field components, charge and current density, kinematics of "electrons"). So he had to find a spacetime description and transformation laws from one frame to the other for all the physical quantities involved in electrodynamics (em. Einstein's argument in the famous 1905 paper on moving bodies was in terms of a symmetry principle, i.e., in modern terms he demanded to find a spacetime description such that (a) the special principle of relativity is fulfilled (tacitly assuming also that in any inertial frame an observer finds the geometry of space to by Euclidean) and (b) the Maxwell equations are form invariant. I think Einstein wanted to give a physical operational definition of the reference frames involved, and indeed there are two frames of reference involved here: the rest frame of the light source and the observer's rest frame.
I think he's distinguishing his theory from both ether theory, where the speed of light is #c# relative to the ether, and ballistic theory, where the speed of light is #c# relative to the emitter.
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