In Vygotsky’s theory, development is conceived of as selfpropulsion creating new higher psychological functions by reorganising lower ones; at the same time, psychological functions being historical-cultural constructions based on semiotic functions, they can come only from exterior. The article shows that the concept of zone of proximal development is the theoretical attempt to bring these two apparently contradictory postulates together. It is the contradiction between internal possibilities and external needs that constitutes the driving force of development.