_Motion is not energy itself, but the by-product yielded by two systems of energies acting on each other._ This by-product will manifest itself, according to circumstances and antecedent conditions, at one time as circular, at another time as elliptical, at another as rhythmical motion, and so forth. In its essential nature this by-product--the movement perceptible to sense--corresponds wholly and completely to a shadow. As a shadow means nothing save that light is present--it is nothing but the by-product of two systems of energies, one giving, the other receiving, light--so “movement” means nothing save that energy is there present. It is nothing but the by-product of two systems of energies. It is absolutely essential that the genuine thinker should make this idea as to the intrinsic nature of all motion his own. As little as it is possible ever to draw from shadows any conclusion as to light itself--saving the one conclusion that it _must be present_--just as little is it possible ever to draw from movements any conclusion as to the energies themselves, saving only that they must be present. The energies themselves withal remain wholly inaccessible. As to whether these are transmitted mediately or immediately, the fact “movement” supplies no information whatever. Here the physicist will say, “That the movements are transmitted mediately is proved to me by experiment, since I can intercept an energy on its way at as many intermediate stations as I choose; hence, as mediate, can track its path.” But this is a grossly erroneous conclusion. To be sure, if I have a magnet here and a needle there, I can intercept the magnetic energy at as many intermediate stations as I choose, and so construct for myself a “path” for the energy. But such a “path” is nothing but a dead line artificially made up of momentary reactions whose continuity is nothing actual and vital, but founded solely upon the minuteness and multiplicity of the moments of section. Again the physicist may object:-- “We can measure exactly the speed with which the energies propagate themselves, as, for example, the time required for light to reach us from the moons of Jupiter.” But this also is an erroneous conclusion. Of course, the fact itself is beyond dispute. But the time here mentioned does not represent the transmission-speed of the energies themselves; it only informs us as to how much delay these have encountered on their way; whether the halting-places have been very numerous and the stay at each a long one. This time which the physicist measures does not give the speed of transmission of the energies, but only the time of their non-transmission. In accord with this is the incorrectness of ordinary physical terminology. The physicist calls light, heat, and so forth, energies themselves. But light is not energy itself, but only a designation for energies that lie for ever beyond our reach.