Nowhere does Binet's genius show more brilliantly than in this work. That he in the midst of a busy life and in addition to all his other work could have acquired so great a knowledge of mental defectives is amazing; the more so when we realize that it was first-hand knowledge gained from observation backed by keen perception, that perception that enabled him to see the truth with a quickness that makes the rest of us, still groping in the dark, question if it was truth Binet saw. But that it was truth we are learning every day. - Summary by Henry H. Goddard