The recently published Vol. 8 of Einstein’s Collected Papers brings together for the first time all extant letters and postcards documenting the famous debate of 1916–18 between Einstein and the Leyden astronomer Willem de Sitter (1872–1934), over, as they referred to it, the relativity of inertia. It was in the course of this debate that the first two relativistic cosmological models were proposed: the “Einstein cylinder world,” filled with a uniform static mass distribution; and the completely empty “De Sitter hyperboloid world” (a name introduced in Weyl 1923 , p. 293). In discussing the latter, Einstein and De Sitter had difficulty distinguishing features of the model from artifacts of its various coordinate representations. The situation was clarified in 1918 in correspondence between Einstein and two of the greatest mathematicians of the era, Hermann Weyl (1885– 1955) in Zurich and Felix Klein (1849–1925) in Göttingen. (Thanks to Klein and David Hilbert (1862–1943), Göttingen in those days was the math capital of the world.) Some of the issues that Einstein discussed with De Sitter also come up in Einstein’s correspondence in 1918 with the German physicist Gustav Mie (1868–1957). The picture that emerges is one of Einstein holding on with great tenacity to two beliefs concerning the universe that guided him in the construction of his cosmological model: first, that the universe is static; and second, that its metric structure is fully determined by matter—in other 2 words, that its metric field satisfies what, in 1918, he called “Mach’s principle.” De Sitter’s vacuum solution of Einstein’s field equations with cosmological term is a counterexample to this principle, and, for this reason, Einstein tried to discard it on various grounds. Two main lines of attack can be discerned: one was to argue that the De Sitter solution is not static; the other was to 3 argue that it has what today would be called an intrinsic singularity, which in turn was used to argue that it is not matter-free...