Now natural science shares with Humboldt the consciousness that the practice of reason in physical research consists exclusively in "perceiving the element of unity in a multitude of forms." But on the other hand, though it does not always admit its belief in speculative introspection as frankly as Humboldt does, it nevertheless proves that it does not fully understand the practice of science and that it believes in a metaphysical as well as a physical science by using the speculative method in the treatment of so-called philosophical topics, in which the element of unity is supposed to be discovered by introspective reason instead of an analysis of multiform sense perceptions, and it demonstrates its lack of unity by being unaware of the unscientific character of disagreements, by believing in a metaphysical science outside of the physical domain. The relations between phenomenon and its nature, cause and effect, matter and force, substance and spirit, are certainly physical ones. But what is there of unity that science teaches about them? Plainly then, the work of science, like that of the farmer, has so far been done only practically, but not scientifically, not with a predetermination of success. Understanding, that is to say the practice of understanding, is well applied in science, I readily admit. But the instrument of this understanding, the faculty of thought, is misunderstood. We find that natural science, instead of applying this faculty scientifically, simply experiments with it. What is the reason for this? Natural science has neglected the critique of reason, the theory of science, logic. Just as the handle and the blade of a knife constitute its general content, so we found that the general content of reason was the universal, the general "itself." We know that it does not produce this content out of itself, but out of given objects, and these objects are the sum of all natural or physical things. The object of reason is, therefore, an infinite, unlimited, absolute quantity. This infinite quantity manifests itself in finite quantities. In the treatment of relatively small quantities of nature the true essence of reason, the true method of understanding, is well recognized. It remains to be demonstrated that the great relations of the world, the treatment of which is still doubtful, are likewise intelligible by the same method. Cause and effect, mind and matter, matter and force, are such great world problems, and they are of a physical character. We shall demonstrate that the most general distinction between reason and its object furnishes the key to the solution of the great world problems. +(a) Cause and Effect.+