Kelsen pointed out that the law speaks of what people ought to do, not of what people are doing. This is the principle of normativity, which makes law different from other forms of social rules. Normativity is the quality of legal rules that impose obligations, guiding behavior rather than merely describing it.

Kelsen's theory builds on philosopher David Hume's argument that we can't deduce "ought"-rules on what ought to be from an "is"-descriptions on how things are. For Kelsen, law isn't a description of people's behavior; it is a system of rules commanding specific actions.

For Kelsen, legal norms are not merely descriptions; rather, they represent obligations to which man is subjected. The statement "stealing is illegal" is not a description of what people do; instead, it says that people are obliged not to steal.

Some Legal Norms: Being internal to the legal system in itself is what legitimates legal norms rather than to appeal to external moral beliefs or social customs. This means a law remains valid even if people disobey it; the "ought" of the law prevails over how things actually are. Thus, law against theft is normative because there is a requirement rather than because people obey it.