Jean Piaget was a Swiss psychologist known for his theory of cognitive development, which outlines how children learn and think as they grow. He proposed that children progress through four stages: sensorimotor, preoperational, concrete operational, and formal operational. Each stage represents a different way of understanding the world, from basic sensory experiences to abstract reasoning. In the sensorimotor stage, infants learn through their senses and actions. The preoperational stage, occurring from ages 2 to 7, is characterized by symbolic thinking but limited logic. The concrete operational stage, from ages 7 to 11, involves logical thinking about concrete events, while the formal operational stage, starting around age 12, allows for abstract and hypothetical reasoning.