First published in English 1968, in this volume Paul Fraisse begins with history, looking at the evolution of experimental psychology, starting with its origins. He then moves on to the establishment of experimental psychology around the world. In the second chapter he discusses the experimental method. In the third chapter Jean Piaget tackles the questions of explanation and parallelism and their problems within experimental psychology. The final chapter by Maurice Reuchlin goes on to discuss measurement in psychology looking at various scales with their experimental conditions and numerical properties.