This book is a continuation of Piaget's investigation into the evolution of cognition and intelligence from infancy to adulthood. Piaget's concern is with the development of knowledge, the organization of thought and its adaptation to experience, and the rules of thought at various levels of problem solving.

In previous volumes Piaget has demonstrated and described three stages of intellectual development: During the sensori-motor stage from birth to about two years the infant learns to distinguish physical objects as external and permanent; the preoperational stage extends to about age six during which period the child is increasingly able to represent the external world by symbols, though his symbolic representations at this level are frequently personal and autistic; between seven and eleven years the child is able to carry out concrete operations—an operation, in the words of Piaget, “is a reversible, internalized action which is bound up