One thought was very naturally suggested by the external circumstances. St. Louis had asked the nations of the world to a celebration of the Louisiana Purchase. Historical thoughts thus gave meaning and importance to the whole undertaking. The pride of one century's development had stimulated the gigantic work from its inception. An immense territory had been transformed from a half wilderness into a land with a rich civilization, and with a central city in which eight thousand factories are at work. No thought lay nearer than to ask how far this century was of similar importance for the changes in the world of thought. How have the sciences developed themselves since the days of the Louisiana Purchase? That is a topic which with complete uniformity might be asked from every special science, and which might thus offer a certain unity of aim to scholars of all scientific denominations. There was indeed no doubt that such an historical question would have to be raised if we were to live up to the commemorative idea of the whole Fair. And yet it seemed still more certain that the retrospective problem did not justify itself as a central topic for a World's Congress. There were sciences for which the story of the last hundred years was merely the last chapter of a history of three thousand years and other sciences whose life history did not begin until one or two decades ago. It would thus be a very external uniformity; the question would have a very different meaning for the various branches of knowledge, and the treatment would be of very unequal interest and importance. More than that, it would not abolish the unrelated character of the endeavors; while the same topic might be given everywhere, yet every science would remain isolated; there would be no internal unity, and thus no inner reason for bringing together the best workers of all spheres. And finally the mere retrospective attitude brings with it the depressing mood of perfunctory activity. Certainly to look back on the advance of a century can be most suggestive for a better understanding of the way which lies before us; and we felt indeed that the occasion for such a backward glance ought not to be missed. Yet there would be something lifeless if the whole meeting were devoted to the consideration of work that had been completed; a kind of necrological sentiment would pervade the whole ceremony, while our chief aim was to serve the progress of knowledge and thus to stimulate living interests.