D O C . 3 7 6 A P R I L 1 9 2 0 3 1 5 “Einstein system” as a whole than is possible on the basis of your confused descrip- tions. (Axiomatization is not appropriate here so one must rely on intuition.) Van Aardenne is also a hopelessly bad describer.)[4] Best regards to all of you! Do come, at last!!!! Yours, Ehrenfest Take van Aardenne along to Christiania![5] 376. From Georg Vogelpohl[1] Osnabrück, 31 Johannismauer, 16 April 1920 Highly estimable Professor, Allow me first to extend my very obliging thanks for the information imparted to me in your letter of February 24th. The facts provided suffice for my purposes. I intentionally waited with my reply in order to be able to pose to you, esteemed Professor, as you had most kindly offered in your letter, any questions raised in the discussions following the talks that I was not capable of answering. Such questions did not arise, however, with one exception, namely: What, according to the general theory of relativity, is supposed to exist beyond the spherical universe? I answered the question with a two-dimensional analogy using the example of an intelligent being, a shadow on a sphere. For him, three dimensions belong in the realm of metaphysics three-dimensional phenomena that he observes in two dimensions (for example, a deformation of the sphere’s surface by a physical object. Further- more, he will certainly perceive in some way or other the world of objects in his flat world as well) at most compel him to assume the existence of a higher dimen- sion than his own. We, however, do not observe any phenomena beyond our three- dimensional world that allow us to conclude the existence of a higher dimension. Accordingly, what lies beyond the universe belongs for us completely in the realm of metaphysics, since physical reality has only 3 dimensions. Metaphysical ques- tions may safely be consigned to philosophers for their kind appraisal. Perhaps, Professor, you will be so good as to give your opinion on these repre- sentations, as this question, which virtually everyone is posing, is of importance and must in any event find an answer. I have also been prompted by the Monist League to speak about the theory of relativity and have accepted this invitation as well. The material here is divided into 4 talks that cover the physical theory in a purely factual manner.[2] I did not show that the efforts of Monism to attribute all natural events to a single source are sig- nificantly supported by the theory of relativity, because the reputation of relativity