On the Electrodynamics of Moving Bodies, no properties of observed facts correspond to a concenpt of absolute rest; ...for all coordinate systems for which mechanical equations hold, the equivalent electrodynamical and optical equations hold also.... In the following we make these assumptions and introduce further assumption - an assumptions which is at first sight quite irreconcilable with the former one - that light is propogated in vacant space, with a velocity c which is independent of the nature of motion of the emitting body. These two assumptions are quite sufficient to give us a simple and consistent theory of electrodynamics of moving bodies on the basis of the Maxwellian theory for bodies at rest.

-- Albert Einstein

The special theory of relativity is a necessary consequence of any assertion that the unity of Physics is essential, for it would be intolerable for all inertial systems to be equivalent from a dynamical point of view yet distinguishable by optical measurements. It now seems incredible that the possiblity of such a discrimination was taken for granted in the nineteenth century, but at the same time it was not easy to see what was more important - the universal validity of the Newtonian principle of relativity or the absolute nature of time.

-- Herman Bondi