Max Planck

“ We imagine that the particle is located at the origin of the coordinates of the "unprimed" system (x, y, z, t) and has the velocity components with respect to this system, and ask for the equations of motion. This question can be answered unequivocally by the fact that we first think of the mass points as the origin of a new reference frame, which moves against the original frame with the constant velocity components . The x-axis of this system may coincide with the direction of the velocity q of the mass point, whose size is expressed by 3) . ”

“ Therein the task is treated to determine the preferred form of the fundamental equations of mechanics, which take the place of the usual Newtonian equations of motion of a free mass point:
when the relativity principle should have general validity. ”

“ However, the question of an electrodynamic explanation of inertia remains open; but instead there arises, on the other hand, the advantage that it's not necessary to ascribe to the electron neither a spherical form nor even any other form in order to arrive at a certain dependence of inertia on speed. ”