Monism Theory :
Professor kelson, Duguit and L.Lauterpacht are the supporters of this theory (Monism Theory). According to them International Law and Municipal Law are components of the same legal system. According to them, law is a unified branch of knowledge, no matter whether it applies to persons for other entities. According to this Theory, International obligations and Municipal rules are facets of the same phenomenon, both deriving ultimately the validity from one basic norm and belonging to the unitary order comprised by the conception of law.
The Monism Theory asserts that the National Law and Municipal Law of the same fundamental nature, and arise from the same unity of the science of law, being manifestations of a single conception of law. both the system have their origin in a 'higher law' founded on the principles of right and wrong
Prof. Kelsen (of the Vienna School) holds that there is unity of law between the legal character of International Law. Duguit carried a step further and held that the subject of International law were not States but the individual members of States.
Monism theory has gained prominence after the first world war and it is advocated by modern writers. Monism finds it's most direct antecedents in the writings of Leon Duguit and Emile Durkheim its recent expositors being Krobbe Kelsen, Kunz, Scelle, Verdross and Wright.
J Lauterpatch held that there is the difference in the basis of these two laws. According to him, "it is true that International Law exists for states, but it is true in that sense that state exists for individual and not individual or States."