Einstein in his formative years (1895–1902) sensed a deep crisis in the foundations of physics. On the one hand, the mechanical view failed to explain electromagnetism, and this failure invited criticisms from the empiricist philosophers, such as Ernst Mach, and from the phenomenalist physicists, such as Wilhelm Ostwald and Georg Helm. These criticisms had a great influence on Einstein's assessment of the foundations of physics. His conclusion was that the mechanical view was hopeless. On the other hand, following Max Planck and Ludwig Boltzmann, who were cautious about the alternative electromagnetic view and also opposed to energeticism, Einstein, unlike Mach and Ostwald, believed in the existence of discrete and unobservable atoms and molecules, and took them as the ontological basis for statistical physics. In particular, Planck's investigations into black body radiation made Einstein recognize a second foundational crisis, a crisis in thermodynamics and electrodynamics, in addition to the one in the mechanical view. Thus it was ‘as if the ground had been pulled out from under one, with no firm foundation to be seen anywhere, upon which one could have built’ (Einstein, 1949).

Einstein's reflections on the foundations of physics were guided by two philosophical trends of the time: critical scepticism of David Hume and Mach, and certain Kantian strains that existed, in various forms, in the works of Helmholtz, Hertz, Planck, and Henri Poincaré. Mach's historico-conceptual criticism of Newton's idea of absolute space shook Einstein's faith in the received principles, and paved for him a way to GTR.