At some point (around 1929-30), Vygotsky came to disagree with Piaget's understanding of learning and development, and held a different theoretical position from Piaget on the topic of inner speech; Piaget asserted that egocentric speech in children "dissolved away" as they matured, while Vygotsky maintained that egocentric speech became internalized, what we now call "inner speech". However, in the early 1930s he radically changed his mind on Piaget's theory and openly praised him for his discovery of the social origin of children's speech, reasoning, and moral judgements. Piaget only read Vygotsky's work after his death. Nearing the end of his life, Vygotsky's later work involved adolescent development. However, his most important and widely known contribution is his theory for the development of "higher psychological functions," which considers human psychological development as emerging through unification of interpersonal connections and actions taken within a given socio-cultural environment (i.e., language, culture, society, and tool-use). 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. At this time, Vygotsky fully revealed his long-time interest in the philosophy of Spinoza, who would remain one of his favorite thinkers throughout his life. A fervent Spinozist in many respects, Vygotsky was profoundly influenced by Spinoza's thought, largely in response to Spinoza's examinations concerning human emotion.