So long as emphasis is laid upon the words “we are taught,” I have no objection. People _have_ taught that, though I strongly object to such teaching, because I object to the idea “Energy in possibility” or “possible Energy” of any kind. I teach the identity of Energy in much the same terms as the identity of Matter; not merely the conservation, with the idea that one quantity can disappear and another quantity reappear. It is not another quantity, but the same; though it may have been locked up for any length of time. But then it has not been usually taught so, and I think you are dealing with what is usual. * * * * * Again, you say on page 227, “Energy suggests ‘doings’ and ‘happenings.’” No, say I, _activity_ suggests doings and happenings, and activity is Energy in transformation. Energy alone is something stored, like Capital. The earth’s rotational energy, for instance, is stored just as really as, and for a longer time than, the vegetation of the carboniferous epoch. Lower down you observe that “Force may be exerted though nothing moves.” Certainly it may, when resisted by an equal opposite force. But I fully admit that a lot of nonsense has been talked about the acceleration measure of force, as if it were the only measure, and that some criticism on this procedure is useful. But I should not speak of “latent” force; it is real force you have in mind, or at least real stress—_i.e._ two equal and opposite forces. It is latent Activity which becomes active when the other factor, viz. Motion, is supplied or allowed—_e.g._ by the release of a bent bow, or a wound-up spring, or a raised weight. So it is also with the Energy of a fly-wheel. That, too, is latent Activity until the other factor, viz. Force, is supplied, _i.e._ when it is employed to overcome resistance, and therefore do work. Otherwise its Motion will be stored to all eternity. In short, activity, or doing of work, has two factors, Force and Motion. When both are present, work is done; when either is present alone, Energy is stored. Static Energy is the Force factor, with the possibility of a certain range of effectiveness understood; like a head of water, for instance, a certain height above the sea. Kinetic Energy is the Motion factor, with a certain inertia or possibility of Force understood; not Motion alone, but a mass in motion, so that it may be able to overcome resistance. There is no real reason why one form of Energy should be considered more “actual” or real than another; our eyes appreciate the one form, our muscles could appreciate the other. * * * * *