Russian psychologist, he made a major contribution to the theory of mental development of the child, the study of the problems of child psychology and educational psychology. Theory and discovery: Blonsky developed a theory of the mental development of the child, which takes into account the influence of the environment and learning on the formation of mental functions. He also studied the problems of child psychology and educational psychology. Significance of works: Blonsky’s research contributed to the understanding of the patterns of mental development of children and the development of methods of teaching and upbringing.