"It might be... that... motion is determined by a medium... In such a case we should have to substitute this medium for Newton's absolute space. ...[I]t is easily demonstrable that the atmosphere is not this motion-determinative medium. We should, therefore, have to picture to ourselves some other medium, filling, say, all space, with respect to... which... we have at present no adequate knowledge. In itself such a state of things would not belong to the impossibilities. It is known, from recent hydrodynamical investigations, that a rigid body experiences resistance in a frictionless fluid only when its velocity changes. ...[T]his result is derived theoretically from the notion of inertia; but it might... also be regarded as the primitive fact from which we... start. ...[W]e might... hope to learn more in the future concerning this hypothetical medium; and from the point of view of science it would be in every respect a more valuable acquisition than the forlorn idea of absolute space."