6 2 D O C . 7 5 J U L Y 1 9 1 9 boys. It is to be hoped that your stay in Switzerland will be beneficial to your health as well. I wanted to write you recently already when I saw your name under an appeal to the intellectual workers of all nations.[3] It is dreadfully difficult to do anything for the sentiments of reconciliation. There are mostly Germans here at the sanatorium. At first, of course, the great questions came under discussion. Then my wife[4] and I always felt how wide a gap there is between everybody. The other people invari- ably had heard other facts. So one does not know whether to point to the error on one’s own side or to be silent about past events and try to talk about more positive new matters. By its behavior in Paris, the Entente did not make pacification easier either.[5] And yet, as much as I detest Parisian intrigues, I could feel virtually no sympathy for the others just because, with such a hue and cry and the slogan that this dictated peace must lead to a new war, they are already starting again with their asinine warmongering. It is sad, and it may possibly even get worse.—The coura- geous statement by Smuts remains a glimmer of hope for me.[6] It doesn’t suit me at all that I became sick at the end of the war, just as I was in- tending to travel abroad to visit you and Bohr. It may take until next spring before I can get away from here again. On the question of the relativity of inertia, I have not yet arrived at any conclu- sion. At first I wanted to size it up as follows: in the absence of matter, or better, at an infinite distance from other masses, a test body has an infinitesimal momentum, that is, an infinitesimal inertial mass.[7] Hence, with finite dx and dt diminishes to zero, in o[ther] w[ords], . However, the solution by Schwarzschild and Droste for the spherically symmetric case, from which the other cases that are an infinite distance away will surely hardly deviate, yields a finite .[8] So I investigated whether one obtains anything else when, instead of taking a permanent center of mass, one takes one that exists only for a split instant. I tried to find the solution for the 4-dimensional pseudohyperspherically symmetric case. (Sorry about the monstrous expression!) This furnishes constant ’s as the only solution. (This solution actually does not belong to an instantaneous flare-up of a center of mass a large spherical shell would rather have to contract upon itself and expand again, similar to a spherical wave of light shrinking into a point and then expanding outward, to exhibit this 4-dim. symmetry.) When I again could find no infinite velocity, I came to believe that the reason for continually finding a finite inertia lay in the mere assumption of any . I saw in an article by Klein (“Über die Integralform der Erh.sätze und die Th. d. räumlich-geschlossenen Welt” [On the Integral Form of the Laws of Conservation and the Theory of a Spatially Closed gabmdxb ds -------- ∑ g44 ∞→ g44 gab ds2