According to Leon Petrażycki’s theory, law is psychical processes of a specific kind, and the sphere of law’s real existence lies in the human psyche. Petrażycki holds that law and legal phenomena can be studied from four different perspectives: (1) a theoretical research of law as a real psychical phenomenon; (2) a practical-dogmatic perspective which deals with law as a system of obligatory norms; (3) a legal-political perspective aimed at the development of desirable, rational rules which should be adopted as law; (4) a sociological perspective which studies law as a product of impulsive-intellectual ‘contagion’ and coordinated behaviour caused by it. Critics of Petrażycki’s theory commit certain methodological mistakes and the major lesson we can draw from these mistakes is that to build a full-blown science of law we need to take into consideration the following methodological tenets: (1) we should not conflate theoretical and practical judgments (statements); (2) we should not conflate law as a real phenomenon (i.e. a psychical and, possibly, trans-subjective reality) with law as an ideal phenomenon; (3) to build a scientific theory of law we need to abide by the principle of adequacy; and (4) legal science should be built as an overarching science in relation to the specific legal sciences, such as civil law, criminal law, international law, etc. The architectonics of the scientific edifice constructed by Petrażycki’s looks as follows: (1) the foundation of this edifice is grounded in Petrażycki’s teaching on logic and scientific methodology; (2) the ground floor of Petrażycki's science is his teaching on emotional psychology; (3) the first floor of his science class is the teaching on law and morality (his ethics); (4) the second floor is Petrażycki's theory of scientific moral and legal policies; (5) the third floor, which was started but not finished by Petrażycki, was supposed to be his teaching on emotional sociology, dealing, inter alia, with the ‘trans-subjective’, social forms of law and moral reality.