Reduce the proposition to its ******st form, and its truth becomes stillmore obvious. Suppose two weights suspended over a pulley, or suppose twomen pulling against each other. The heavier weight will descend, and thestronger man will draw the weaker towards him. If asked how we know whichis the heavier weight or the stronger man, we can only reply that it is theone producing motion in the direction of its pull. But if of two opposingtractions we can know one as greater than the other only by the motion itgenerates in its own direction, then the assertion that motion occurs inthe direction of greatest traction is a truism. When, going a step furtherback, we seek a warrant for the assumption that of the two conflicting forces,the one which produces motion in its own direction is the greatest, we findno other than the consciousness that such part of the greater force as isunneutralized by the lesser, must produce its effect -- the consciousnessthat this residuary force cannot disappear, but must manifest itself in someequivalent change -- the consciousness that force is persistent. Here too,as before, it may be remarked that no number of varied illustrations, likethose of which this chapter mainly consists, can give greater certainty tothe conclusion thus immediately drawn from the ultimate datum of consciousness.
For in all cases, as in the ****** ones just given, we can identify the greatestforce only by the resulting motion.
From this same primordial truth, too, may be deduced the principle thatmotion once set up along any line, becomes itself a cause of subsequent motionalong that line. The mechanical axiom that, if left to itself, matter movingin any direction will continue in that direction with undiminished velocity,is but an indirect assertion of the persistence of that kind of force calledenergy; since it is an assertion that the energy manifested in the transferof a body along a certain length of a certain line in a certain time, cannotdisappear without producing some equal manifestation: a manifestation which,in the absence of conflicting forces, must be a further transfer in the samedirection at the same velocity. In the case of matter traversing matter alike inference is necessitated. Here however the actions are complicated.
A liquid that follows a certain channel through or over a solid, as wateralong the Earth's surface, loses part of its motion in the shape of heat,through friction and collision with the matters forming its bed. A furtheramount may be absorbed in overcoming the forces it liberates; as when itloosens a mass which falls into its channel. But after these deductions,any further deduction from the energy embodied in the motion of the water,is at the expense of a reaction on the channel which diminishes its obstructivepower: such reaction being shown in the motion acquired by the detached portionscarried away. The cutting out of river-courses perpetually illustrates thistruth. Still more involved is the case of motion passing through matter byimpulse from part to part; as a nervous discharge through animal tissue.
There are conceivable anomalies. Some chemical change wrought along the routetraversed, may render it less fit than before for conveying a current. Orsome obstructive form of force may be generated; as in metals, the conductingpower of which is, for the time, decreased by the heat which the electriccurrent produces. The real question is, however, what structural modification,if any, is produced throughout the matter traversed, apart from incidentaldisturbing forces -- apart from everything but the necessary resistance ofthe matter: that, namely which results from the inertia of its units. Ifwe confine our attention to that part of the motion which, escaping transformation,continues its course, then the persistence of force necessitates that asmuch of it as is taken up in changing the positions of the units, must leavethese by so much less able to obstruct subsequent motion in the same direction.
Thus in all the changes displayed by the Solar System, in all those whichare going on in the Earth's crust, in all processes of organic developmentand function, in all mental actions and the effects they work on the body,and in all modifications of structure and activity in societies, the impliedmovements are of necessity determined in the manner above set forth. Thetruth set forth holds not only of one class, or of some classes, of phenomena,but it is among those universal truths by which our knowledge of phenomenain general is unified.
