We should perhaps apologize for inflicting another volume on the patient reader. And yet, in a sense, this work is long overdue. Elsewhere, in our discussions on the development of children's thinking in relation to problems of number, quantity and space, and again in dealing with the notion of chance and inductive reasoning, we have spoken of the formation of elementary logical operations. But the development of these structures as such deserves a separate study, and, from a logical point of view, we should have started with it.