According to Kelsen, law is a "normative science". Explain.

Kelsen described law as a "normative science" as distinguished from natural sciences which are based on cause and effect, such as law of gravitation. The laws of natural science are capable of being accurately described, determined and discovered whereas the science of law is knowledge of what law ought to be. According to Kelsen, 'law is a primary norm which stipulates sanction'. According to Kelsen, 'norm (sanction) is rules forbidding or prescribing a certain behaviour'. He saw legal order as the hierarchy of norms having sanction, and jurisprudence was the study of these norms which comprised legal order. Kelsen distinguished moral norm with legal norm and said that though moral norms are 'ought' prepositions, a violation of it does not have any penal fallout. The 'ought' in the legal norm refers to the sanction to be applied for violation of law. According to Kelsen, we attach legal-normative meaning to certain actions and not to others depending on whether that event is accorded any legal-normative by any other legal norm. This second norm gains its validity from some other norm that is placed above it. The successive authorizations come to an end in the highest possible norm which was termed by Kelsen as 'Grundnorm'. Thus, Kelsen's pure theory of law is based on pyramidical structure of hierarchy of norms which derive their validity from the basic norm. Grundnorm or basic norm determines the content and gives validity to other norms derived from it. Under Kelsen's pure theory, the Grundnorm does not derive its validity from any other norm and its validity must be presupposed. In his view the basic norm is the result of social, economic, political and other conditions and it is supposed to be valid by itself.