Piaget's theory of cognitive development describes how human intelligence develops in a series of stages from infancy to adulthood. The stages are sensorimotor (birth to age 2), preoperational (ages 2 to 7), concrete operational (ages 7 to 11), and formal operational (ages 11 to adulthood). At each stage, children construct an understanding of the world through interactions with objects and people. Piaget believed that cognitive development relies on biological maturation and active learning through discovery rather than passive absorption.