2 5 4 D O C U M E N T 3 1 9 A U G U S T 1 9 2 2 theory are defined for so-called “quasi-periodic” mechanical systems. This general rule has proven its value in so very subtle and varied special cases (e.g., Epstein’s theory of the Stark effect)[18] that its general correctness has become quite probable. From the general theoretical point of view, the following is especially puzzling. On one hand—as already pointed out—mechanics does not appear to be generally valid, as statistical mechanics, which is based on it, leads to results that contradict experience (e.g., specific heats of solids). On the other hand, the mechanical laws most astonishingly stand the test within the scope of validity of the above rule. Are there only supposed to be quasi-periodic elementary processes in nature or more generally only such mechanical systems as have as many complete integrals as degrees of freedom? Such a thought seems absurd in view of the kinetic theory of gases. The problem of how the validity of classical mechanics (and electrodynam- ics) is restricted by the demands of quantization is shrouded in as deep darkness today as it was 15 years ago. It has often been noted that in the present state of our knowledge, the possibility of representing natural laws by differential equations appears doubtful.[19] Indeed, according to the quantum rule just indicated, a complete cycle of the system must be considered in order to be able to judge whether a particular state of the system is quantum-theoretically permissible or not. In order to really do justice to the quantum relations, a new mathematical language seems to be necessary in any case it seems preposterous to express the laws through a combination of differential laws and integral conditions as we do today. Once again the foundations of theoret- ical physics are shaken and experience is calling for the expression of a higher level of lawfulness. When will the saving idea be granted us? Happy are those who may live to see it. 319. From Hantaro Nagaoka Science College, Imperial University, Tokyo, Japan, 2 August 1922 Highly esteemed Colleague, Your valued letter of the 22nd of May pleased me greatly,[1] because your arrival in Tokyo is already beyond doubt and your hope of engaging in closer relations with scholars of the Far East has become clear. Sadly, I fear that you will ultimately say: “There are no scholars in the Far East!” News about your oriental voyage has much roused the interest of the Japanese public I have often been asked to give popular talks about the principle of relativity.[2] Apparently people have a great curiosity about the concept of space [p. 8]