Kelsen is known for his "Pure Theory of Law", which argues that the concept of justice lies outside the realm of traditional legal theory. He also believes that primitive man sees everything as normative, and that the basic norm that gives validity to legal systems cannot be pinpointed. Additionally, he argues that laws are most effective when they restrain people rather than encourage them. However, his theory has faced criticism, particularly from H.L.A. Hart who did not succeed in proving Kelsen's monism wrong. Furthermore, Kelsen's main focus is on the concept of norms and their hierarchy in jurisprudence. Another influential figure in the debate on legal theory is Eugen Ehrlich, who believed that the true essence of law can be found in living law and judicial decisions.