"In this volume we have tried to present, as briefly and as clearly as possible, a synthesis, or summing up, of our work in child psychology. A book such as this seemed to us particularly desirable since our published studies have been spread out over a number of volumes, some of them quite lengthy and some of them fairly difficult to read. This little book, of course, is not meant to be a substitute for reading the other volumes. But it represents, we believe, a useful introduction to the questions we have studied and will enable the reader to gain an adequate understanding of what we have learned in our investigation."--Jean Piaget (March 1969).