Lev Vygotsky
Major themes of research

Vygotsky was a pioneering psychologist with interests in extremely diverse fields: his work covered topics such as the origin and the psychology of art, development of higher mental functions, philosophy of science and the methodology of psychological research, the relation between learning and human development, concept formation, interrelation between language and thought development, play as a psychological phenomenon, learning disabilities, and abnormal human development (aka defectology). His philosophical framework includes interpretations of the cognitive role of mediation tools, as well as the re-interpretation of well-known concepts in psychology such as internalization of knowledge. Vygotsky introduced the notion of zone of proximal development, a metaphor capable of describing the potential of human cognitive development.

His most important and widely known contribution is his theory for the development of "higher psychological functions," which emerge through unification of interpersonal connections and actions taken within a given socio-cultural environment (i.e. language, culture, society, and tool-use). 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.

While Vygotsky never met Jean Piaget, he had read a number of his works and agreed on some of his perspectives on learning. 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 thought that egocentric speech follows from inner speech and "dissolved away" as children matured. Vygotsky showed that egocentric speech became inner speech, and then called "thoughts". Piaget only read Vygotsky's work after his death and openly praised him for his discovery of the social origin of children's thoughts, reasoning, and moral judgements.
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