I N T R O D U C T I O N T O V O L U M E 1 3 x l v there exist nonstatic solutions to Einstein’s general relativistic equations for the universe. He showed that it was possible to have a universe whose curvature is con- stant in space but changes with time. Einstein, however, was convinced that such a universe is impossible. He be- lieved he had found a mistake in Friedmann’s mathematical arguments. His “Com- ment on Friedmann’s Paper: ‘On the Curvature of Space’” (Einstein 1922p, Doc. 340) points to this problem. More than two months later, Friedmann, learning of Einstein’s “Comment” from a friend, reacted with a long letter to Einstein, point- ing out errors in the “Comment” (Doc. 390). Our volume ends before the debate is settled, but eventually Einstein, after further persuasion, published a correction to his former note on 31 May 1923 (Einstein 1923g). Franz Selety, an independent scholar from Vienna, who had earlier correspond- ed with Einstein,[10] wrote a long paper on “Contributions to the Cosmological Sys- tem,” published in Annalen der Physik. Einstein immediately replied (Einstein 1922q, Doc. 370), criticizing the assumption, held by Selety and others, that our Milky Way is one of many similar systems and that the spiral nebulae visible to us are indeed similar systems (see also Docs. 59 and 371). And in yet another comment on a paper by Erich Trefftz on the cosmological im- plications of general relativity, Einstein engaged with a solution to the vacuum field equations of Einstein 1919a (Vol. 7, Doc. 17), which is also a solution to the orig- inal vacuum field equations with a cosmological term. Trefftz’s solution, which was also discussed by Friedrich Kottler and Hermann Weyl and is known today as the Kottler-Trefftz-Weyl (KTW) solution, was interpreted by Trefftz as a static so- lution of spherical spatial topology. This solution had only two singularities, and two masses were associated with these singularites. The solution also had no other masses, i.e., it was a static two-particle solution. Not only would static two-particle solutions have direct cosmological implications, but it was uncertain whether they existed at all (they do not exist). In any case, Einstein criticized Trefftz’s interpre- tation as physically not viable. The debate continued past the time period covered by this volume. In these published comments by Einstein and in his correspondence about the cosmological interpretation of the theory of general relativity, one can observe a subtle but decisive shift in his views on the role of Mach’s principle. It no longer functions as a constitutive heuristic principle for the conceptual foundation of the theory but rather becomes a selection principle for different solutions to the same set of equations. Einstein published two papers in an effort to unify the electromagnetic with the gravitational field. The first, Einstein and Grommer 1923a (Doc. 12), was received 10 January 1922 and published during 1923. The paper was based on a calculation