  Schlick applied these insights to the fact that no physical means suffice to distinguish inertial frames or, in other words, no (uniform, rectilinear) motion can be detected relative to the ether. There are two alternative responses to this situation. The first alternative, due to Lorentz and Fitzgerald, accommodates experimental findings through the postulation of compensating contractions of moving bodies in the direction of motion. Buttressed by additional auxiliary hypotheses, the Lorentz-Fitzgerald hypothesis preserves the absolute space and time of Euclid and Newton, as well as Galilean kinematics, while explicating experimental failures to detect the absolute rest of the ether by positing a real effect of absolute motion on length. The alternative presented by Einstein in the Special Theory was simply to deny the presupposition of an absolute time reference, allowing that two spatially separated events may be temporally ordered in one way for a given system of reference and may also be ordered differently for a distinct, yet equally legitimate system. Contractions of length are then a consequence of the relativity of reference frames: the length of a measuring rod depends on its velocity for a given frame of reference (Schlick 1979a, 160–1). Consequently, the facts of observation are accommodated equally well by the Principle of Special Relativity as they are by the Lorentz-Fitzgerald hypotheses. In other words, they are equivalent or “both theories do the same thing”.  (Schlick 1979a, 162) The principal advantage of Einstein’s approach is that his solution is clearly the simplest. At this juncture, it should be noted that Schlick did not argue that the choice between the available alternatives is conventional because they are empirically equivalent, implying all the same observational consequences. Rather, Schlick repeatedly urged that there is an underlying physical equivalence from which the empirical or observational equivalence follows. And the fact that it is the physical rather than observational equivalence which serves as premise of his argument is especially evident from his use of his earlier analysis of the concept of truth to explicate the equivalence.