Kelsen is known for the most rigorous development of a ‘positivist’ theory of law that rigorously excludes from its analysis any ethical, political, or historical considerations, and finds the essence of the legal order in the ‘black letter’ or laid-down law. A system of law is based on a Grundnorm or ground rule, from which flows the validity of other statements of law in the system. The ground rule might be that some particular dictates or propositions, such as those of the sovereign, are to be obeyed. The Grundnorm can only be changed by political revolution. The theory is best known in its development in the Allgemeine Staatslehre (1925, trs. and revised as General Theory of Law and State, 1945).