Jean Piaget was a Swiss philosopher born in 1896 who was interested in biology and the natural world from a young age. He published several papers before high school and over 60 books in his lifetime. Piaget's theory of cognitive development proposed that children progress through four distinct stages - sensorimotor, preoperational, concrete operational, and formal operational - with learning limited by each stage. Piaget also believed moral development occurred in two stages, from heteronomous morality where rules are seen as unchangeable, to autonomous morality where rules are understood to be flexible and made by mutual agreement. While influential, Piaget's theories have also received some criticism regarding aspects like the ages associated with each stage of development.