When after the first third of the nineteenth century the great philosophic movement which found its climax in Hegelianism came to disaster in consequence of its absurd neglect of hard solid facts, the era of naturalism began its triumph with contempt for all philosophy and for all deeper unity. Idealism and philosophy were stigmatized as the enemies of true science and natural science had its great day. The rapid progress of physics and chemistry fascinated the world and produced modern technique; the sciences of life, physiology, biology, medicine, followed; and the scientific method was carried over from body to mind, and gave us at the end of the nineteenth century modern psychology and sociology. The lifeless and the living, the physical and the mental, the individual and the social, all had been conquered by analytical methods. But just when the climax was reached and all had been analyzed and explained, the time was ripe for disillusion, and the lack of deeper unity began to be felt with alarm in every quarter. For seventy years there had been nowhere so much philosophizing going on as suddenly sprung up among the scientists of the last decade. The physicists and the mathematicians, the chemists and the biologists, the geologists and the astronomers, and, on the other side, the historians and the economists, the psychologists and the sociologists, the jurists and the theologians--all suddenly found themselves again in the midst of discussions on fundamental principles and methods, on general categories and conditions of knowledge, in short, in the midst of the despised philosophy. And with those discussions has come the demand for correlation. Everywhere have arisen leaders who have brought unconnected sciences together and emphasized the unity of large divisions. The time seems to have come again when the wave of naturalism and realism is ebbing, and a new idealistic philosophical tide is swelling, just as they have always alternated in the civilization of two thousand years. No one dreams, of course, that the great synthetic apperception, for which our modern time seems ripe, will come through the delivery of some hundred addresses, or the discussions of some hundred audiences. An ultimate unity demands the gigantic thought of a single genius, and the work of the many can, after all, be merely the preparation for the final work of the one. And yet history shows that the one will never come if the many have not done their share. What is needed is to fill the sciences of our time with the growing consciousness of belonging together, with the longing for fundamental principles, with the conviction that the desire for correlation is not the fancy of dreamers, but the immediate need of the leaders of thought. And in this preparatory work the St. Louis Congress of Arts and Science seemed indeed called for an important part when it was committed to this topic of correlation.