"It has seemed to me useful to develop a theory of electromagnetic phenomena based on the idea of a ponderable matter perfectly permeable to the ether and able to move without communicating to the latter the least motion. Certain facts of optics may be invoked in support of this hypothesis and, although doubt is still permitted, it is certainly important to examine all the consequences of this view. Unfortunately, a quite serious difficulty presents itself from the beginning. How, in fact, is one to form a precise idea of a body which, moving in the bosom of the ether and consequently traversed by this medium, is at the same time the seat of an electric current or a dielectric phenomena? To surmount this difficulty, so far as I could, I have sought to reduce all the phenomena to a single one, the simplest of all, which is nothing else than the motion of an electrified body. One will see that, without any deeper study of the relation between ponderable matter and the ether, one can establish a system of equations suited to describe what passes in a system of such bodies. These equations accommodate very varied applications, which will be the object of the subsequent chapters." Hendrik Lorentz