Hans Kelsen is considered one of the preeminent jurists of the 20th century. He is most influential in Europe, where his notion of a Grundnorm or a "presupposed" ultimate and basic legal norm, still retains some influence. It is a hypothetical norm on which all subsequent levels of a legal system such as constitutional law and "simple" law are based. Kelsen's pure theory of law described the law as being a set of social facts, which are normatively binding too. Law's normativity, meaning that we must obey it, derives from a basic rule which sits outside the law we can alter. It is a rule proscribing the validity of all others.
Kelsen was a Professor around Europe, notably the University of Vienna. In 1940, he moved to the United States, giving the Oliver Wendell Holmes Lectures at Harvard Law School in 1942 and becoming a full professor at the department of political science at the University of California, Berkeley in 1945. During those years, he increasingly dealt with issues of international law and international institutions such as the United Nations.