[Undated]

The contents and method of the theory of relativity can, despite the variety of experimental physical facts on which the theory is based, be outlined in a few words. In contrast to the fact, known since ancient times, that movement is perceptible only as relative movement, physics was based on the notion of absolute movement. Optics had assumed that one state of movement, luminous ether, is distinct from all others. All movements of bodies were supposed to be related to the luminous ether, which was the incarnation of absolute motionlessness. If a fixed formal luminous ether filled everything in space, then the movements of bodies would be related to it and one could in this physical sense speak of “absolute movement” and ground mechanics on this notion. But after efforts to discover the privileged state of movement of this hypothetical luminous ether through physical experiments had failed, it seemed that the problem should be restated. That is what the theory of relativity did systematically. It assumed that there are no privileged physical states of movement and asked what consequences could be drawn from this supposition concerning the laws of nature. The method of the theory of relativity is analogous to the method of thermodynamics; for the latter is nothing more than the systematic answer to the question: how must the laws of nature be constructed in order to rule out the possibility of bringing about perpetual motion?

A further characteristic of the theory of relativity is an epistemological point of view. In physics no concept is necessary or justifiable on an a priori basis. A concept acquires a right to existence solely through its obvious and unequivocal place in a chain of events relating to physical experiences. That is why the theory of relativity rejects concepts of absolute simultaneity, absolute speed, absolute acceleration, etc.; they can have no unequivocal link with experiences. Similarly, the notions of “plane,” and “straight line,” and the like, which form the basis of Euclidian geometry, had to be discarded. Every physical concept must be defined in such a way that it can be used to determine in principle whether or not it fits the concrete case.