 This is why it is so important that Einstein’s 1905 paper begins with a critical analysis of the entire notion of a frame of reference. It is tacitly assumed by Lorentz’s theory, and classical electrodynamics generally, that we have a reference-frame in which we can measure the velocity of light, and that there is a distinguished frame—at rest with respect to the ether—in which its true velocity would be measured. But how is such a reference-frame determined? The distances between points in space can only be determined if it is possible to determine which events are simultaneous. In practice this is always done by light-signalling, if only in the informal sense that we identify simultaneous events when we see them at the same time. But if the spatial frame of reference is determined by light-signals, and is then to be used to measure the speed of light, we would appear to be going in a circle. For this reason, Poincaré concluded that determining the speed of light is partly a matter of convention (1898). Before Einstein, however, it was tacitly assumed that, while light-signalling is useful and practical, it is not essential to the definition of simultaneity. There would be, therefore, a fact of the matter about which events are simultaneous that is independent of this method of signalling. This assumption was actually made explicit by James Thomson. He recognized—as few did before Poincaré and Einstein—that the measurement of distance involves