Body is distinguishable from space by its power of affecting our senses,and, in the last resort, by its opposition to our efforts. We can conceiveof body only by joining in thought extension and resistance: take away resistance,and there remains only space. In what way this force which produces space-occupancyis conditioned we do not know. The mode of force which is revealed to usonly by opposition to our own powers, may have for one of its factors themode of force which reveals itself by the changes initiated in our consciousness.

That the space a body occupies is in part determined by the degree of thatactivity of its molecules known as heat, is a familiar truth. Moreover, suchmolecular rearrangement as occurs when water is changed into ice, is shownto be accompanied by an evolution of force which may burst the containingvessel and give motion to the fragments. Nevertheless, the forms of our experienceoblige us to distinguish between two modes of force; the one not a workerof change and the other a worker of change, actual or potential. The firstof these -- the space-occupying kind of force -- has no specific name.

For the second kind of force, the specific name now accepted is "Energy."That which in the last chapter was spoken of as perceptible activity, iscalled by physicists, "actual energy;" and that which was therespoken of as latent activity, they call "potential energy." Whileincluding the mode of activity shown in molar motion, Energy includes alsothe several modes of activity into which molar motion is transformable --heat, light, etc. It is the common name for the power shown alike in themovements of masses and in the movements of molecules. To our perceptionsthis second kind of force differs from the first kind as being not intrinsicbut extrinsic.

In aggregated matter as presented to sight and touch, this antithesisis, as above implied, much obscured. Especially in a compound substance,both the latent energy locked up in the chemically-combined molecules andthe actual energy made perceptible to us as heat, complicate the manifestationsof intrinsic force by the manifestations of extrinsic force. But the antithesis,here partially hidden, is clearly seen on reducing the data to their lowestterms -- a unit of matter, or atom, and its motion. The force by which itexists is passive but independent; while the force by which it moves is activebut dependent on its past and present relations to other atoms. These twocannot be identified in our thoughts. For as it is impossible to think ofmotion without something that moves, so it is impossible to think of energywithout something possessing the energy.

While recognizing this fundamental distinction between that intrinsicforce by which body manifests itself as occupying space, and that extrinsicforce distinguished as energy, I here treat of them together as being alikepersistent. And I thus treat of them together partly because, in our consciousnessof them, there is the same essential element. The sense of effort is oursubjective symbol for objective force in general, passive and active. Powerof resisting that which we know as our own muscular strain, is the ultimateelement in our idea of body as distinguished from space; and any motor energywhich we give to body, or receive from it, is thought of as equal to a certainamount of muscular strain. The two consciousnesses differ essentially inthis, that the feeling of effort common to them is in the last case joinedwith consciousness of change of position, but in the first case is not.*

There is, however, a further and more important reason for here dealingwith the proposition that Force under each of these forms persists. We haveto examine its warrant. §61. A little more patience is asked. We must reconsider the reasoningby which the indestructibility of Matter and the continuity of Motion areestablished, that we may see how impossible it is to arrive by parallel reasoningat the Persistence of Force.

In all three cases the question is one of quantity. Does the Matter orMotion, or Force, ever diminish in quantity? Quantitative science impliesmeasurement, and measurement implies a unit of measure. The units of measurefrom which all others of any exactness are derived, are units of linear extension.