An old principle of inertia can explain special relativity in a new way. With a proper understanding of motion, the speed of light is constant c only with reference to the source of emission, while the magnitude of its average velocity remains constant c regardless of the motion of the emitter. This interpretation on the invariance of light speed was published in a previous report, but the purpose now is to make that presentation more clear. And for something new, a self-sufficient cause for inertia is to be explored.
Keywords: Inertia, Circular Motion, Special Relativity