Jean Piaget (1896-1980) was the most influential developmental psychologist in history. His theory of cognitive development proposed that children are not miniature adults who simply know less, but active constructors of knowledge who think in qualitatively different ways at different ages. Through decades of meticulous observation and clever experiments, Piaget mapped the development of logical thinking, mathematical understanding, moral reasoning, and scientific thought from infancy through adolescence.
Stages of Development
Piaget proposed four major stages. The sensorimotor stage (birth to ~2 years): infants understand the world through direct sensory experience and motor actions, developing object permanence (understanding that objects continue to exist when hidden). The preoperational stage (~2-7 years): children develop symbolic thought and language but struggle with conservation (understanding that quantity is unchanged by superficial transformations), egocentrism, and logical reasoning. The concrete operational stage (~7-11 years): children master conservation, classification, and logical reasoning about concrete objects. The formal operational stage (~11+ years): adolescents develop abstract reasoning, hypothetical thinking, and systematic problem solving.
Piaget proposed that development is driven by assimilation (incorporating new experiences into existing schemas), accommodation (modifying schemas to fit new experiences), and equilibration (the drive to resolve cognitive conflicts between expectations and reality). While subsequent research has shown that Piaget underestimated infant competence, overestimated the discreteness of stages, and underemphasized social and cultural factors, his core insights endure: children actively construct understanding, development involves qualitative changes in thinking, and cognitive conflict drives learning. His influence on education — particularly constructivist approaches — has been enormous.