The Classic Edition of Dolph Kohnstamm’s Jean Piaget, Children and the Class-Inclusion Problem, first published in 1967, includes a new introduction by the author, describing for readers the original context for his work, how the field has moved forward and the ongoing relevance of this volume. This enduring text offers a critical study of a cornerstone of Piaget’s theory that a child's ability to solve problems of class-inclusion marks the beginning of the period of concrete (logical) operations at about 7 or 8 years of age. Kohnstamm's experiments show, however, that, with a teaching method that provokes children’s authentic logical thinking processes, most children of 5 can already learn to solve a variety of class-inclusion problems, up to a level where they can even invent similar but new problems themselves. These results question the basic assumption of Piaget's theory that logical operations can only develop in firmly connected groupings of operations. Kohnstamm argues that experimenters must, therefore, show that children who come to master one kind of operation should also show transference to other operations of the same grouping. This insightful volume questions the real existence in brain functioning of Piaget’s families of logical operations. No experimental proof of such families has ever been demonstrated, and thus is solely an assumption in Piaget’s theory. This challenge to Piaget's theory is an invaluable resource for students and scholars of cognitive, developmental and educational psychology.