Jean Piaget was a Swiss psychologist who studied how children's cognitive abilities develop from birth through adulthood. He identified four stages of development: sensorimotor, preoperational, concrete operational, and formal operational. According to Piaget, learning occurs through active discovery and interaction with the environment as a child constructs their understanding of the world. Piaget's theory emphasizes that teachers should focus on student-centered, active learning through experimentation and social interaction, rather than direct instruction, to best facilitate cognitive development.