1. Introduction. It is an amazing thing that, unlike pebbles, clouds and stars, the smallest building stones of Nature—the electrons, the protons and the neutrons—are all completely uniform. There seems to be a necessity in Nature for systems of a very special form, and one can wonder now what kind of necessity this is. I believe that this can only be a mathematical necessity, that consequently the principle of Nature is a mathematical one. How else could Nature obey the mathematical laws of physics if it were not because of her own principle? It can be argued that these mathematical laws, like Newtonian mechanics, the classical theory of electromagnetism and the quantum theory of the electron are all some form of beginner's luck, and that continued investigation will reveal Nature as more and more mysterious and irrational. If that were true, one might as well give up the pursuit of physics completely! But on one hand modern physics has discovered what we believe to be the elementary particles of the material world, and on the other hand it comes out that at least a part of these elementary forms obey mathematic laws to a very high degree of accuracy.