A system of methodological and theoretical principles of research of mental phenomena, according to which the main subject of research is activity being an intermediate link for all mental processes. The theory was developed in 1920 - 1930 based on the constructivist approach of L.S. Vygotsky. The founders of the theory are A.N. Leontiev and S.L. Rubinstein. The basic thesis is formulated in the following way: consciousness does not determine activity, but activity determines consciousness. Consciousness can be known only through subjective relations, and through the activity of a person, in the process of which a person develops.