This book, the last one written by Piaget, presents a new line of empirical studies based on a revised formulation of his theory of the development of logical reasoning. The amended theory overcomes many problems and criticisms of his earlier formulations by providing a fresh explanation for the origin of mental operations and mental organization based on the concept of meaning. It also offers a more elegant vision of the continuity in mental development from birth to adulthood. As the final revision of Piaget's theory -- and one that opens up new areas of inquiry -- this book calls for a reinterpretation of his earlier work -- a task which will occupy scholars for decades to come.