The synthesis of many intellectual development theories and conceptual generalization of a lot of experimental data describing ontogenetic development of all the psychic functions is offered. The synthesis is based on one of general principles of big systems development - the principle of system differentiation. Psychological-pedagogical aspects of this approach are considered. The capacity of human brain to separate reflection of multi-dimension objects and situations, to differentiated functional separation of similar arousals is one of natural prerequisites of intellect.