 Most of Einstein’s early papers (1902–1911) prior to his nearly exclusive concentration on a relativitistic theory of gravitation (1909–1915) are devoted not to the theory of relativity but with problems posed to classical physics by Planck’s discovery of his eponymous energy constant in 1900. These early works reveal Einstein to be a strong supporter of Ludwig Boltzmann rather than Ernst Mach in the debate over atomism at the turn of the century (Ryckman 2017: chapter 3). Yet in 1912, Einstein’s name together with those of Göttingen mathematicians David Hilbert and Felix Klein, was prominently displayed (in the Naturwissenschaftliche Rundschau 27: 336) among those joining Mach’s in a call for the formation of a “Society for Positivist Philosophy”. Citing the pressing need of science “but also of our age in general” for a “comprehensive world view based on the material facts accumulated in the individual sciences”, the appeal appears above all to have been an orchestrated attempt to buttress Mach’s positivist conception of science in the face of recent criticisms by Max Planck, then Germany’s leading theoretical physicist. More a declaration of allegiance than an act of scholarly neutrality, it provides evidence of Einstein’s youthful enthusiasm for at least certain of Mach’s writings. Late in life (1949a: 21), Einstein wrote of the “profound influence” that Mach’s Science of Mechanics (1883) exercised upon him as a student as well as of the very great influence in his younger years of “Mach’s epistemological position”. Already in the special theory of relativity (1905), Einstein’s operational definition of the “simultaneity” of distantly separated events, whereby distant clocks are synchronized by sending and receiving light signals, is closely modeled on the operational definition of mass in Mach’s Mechanics. Moreover, occasional epistemological and methodological pronouncements seem to indicate agreement with core parts of Mach’s positivist doctrine of meaningfulness, e.g., “The concept does not exist for the physicist until he has the possibility of discovering whether or not it is fulfilled in an actual case” (1917a [1955: 22]). Thus the general theory of relativity might be seen as fully compliant with Mach’s characterization of theoretical concepts as merely economical shorthand for concrete observations or operations.