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

σ
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