consciousness of insecurity which I have not yet got over has made me cautious. Actual blunders respecting facts up to the present known, and incorrect presentations of theories thus far recognised cannot be proved against me. In this relation just one great mathematician, who is laboring under a mistake, has complained to Marx in a letter that I have made a mischievous attack upon the honor of the square root of minus one.
As regards my review of mathematics and the natural science it was necessary for me to reassure myself on some special points—since I had no doubts about the truth of the general proposition—that in nature the same dialectic laws of progress fulfill themselves amid all the apparent confusion of innumerable changes as dominate the apparently accidental in nature; the same laws whose threads traverse the progressive history of human thought, and little by little come to the consciousness of thinking men. These were first developed by Hegel in a comprehensive fashion but in a mystical form. Our efforts were directed towards stripping away this mystical form and making them evident in their full simplicity and universal reality. It was self evident that the old philosophies of nature—in spite of all their actual value and fruitful suggestiveness—could be of no value to us. There was an error in the Hegelian form, as shown in this book, in that it recognised no progression of nature in time, no "one after another" (Nacheinander) but merely "one besides another" (Nebeneinander). This was due on the one hand to the Hegelian system itself which ascribed to the Spirit (Geist) alone a progressive historical development, but on the other hand, the general attitude of the natural sciences was responsible. So Hegel fell far behind Kant in this respect for the latter had al-
