Already the English sceptic Hume felt that true causes are different from assumed causes. According to him the concept of a cause contains nothing but the experience of that which generally precedes a certain phenomenon. Kant rightfully remarks on the other hand that the conception of cause and effect expresses a far more intimate relation than that indicated by a loose and accidental succession, and that the concept of a cause rather comprises that of a certain effect as a necessity and strict general result. Therefore he claimed that there must be something _a priori_ in reason which cannot be experienced and which extends beyond experience. We reply to the materialists who deny all autonomy of the mind and hope to detect causes by experience alone that the general necessity which presupposes the relation of cause and effect represents an impossible experience. And we reply to the idealists: Although reason explores causes which cannot be experienced, this research cannot take place _a priori_, but only _a posteriori_, only on the basis of empirically given effects. It is true that the mind alone discovers the imperceptible and abstract generality, but it does so only within the circle of certain given sense perceptions. +(b) Matter and Mind.+ The understanding of the general dependence of the faculty of thought on material sense perceptions will restore to objective reality that right which has long been denied to it by ideas and opinions. Nature with its varied concrete phenomena which had been crowded out of human considerations by philosophical and religious imaginings, and which has been scientifically re-established again on special fields by the development of natural sciences, gains general theoretical recognition by the understanding of the functions of the brain. Hitherto natural science has chosen for its object only special matters, special causes, special forces, but has remained ignorant in general questions of so-called natural philosophy regarding the cause of all things, of matter, of force in general. The actual existence of this ignorance is revealed by that great contradiction between idealism and materialism which pervades all works of science like a red thread. "May I succeed in this letter in strengthening the conviction that chemistry as an independent science represents one of the most powerful means for the higher cultivation of the mind, that its study is useful not alone for the promotion of the material interests of mankind, but because it permits a deeper penetration of the wonders of creation, with which our existence, our welfare, and our development are intimately connected."