In 1905, Einstein was not able to explain his postulate c=const differently than only by symmetry. If a person moves an electrical conductor in the presence of a magnet, current will appear in the conductor. If the person moves the magnet now in the presence of the conductor, the current will again appear in the conductor - regardless of the fact that he homocentrically explains that it is once because of the electromotive force, another time because of the magnetomotive force, depending on what he holds in his hand. Yes, the unique electromagnetic force is transmitted at the same speed whether the magnet or conductor coordinate system is stationary. Equal and symmetrical. And precisely because of that equality, that is, mutual relativity, that Einstein’s explanation with the train and lightning on the railway embankment is not valid. Because of tacit homocentrism also the twin paradox is only a pseudo paradox, because of a misunderstanding of the postulate c=const. The real explanation for c=const is c2 inertia, c2 as a measure of inertia, not mass. So can such an explanation open the way to understanding that deep connection ε0μ0m between electromagnetism and gravity, and finally perhaps make the graviton hypothesis redundant precisely because the inertial and gravitational masses are not only equal but identical—that is, if they really are? I specifically summarize my five previous articles on viXra.org here with the intention of showing that the title of this article is appropriate: it suggests that Einstein’s theory of relativity should be understood via c2 inertia and not via assuming masses already realized, whereby the light speed c=const appears to us post factum only as kinematic quality.