Jean Piaget (1896–1980) was a Swiss psychologist whose study of how children think reshaped developmental psychology and education alike. He asked an unusual question for his time: not how much children know, but how knowledge itself takes shape and grows inside a mind. His answer, a theory of stages and self-correcting mental structures, made the child's reasoning a serious scientific object rather than a defective version of adult thought.
Born in Neuchâtel, Piaget was a prodigy in natural history, publishing notes on molluscs as a teenager and earning a doctorate in zoology in 1918. He then turned to psychology, studying briefly in Zürich and Paris, where work on standardising children's intelligence tests gave him his decisive insight: the children's wrong answers were more revealing than their right ones, because they exposed a different and consistent way of reasoning. From 1929 he was associated with the University of Geneva, his base for the rest of his life, and in 1955 he founded and directed the International Centre for Genetic Epistemology there. He named his lifelong project genetic epistemology, the study of how knowledge develops, and pursued it through close, unstandardised clinical interviews and observation, famously testing ideas on his own three children. By his death he had produced more than sixty books and hundreds of articles, and had received the Erasmus Prize (1972) and the Balzan Prize (1979).
Piaget gave teachers a developmental map and a rationale for active, discovery-oriented learning: if children construct understanding by acting on the world, then arranging rich experiences matters more than transmitting facts. His constructivism underlies inquiry methods and much of how primary curricula are sequenced. His account also stands as the great individualist counterpoint to the socially mediated learning of Lev Vygotsky, a contrast that still organises debate in the field, while Jerome Bruner built on Piagetian foundations to argue that any subject can be taught honestly to a child at any stage in some intellectually adequate form. The schema, his unit of organised knowledge, fed directly into later schema theory in cognitive psychology. Later research has revised many of his specific claims, especially the ages and the universality of his highest stage, yet the core image of the child as an active builder of meaning has proved durable.