2 2 D O C . 2 7 A P R I L 1 9 1 9 exceed 8 printed pages, since the Academy no longer accepts longer papers by non- members because of the enormous printing costs.[7] In assuring you of my great interest in your idea, I am with kind regards, yours, A. Einstein. 27. From Felix Klein Göttingen, 22 April 1919 Esteemed Colleague, My reply to your kind postcards[1] is rather meager. That’s because I have been distracted from working on your pieces for a few weeks, in that I was forced to re- turn to my early essays on linear geometry for the sake of a reprinting of my old papers to please my friends.[2] It has become evident that members of the younger generation, who are generally of help to me, do not know about these things at all anymore, and so I held a series of appropriate lectures with discussions about the subsequent related literature, which took up all of my time. My intention now is to highlight similarly everything that is connected with my Erlangen program of 1872.[3] In this context I hope to succeed in giving a condensed presentation of pre- cisely your theories (from my mathematical standpoint) as well.[4] From the outset I do feel that I am in agreement with you in principle, as far as the range of indivi- dual assumptions is concerned: in contrast to the majority of your followers, who see the latest form of your theories as final and binding, you have maintained the freedom to look for increasingly refined formulations of the general foundations and simultaneously, in accordance with each individual problem under considera- tion, for specific assumptions that sufficiently approximate the relevant circum- stances. In heartily concurring with you in my own way of thinking, I also welcome in particular your new speculations (Hamilton’s principle is not a conceptual neces- sity [Denknotwendigkeit] for me).[5] But I do not yet see how far this will lead. I presume I may keep the correction proofs? A few more details: 1. What you write about the topology [analysis situs] of the elliptic plane was thoroughly and variously discussed at the t[ime] (1876) between me and Schläfli (Math. Ann. 7, p. 550).[6] The rel[evant] relationships seem odd to us only because they are not familiar to us from daily experience. 2. The energy components of the gravitational field, which I call , appear as such, as I subsequently noticed, in Fokker’s paper in early 1917. But Fokker is un- justified there in distinguishing them from Lorentz’s components (which do look Uτ σ