 During the decade 1905–1915, Einstein had more or less explicitly assumed that in a good theory there are certain individual parts that can be directly coordinated with the behavior of physically-existent objects used as probes. A theory can be said to be ‘true or false’ if such objects respectively behave or do not behave as predicted. In special relativity, as in classical mechanics, the fundamental geometrical/kinematical variables, the space and time coordinates, are measured with rods and clocks separately from the other non-geometrical variables, say, charge electric field strengths, which were supposed to be defined by measuring the force on a charge test particle. In general relativity, coordinates are no longer directly measurable independently from the gravitational field. Still, the line element \(ds\) (distance between nearby spacetime points) was supposed to have a ‘natural’ distance that can be measured with rods and clocks. In the late 1910s, pressed by the epistemological objections raised by different interlocutors—in particular Hermann Weyl (Ryckman 2005) and the young Wolfgang Pauli (Stachel, 2005)—Einstein was forced to recognize that this epistemological model was at most a provisional compromise. In principle rod- and clock-like structures should emerge as solutions of a future relativistic theory of matter, possibly a field theory encompassing gravitation and electromagnetism. In this context, the sharp distinction between rods and clocks that serve to define the geometrical/kinematical structure of the theory and other material systems would become questionable. Einstein regarded such distinction as provisionally necessary, give the current state of physics. However, he recognized that in principle a physical theory should construct rods and clocks as solutions to its equations (see Ryckman 2017, ch. VII for an overview on Einstein view on the relation between geometry and experience). 