The “Encyclopædia” seems to us to-day but a small portion of the achievement of Diderot’s life, though it represents the part that he played in relation to the science of his time. His place in science has sometimes been wrongly stated. It has been said, for instance, that he anticipated Lamarck and Darwin. It is true that he wrote, “The need produces the organ; the organization determines the function,” and that this contains the germ of Lamarck’s doctrine; and again, “The world is the abode of the strong,” and that this may be said to be the germ of the doctrine of natural selection; but at both points he was simply putting into epigrammatic form the conceptions of the greatest scientific genius of his age and country, Buffon, the only man of that time who was cast in the same massive mould, and to whom Diderot could turn with fraternal delight and admiration. It is to Buffon also, and not to Diderot, that the honour of anticipating Lyell belongs. It is in his Baconian thoughts on the interpretation of nature, and again in such a comprehensive collection of data as his notes on physiology, discovered of recent years, that Diderot’s searching and inquisitive scientific spirit appears. He frequently startles us by the way in which he vividly realizes and follows out to their legitimate conclusions those floating ideas of his time which we are working out to-day. Above all, and from the first, he clearly grasps the fundamental value of the human body and its processes in the interpretation of mental phenomena; in one of his comparatively early works, the “Lettre sur les Aveugles,” he remarks that he has never doubted that “our most purely intellectual ideas are closely related to the conformation of our bodies.” “How difficult it is,” he says elsewhere, “to be a good philosopher and a good moralist without being anatomist, naturalist, physiologist, and doctor.” Holding firmly by this clue, he was constantly trying to fathom the mysteries of the soul and to picture the processes of life; it is because he has realized that this can only be done fruitfully from the physiological side that the “Rêve de d’Alembert,” his most brilliant effort in this direction, is interesting after the lapse of a century.