The first step towards such explanation is, of course, in psychology, as in all other sciences, the careful observation of regularities. It quickly leads us to formulate some general laws. Psychology has known, for instance, for two thousand years, that if we have perceived two things together, and later we see the one again, the new perception brings us a memory image of the other thing. If we saw a man's face and heard at the same time his name, seeing his face may later awaken in us the memory of his name, or the hearing of his name may later awaken in us a reproduced memory image of his face. On such a basis, for instance, we formulate some general laws of association of ideas, and as soon as we have such laws laid down, we consider the appearance of such a memory image by association as sufficiently explained. We feel that it gives us sufficient basis to predict that in the future this idea will stir up in us the other idea. Psychology has formulated plenty of such general statements, and they serve well for a first orientation. Yet can this ever be considered as a last word of scientific explanation of psychical facts? Can psychology really in this way reach an ideal similar to that of scientific astronomy or chemistry? Would the scientist of nature ever be satisfied with this kind of explanation, which is nothing but generalization of certain sequences? Does not the explanation of the naturalist contain an entirely different element? He does not merely want to say that this effect has sometimes been observed and that there is thus probability that it will come again, when similar causes are given. No, the physicist wants to understand those connections of cause and effect as necessary ones. He tries to find sequences which cannot be otherwise because they cannot be thought in any other way. Therefore he is not satisfied with complex regularities, but analyzes them until he can bring them down to simple physical connections, and these physical connections finally to mechanical processes, which realize for us logical necessities. That matter lasts and cannot disappear is such a presupposition, which comes to us with the necessity of logical thinking. We simply cannot think it otherwise. And the whole idea of natural science is to conceive the physical universe in such a way that all changes in the outer world can be understood as the movements of its parts in accordance with such necessary physical axioms. If we knew all the atoms of the present status of the universe, and we knew every present movement of every atom, we should be able to foresee the position of every atom in the next moment and in the following moment and in all following moments, and all that by the necessary continuation of the substance and its energies. That alone is the background of all special physical inquiry, and we rely on the special laws of physics and chemistry, because we trust that this universe, as a whole, could be ultimately understood as