 Vygotsky eventually came to dialogue with the mainstream Gestalt line of thought and adopted a more holistic approach to understanding development.Under the increasing influence of the holistic thinking of the scholars primarily associated with the German-American Gestalt psychology movement, Vygotsky adopted their views on "psychological systems" and—inspired by Kurt Lewin's "Topological (and vector) psychology"—introduced the enigmatic construct of the "zone of proximal development".It was during this period that he identified the play of young children as their "leading activity", which he understood to be the main source of preschoolers' psychological development, and which he viewed as an expression of an inseparable unity of emotional, volitional, and cognitive development.