of Law, Kelsen toys with the idea that perhaps changes in the basic norms of municipal legal systems legally derive from the basic norm of public international law. It is a basic principle of international law that state sovereignty is determined by actual control over a territory/population (PT1 61–62, though in PT2, 214–215, the idea is presented with greater hesitation; notably, some commentators argue that Kelsen took the idea of a universal legal order much more seriously than suggested here—see Green 2016).  But this led Kelsen to the rather uncomfortable conclusion that there is only one basic norm in the entire world, namely, the basic norm of public international law. Be this as it may, the main worry lies elsewhere. The worry stems from the fact that it is very difficult, if not impossible, to maintain both a profound relativist and an anti-reductionist position with respect to a given normative domain. If you hold the view that the validity of a type of norms is entirely relative to a certain vantage point—in other words, if what is involved here is only the actual conduct, beliefs/presuppositions and attitudes of people—it becomes very difficult to detach the explanation of that normative validity from the facts that constitute the relevant point of view (namely, the facts about people’s actions, beliefs, attitudes, etc). This is basically what was meant earlier by the comment that Kelsen had no option but to admit that the validity of the basic norm is conditional on its efficacy. The normative relativism which is inherent in Kelsen’s conception forces him to ground the content of the basic norm in the social facts that constitute its content, namely, the facts about actions, beliefs, and attitudes actually entertained by the population in question. And this makes it very questionable that reductionism can be avoided. In fact, what Kelsen really offered us here is an invitation to provide a reductive explanation of the concept of legal validity in terms of some set of social facts, the facts that constitute the content of any given basic norm. (Which is precisely the kind of reduction H.L.A. Hart later offered in his account of the Rules of Recognition as social rules [see Hart 1961, at p. 105, where Hart alludes to the difference between his conception of the rules of recognition and Kelsen’s idea of the basic norm.])