This leaving out of account of the actual energies is only possible in the world of reactions. Here it is possible, and therefore also legitimate, to regard any kind of process as a something constant and complete, as a product, and correspondingly to treat it as such. Every physicist knows that the grocer’s pound weight, as well as the grain of his own scales, rigorously tested, to-morrow are no longer the same as they were to-day. Nevertheless we make a compromise with actuality and act as though they were the same. It suffices for all practical purposes, and so is permissible. Here one is not at all aiming at a world-theory; one only seeks to measure and weigh, and satisfy certain needs. This compromise with actuality--the looking upon things as finished, completed--is forced upon us by the idea of identity, with which all mental life, without exception, operates. And the physicist accommodates himself to this idea with his concept of “body.” Body, in the physical acceptation of the word, is nowhere to be found in actuality; none the less the physicist is justified in making use of this idea so long as, in the pursuit of his aims, he can do so with advantage--that is, so long as it is a question of measuring and determining in advance. The re-actual point of view of science involves as logical correlate the merging in one, of “motion” as manifesting itself to sense, and “energy.” Aught else corresponding to energy besides motion itself is not to be found in the re-actual world of the physicist. Here motion is energy itself. Under these two preliminary conditions--the regarding of things as “bodies,” and their motions as energies themselves--the play of world-events displays itself in its entirety to perception by the senses; and every effect is something mediate, possible of being followed up in space and time. But the movements that are perceptible to sense are just as little the energies themselves as “bodies,” in the physical acceptation of the word, are actuality. The sensible motion is not the energy; it is only the evidence that energies are present. When two electro-magnets, placed in a certain position with reference to each other, go through circular movements, this does not mean that these circular movements are the energies themselves; it only means that energies are there present, and of themselves prove themselves such by producing effects. When a geyser discharges water every hour, it does not mean that this kind of action is energy itself; it means nothing more than that energies _are there present_, and as such are at work. The earth’s course round the sun does not represent energy itself; it means nothing more than that energies are there present, and as such are at work.