That a phenomenon must be based on nature, or an effect on a cause, is a fact identical with "causality" which I already promised to discuss in the preceding letter. This same problem may also be expressed in the words: Where there are predicates, there must be a subject that carries them. In order to make quite sure that I will not be misunderstood, I emphasize once more the fact that I am not raising any doubt as to the correctness of this conclusion, but only to the metaphysical application of this conclusion after the Kantian manner which consists in making the same use of it as a clergyman who tries to prove that his theology is innate in reason. Our conception of logic wishes to show that all causes and effects are matter of the same kind, and that our faculty of reasoning is a matter of fact thing which brooks no mysteries or metaphysical dreams. SIXTEENTH LETTER Now let me illustrate the interconnection of all things, or the world-unit, by discussing the question of causality. We know that everything has its cause. We know that this is also true on the Moon or on Uranus, although we have not acquired this knowledge by experience on those world bodies. Thus it seemed that the intellect was a mysterious receptacle containing innate wisdom. The same receptacle also contains, for instance, the truth that all white horses are white and all black horses black. We do not know anything about the color of other horses in other countries, but the color of black and white horses we know even if we have never seen them in other countries. It is thus apparent that our intellect is an instrument which reaches beyond experience. For this reason there would seem to be no telling where the supply of such miraculous revelations would stop and into what mysterious worlds the intellect passing beyond the limits of experience would lead us. In order that the human intellect may not appear transcendental, in order to give it its place in the general classification of natural forces, we must investigate the nature of causality and so-called _a priori_ knowledge. Kindly observe in the first place that a thing is just as wonderful _after_ it is explained as it was before its explanation. A scientific explanation of a thing ought not to do away with our admiration, but only to reduce it to reasonable bounds. The intellect may very well be regarded as something wonderful, but its wondrous quality should be reduced to the measure of all things which are none of them any less wonderful. After you have explained what water is, after you have learned that it is composed of two chemical elements, after you have realized all its qualities thoroughly, it still remains a wonderful, divine, fluid.