The Grundnorm (basic norm) is a presupposed ultimate rule that validates an entire legal system. Kelsen argued that every legal norm derives its validity from a higher norm, and this chain must end with a fundamental norm that itself cannot be derived from a higher legal authority. The Grundnorm is not a factual command but a transcendental-logical presupposition—we must assume it to make sense of the legal system as a coherent set of binding rules. For example, in a national constitution, the Grundnorm might be 'the constitution is valid', but its content is historically determined.

Kelsen advocated a concentrated model: a specialized constitutional court with monopoly power to strike down legislation, operating through abstract review (deciding laws in the abstract, not only in concrete cases). The American model is decentralized—any court can declare a law unconstitutional only in the context of an actual dispute. Kelsen's model, adopted across Europe after WWII, enhances legal certainty and uniformity but also elevates a judicial body with quasi-legislative powers. He viewed this as a logical extension of the hierarchy of norms, where the constitution stands above ordinary statutes.

Both are positivists, but Kelsen's system is built on a single Grundnorm presupposed as valid, while Hart uses a 'rule of recognition' that is a social fact—a practice officials accept as the criteria for validity. Hart's rule of recognition is empirically observable, whereas Kelsen's Grundnorm is a logical presupposition. Moreover, Kelsen reduces law to a normative science, excluding morality and sociology entirely; Hart allows a minimal core of natural law and acknowledges the role of social acceptance. Kelsen is more radically normative and formalist.

Kelsen was the primary drafter of the Austrian Federal Constitution of 1920, which established one of the world's first constitutional courts. He designed a centralized court with jurisdiction to review the constitutionality of statutes, making it the prototype for postwar European constitutional review. Kelsen served as a justice on that court from 1920 to 1930, until political pressure forced his removal. His practical work cemented his theoretical conviction that constitutional courts are essential guardians of the hierarchical legal order.

Kelsen, though born Jewish and later converted to Catholicism, faced rising anti-Semitism and was dismissed from the University of Cologne in 1933 after the Nazi seizure of power. He fled to Geneva, then to Harvard and Berkeley (where he taught from 1945). This displacement shifted his focus to international law and legal theory in the American context, producing *General Theory of Law and State* (1945) and later works on the unity of domestic and international law. His experience reinforced his commitment to legal science as a bulwark against ideological tyranny.

Kelsen argued that international law is a genuine legal system, not merely morality or politics, because it consists of valid norms arranged in a hierarchy. He controversially advocated a monist view where the international legal order is supreme over national legal systems—the Grundnorm of international law validates all domestic constitutions. This view directly influenced the legal architecture of institutions like the UN and the EU. However, critics argue that international law lacks central enforcement, making Kelsen's normative claim too abstract.

Many assume 'pure' means 'value-free' in a nihilistic sense—that Kelsen ignores justice or morality entirely. In fact, he rigorously separates law from morality to maintain science, but he acknowledges that moral values can become legal content when positivized by the legislature. The 'purity' refers to the method: law must be studied on its own terms, not confused with politics, sociology, or ethics. Kelsen himself was a liberal democrat who believed law should be used to realize justice, but he insisted that legal science cannot prescribe what is just—that is a political decision for the lawmaker.