Almost all natural sciences have a sort of naïveté in their own spheres; they all stand on the ground of what has been called a naïve realism, as long as they are, so to say, at home. That in no way prejudices their own progress, but it seems to stand in the way of establishing contact with any higher form of human knowledge than themselves. One may be a first-rate organic chemist even when looking upon the atoms as small billiard balls, and one may make brilliant discoveries about the behaviour of animals even when regarding them in the most anthropomorphic manner--granted that one is a good observer; but it can hardly be admitted that our chemist would do much to advance the theory of matter, or our biologist to solve the problem of the relations between body and mind. It is only by the aid of philosophy, or I would rather say by keeping in constant touch with it, that natural sciences are able to acquire any significance for what might be called *the* science of nature in the most simple form. Unhappily the term “natural philosophy” is restricted in English to theoretical physics. This is not without a high degree of justification, for theoretical physics has indeed lost its naïveté and become a philosophy of nature; but it nevertheless is very unfortunate that this use of the term “natural philosophy” is established in this country, as we now have no proper general term descriptive of a natural science that is in permanent relation to philosophy, a natural science which does not use a single concept without justifying it epistemologically, *i.e.* what in German, for instance, would simply be called “Naturphilosophie.” Let us call it philosophy of nature; then we may say that only by becoming a true philosophy of nature are natural sciences of all sorts able to contribute to the highest questions which man’s spirit of inquiry can suggest. These highest questions themselves are the outcome of the combination of the highest results of all branches of philosophy, just as our philosophy of nature originated in the discussion of the results of all the separate natural sciences. Are those highest questions not only to be asked, are they to be also solved? To be solved in a way which does not exceed the limits of philosophy as the domain of actual understanding? The beginning of a long series of studies is not the right place to decide this important question; and so, for the present certainly, “natural theology” must remain a problem. In other words: it must remain an open question at the beginning of our studies, whether after all there can be any final general answer, free from contradictions, applicable to the totality of questions asked by all the branches of philosophy.