The author discusses P.Ya. Galperin's understanding of the nature of psychological phenomena, mechanisms of their formation and development. This approach is systemic and integrated which makes it quite unique. The principal components of the approach are: a conception of the subject of psychology, objective necessity of the mind, laws of its development in phylo-, anthropo- and onto-genesis, regularities of the formation of mental activity. The article also describes areas of practical application of the main principles of P.Ya. Galperin's theory.