A particular planet is identified by its constant power to affect oureyes in a special way. Further, such planet has not been seen to move bythe astronomer; but its motion is inferred from a comparison of its presentposition with the position it before occupied. This comparison proves tobe a comparison between the different impressions produced on him by thedifferent adjustments of his observing instruments. And the validity of theinferences. drawn depends on the truth of the assumption that these massesof matter, celestial and terrestrial, continue to affect his senses in thesame ways under the same conditions. On going a step further back, it turnsout that difference in the adjustment of his observing instrument, and byimplication in the planet's position, is meaningless until shown to correspondwith a certain calculated position which the planet must occupy, supposingthat no motion has been lost. And if, finally, we examine the implied calculation,we find that it takes into account those accelerations and retardations whichellipticity of the orbit involves, as well as those variations of motioncaused by adjacent planets -- we find, that is, that the motion is concludedto be indestructible not from the uniform velocity of the planet, but fromthe constant quantity of motion exhibited after allowances have been madefor the motions communicated to, or received from, other celestial bodies.
And when we ask how this is estimated, we discover that the estimate assumescertain laws of force or energy; which laws, one and all, embody the postulatethat energy cannot be destroyed.
Similarly with the a priori conclusion that Motion is continuous. Thatwhich defies suppression in thought (disciplined thought, of course), isthe force which the motion indicates. We can imagine retardation to resultfrom the actions of other bodies. But to imagine this we must imagine lossof some of the energy implied by the motion. We are obliged to conceive thisenergy as impressed in the shape of reaction on the bodies causing the retardation.
And the motion communicated to them, we are compelled to regard as a productof the communicated energy. We can mentally diminish the velocity or space-elementof motion, by diffusing the momentum or force-element over a larger massof matter; but the quantity of this force-element is unchangeable in thought.*
Chapter 6
The Persistence of Force (*)
§60. In the foregoing two chapters, manifestations of force of twofundamentally-different classes have been dealt with -- the force by whichmatter demonstrates itself to us as existing, and the force by which it demonstratesitself to us as acting.
