a fundamental new definition of space and time (all time and space coordinates in all reference frames are on an equal footing, so there is no physical basis for distinguishing "true" from "apparent" time) and makes the aether an unnecessary concept, at least in regard to inertial motion. Einstein identified two fundamental principles, the principle of relativity and the principle of the constancy of light (light principle), which served as the axiomatic basis of his theory. To better understand Einstein's step, a summary of the situation before 1905, as it was described above, shall be given (it must be remarked that Einstein was familiar with the 1895 theory of Lorentz, and Science and Hypothesis by Poincaré, but not their papers of 1904–1905): a) Maxwell's electrodynamics, as presented by Lorentz in 1895, was the most successful theory at this time. Here, the speed of light is constant in all directions in the stationary aether and completely independent of the velocity of the source; b) The inability to find an absolute state of motion, i.e. the validity of the relativity principle as the consequence of the negative results of all aether drift experiments and effects like the moving magnet and conductor problem which only depend on relative motion; c) The Fizeau experiment; d) The aberration of light; with the following consequences for the speed of light and the theories known at that time: The speed of light is not composed of the speed of light in vacuum and the velocity of a preferred frame of reference, by b. This contradicts the theory of the (nearly) stationary aether. The speed of light is not composed of the speed of light in vacuum and the velocity of the light source, by a and c. This contradicts the emission theory. The speed of light is not composed of the speed of light in vacuum and the velocity of an aether that would be dragged within or in the vicinity of matter, by a, c, and d. This contradicts the hypothesis of the complete aether drag. The speed of light in moving media is not composed of the speed of light when the medium is at rest and the velocity of the medium, but is determined by Fresnel's dragging coefficient, by c. In order to make the principle of relativity as required by Poincaré an exact law of nature in the immobile aether theory of Lorentz,