Jean Piaget was a Swiss philosopher and natural scientist born in 1896 who studied children's cognitive development. He identified four stages of development: sensorimotor (birth to age 2), preoperational (ages 2 to 7), concrete operational (ages 7 to 11), and formal operational (ages 11 to 15). Piaget's theory emphasizes that children build understanding of reality through interactions with their environment and that curriculum should be developmentally appropriate to enhance logical and conceptual growth.