The idea of a Pure Theory of Law was propounded by the formidable Austrian jurist and philosopher Hans Kelsen (1881–1973). Kelsen was a legal theorist at the beginning of the 20th century.

Propounder of the Theory:

Kelsen:
Hans Kelsen, an Austrian jurist, was the propounder of the theory that 'law and state are the same'. He is known for his contributions to legal theory, particularly the theory of law as a hierarchical system of norms.
Explanation of the Theory:

Law and State as the Same:
Kelsen argued that law and the state are inseparable. According to him, the state is the ultimate source of law, and all legal norms derive their validity from the state. In other words, without the state, there would be no law. Kelsen believed that the state creates law through its legislative and judicial functions.

Hierarchical System of Norms:
Kelsen also proposed the idea of a hierarchical system of norms, where higher norms give validity to lower norms. He introduced the concept of the 'basic norm' or 'Grundnorm', which serves as the foundation of the legal system. The basic norm is not created by any higher norm but is presupposed by the legal system itself.

Criticism:
While Kelsen's theory has been influential in legal philosophy, it has also faced criticism. Some scholars argue that it oversimplifies the complex relationship between law and the state. Critics also point out that the theory does not adequately address the role of morality in law.
In conclusion, Hans Kelsen's theory that 'law and state are the same' has been a significant contribution to legal theory, emphasizing the interconnection between the state and the legal system.