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.*<* This exposition differs in its point of view from the expositionsordinarily given; and some of the words employed, such as strain, have somewhatlarger implications. Unable to learn anything about the nature of Force,physicists have, of late years, formulated ultimate physical truths in suchways as often tacitly to exclude the consciousness of Force: conceiving cause,as Hume proposed, in terms of antecedence and sequence only. "Potentialenergy," for example, is defined as constituted by such relations inspace as permit masses to generate in one another certain motions, but asbeing in itself nothing. While this mode of conceiving the phenomena sufficesfor physical inquiries. It does not suffice for the purposes of philosophy.
In the Principles of Psychology, §§ 347-350, I have shownthat our ideas of Body, Space, Motion, are derived from our ideas of musculartension, which are the ultimate symbols into which all our other mental symbolsare interpretable. Hence to formulate phenomena in the proximate terms ofBody, Space, Motion, while discharging from the concepts the consciousnessof Force, is to acknowledge the superstructure while ignoring the foundation.
When, in 1875, I recast the foregoing chapter, and set forth more fullythe doctrine contained in the answering chapters of preceding editions, Isupposed myself to be alone in dissenting from the prevailing doctrine. Buta year after, in the Philosophical Magazine for October, 1876, I wasglad to see the same view enunciated and defended by Dr. Croll, in an essay"On the Transformation of Gravity." I commend his arguments tothose who are not convinced by the arguments used above.
Let me add a remark concerning the nature of the question at issue. Itis assumed that, as a matter of course, it is a question falling within thesphere of the mathematicians and physicists. I demur to the assumption. Itis a question falling within the sphere of the psychologists -- a questionconcerning the right interpretation of our ideas.>
Chapter 6
The Persistence of Force (*)
<* Some explanation of this title is needful. In the text itself aregiven the reasons for using the word "force" instead of the word"energy" and here I must say why I think "persistence"preferable to "conservation." Some two years ago (this was writtenin 1861) I expressed to my friend Prof. Huxley, my dissatisfaction with the(then) current expression -- "Conservation of Force," assigningas reasons, first, that the word "conservation" implies a conserverand an act of conserving; and, second, that it does not imply the existenceof the force before the particular manifestation of it which is contemplated.
And I may now add, as a further fault, the tacit assumption that, withoutsome act of conservation, force would disappear. All these implications areat variance with the conception to be conveyed. In place of "conservation"Prof. Huxley suggested persistence. This meets most of the objections; andthough it may be urged that it does not directly imply pre-existence of theforce at any time manifested, yet no word less faulty in this respect canbe found. In the absence of a word coined for the Purpose, it seems the best;and as such I adopt it.> §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.
