Piaget’s first book on psychology, in which he and his collaborators use the clinical method and other strategies to obtain data. Showing the strong relation between language and the mechanisms of thought—egocentrism and syncretism—it illustrates the specificity of child logic in comparison with adult logic. The book is expanded with Judgment and Reasoning in the Child (1924), in collaboration with F. Berguer, E. Cartalis, S. Escher, M. Fiaux, L. Gonet, L. Hahnloser, U. Hanhart, O. Matthes, S. Perret and M. Roud.