Piaget’s cognitive-developmental theory emphasizes an active individual whose mind consists of rich structures of knowledge. According to Piaget, children move through four stages, from the baby’s sensorimotor action patterns to the elaborate, abstract reasoning system of the adolescent. Although some of Piaget’s conclusions have been challenged, his work has had an overwhelming impact, stimulating a wealth of research on children’s thinking and encouraging educational philosophies and programs that emphasize discovery learning.