Examples of this sort (asymmetries in the prevailing view of electrodynamic phenomena), together with the unsuccessful attempts to discover any motion of the earth relative to the “light medium,” suggest that the phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest.  They suggest rather that . . . the same laws of electrodynamics and optics will be valid for all frames of reference for which the laws of mechanics hold good.  We will raise this conjecture (the purport of which will hereafter be called the “Principle of Relativity”) to the status of a postulate, and also introduce another postulate which is only apparently irreconcilable with the former, namely that light is always propagated in empty space with a definite velocity c which is independent of the state of motion of the emitting body.  These two postulates suffice for the attainment of a simple and consistent theory of the electrodynamics of moving bodies based on Maxwell’s theory for stationary bodies.  The introduction of the “luminiferous ether” will prove to be superfluous inasmuch as the view here to be developed will not require an “absolute stationary space” provided with special properties, nor assign a velocity vector to a point in space in which electromagnetic processes take place.

 

Albert Einstein (1905)