The famous German legal philosopher G. Radbruch has gone through a complex scientific path which influenced both his worldview and methodological foundations of the concepts proposed by him. The study traces the evolution of Radbruch’s philosophy of law, which was formed under the influence of representatives of neo-Kantianism, in particular G. Simmel, H. Rickert, E. Lask and the school of «free law” – G. Kantorovich, as well as historical events – the domination of Nazism in Germany. The authors reveal the peculiarities of G. Radbruch’s methodology; his position is based on the neo-Kantian methodology of distinguishing between “what is” and “what ought to be”, on the methodology of the philosophy of values and the principle that it is logically impossible from what is to derive what is valuable, what is right, what ought to be. G. Radbruch viewed law as a cultural phenomenon, a value fact that differs from other normative systems by a special value that is important for the realization of justice. The law as a phenomenon has a positive form ( statute, custom, international agreement) and social significance (regulates human behaviour and relations). But law also has a normative character (it is an evaluation and a requirement for reality), it also has a social purpose (which is to achieve the public good), it has an individual purpose (which is to realise and ensure human rights) and a nature of universality, and therefore equality and it should embody justice. The authors examine the well-known and fundamental concept of the “idea of law” which is fundamental to Radbruch’s philosophy and consists of three elements: justice, expediency (sustability), and legal certainty. The idea of law is an essential requirement for positive law; it legitimizes it, makes it “correct law” and “lawfull law (statute)”. Legislation that does not embody the idea of law essentially loses its legal nature. Justice is the formal equal treatment of equals and unequal treatment of unequals. Legal certainty reflects the social and individual purpose of law and justifies human freedom, equality and uniqueness on the basis of human value. Expediency (sustability) is the clarity, certainty, and comprehensibility of the law and should be understood as a basis for ensuring justice. G. Radbruch’s concept of the “idea of law” is essentially a modern theory of the legitimation of positive law.