 The main empirical evidence bearing on this issue consists of the patterns of acceptance and rejection of competing theories by working scientists. Einstein’s development of Special Relativity—and its impact on the hypothesis of the existence of the electromagnetic ether—is one of the episodes most often cited (by both philosophers and scientists) as an example of Occam’s Razor in action (see Sober 1981, p. 153). The ether is by hypothesis a fixed medium and reference frame for the propagation of light (and other electromagnetic waves). The Special Theory of Relativity includes the radical postulate that the speed of a light ray through a vacuum is constant relative to an observer no matter what the state of motion of the observer. Given this assumption, the notion of a universal reference frame is incoherent. Hence Special Relativity implies that the ether does not exist.