His anticipation, however, was not only bold; it was fortunate. Kinetic energy may really be transformed from molar to molecular motion, and suffer no variation. It is conserved. On the other hand, it may not. It may altogether cease, and what becomes of conservation then? The scientific formula which satisfies both the facts of the case and our desire for conservation is well known.[13] Energy, we are taught, is of two kinds: kinetic and potential energy—energy in act and energy in possibility. Each may turn into the other, and is continually so turning. Each, therefore, may vary in quantity, and does vary in quantity. It is only their sum which is indestructible. Few scientific generalisations have been more fruitful; few have been accepted on more slender evidence; none are more certain; none more clearly illustrate our natural appetite for beliefs of conservation. For, indeed, to the over-critical this sort of conservation must needs leave something to be desired. When we assert the indestructibility of matter we mean that a real entity continues through time unchanged in quantity. But the word has a less obvious meaning when it is applied to energy. The propriety of describing motion as energy seems indeed clear enough; and if all energy were energy of motion, and if energy of motion were always conserved, the conservation of energy would be on all fours with the conservation of matter. But this is not the case. In spite of Leibnitz, the amount of _vis viva_ is not indestructible. What, then, happens when some of it is destroyed? In that case, says science, energy changes its form but not its quantity. Energy of motion becomes energy of position. What was kinetic becomes potential; and, as the transformation is effected without loss, the principle of conservation is saved. When, however, energy thus becomes potential, in what sense does it still exist, and why do we still call it energy? Energy suggests “doings” and “happenings.” In the case of “potential” energy there are no “doings” and no “happenings.” It is “stored”; and stored it may for ever remain, hibernating (as it were) to all eternity, neither changing nor causing change. I do not quarrel with this; but I ask myself why “energy” should be treated more leniently than “force.” Though force is now known not to be “conserved,” ordinary thought attributes to it a certain continuity of existence even when it does not show itself in motion. Force may be exerted though nothing moves; as, for example, by a book pressing on a table. But this view is profoundly unsatisfactory to many scientific thinkers. For them force is nothing apart from “acceleration”; it does not represent a cause, it only measures an effect. And if in our ordinary moments we think otherwise, this (they think) is simply because we illegitimately attribute to matter something which corresponds to muscular effort in man.