THE CONTINUITY OF MOTION. 193 



cated to, or received from, other celestial bodies. And 

 when we ask how this communicated motion is estimated, 

 we discover that the estimate is based on certain laws of 

 force; which laws, one and all, embody the postulate that 

 force cannot be destroyed. Without the axiom that action 

 and re-action are equal and opposite, astronomy could not 

 make its exact predictions. 



Similarly with the a priori conclusion that Motion is 

 continuous. That which defies suppression in thought, is 

 really the force which the motion indicates. We can imag- 

 ine retardation to result from the action of external bodies. 

 But to imagine this, is not possible without imagining ab- 

 straction of the force implied by the motion. We are 

 obliged to conceive this force as impressed in the shape of 

 reaction on the bodies that cause the arrest. And the mo- 

 tion communicated to them, we are compelled to regard, 

 not as directly communicated, but as a product of the com- 

 municated force. We can mentally diminish the velocity 

 or space-element of motion, by diffusing the momentum 

 or force-element over a larger mass of matter; but the 

 quantity of this force-element, which we regard as the cause 

 of the motion, is unchangeable in thought.* 



* It is needful to state that this exposition differs in its point of view 

 from the expositions ordinarily given; and that some of the words employed, 

 such as strain, have somewhat larger implications. Unable to learn anything 

 about the nature of Force, physicists have, of late years, formulated ultimate 

 physical truths in such ways as often tacitly to exclude the consciousness of 

 Force : conceiving cause, as Hume proposed, in terms of antecedence and 

 sequence only. " Potential energy," for example, is defined as constituted by 

 such relations in space as permit masses to generate in one another certain 

 motions, but as being in itself nothing. While this mode of conceiving the 

 phenomena suffices for physical inquiries, it does not suffice for the purposes 

 of philosophy. After referring to the Principles of Psychology, §§ 347-350, 

 the reader will understand what I mean by saying that since our ideas of 

 Body, Space, Motion, are derived from our ideas of muscular tension, which 

 are the ultimate symbols into which all our other mental symbols are inter- 

 pretable, to formulate phenomena in the proximate terms of Body, Space, 

 Motion, while discharging from the concepts the consciousness of Force, is to 

 acknowledge the superstructure while ignoring the foundation. 



