196 THE PERSISTENCE OF FORCE. 



is clearly seen on reducing the data to their lowest terms 

 a unit of matter, or atom, and its motion. The force by 

 which it exists is passive hut independent $ while the force 

 by which it moves is active but dependent on its past and 

 present relations to other atoms. These two cannot be iden- 

 tified in our thoughts. For as it is impossible to think of 

 motion without something that moves; so it is impossible 

 to think of energy without something possessing the 

 energy. 



While recognizing this fundamental distinction be- 

 tween that intrinsic force by which body manifests itself as 

 occupying space, and that extrinsic force distinguished as 

 energy; I here -treat of them together as being alike per- 

 sistent. And I thus treat of them together partly for the 

 reason that, in our consciousness of them, there is the same 

 essential element. The sense of effort is our subjective 

 symbol for objective force in general, passive and active. 

 Power of neutralizing that which we know as our own 

 muscular strain, is the ultimate element in our idea of body 

 as distinguished from space; and any energy which we can 

 give to body, or receive from it, is thought of as equal to a 

 certain amount of muscular strain. The two conscious- 

 nesses differ essentially in this, that the feeling of effort 

 common to the two is in the last case joined with conscious- 

 ness of change of position, but in the first case is not.* 



* In respect to the fundamental distinction here made between the space- 

 occupying kind of force, and the kind of force shown by various modes of 

 activity, I am, as in the last chapter, at issue with some of my scientific 

 friends. They do not admit that the conception of force is involved in the 

 conception of a unit of matter. From the psychological point of view, how- 

 ever, Matter, in all its properties, is the unknown cause of the sensations it 

 produces in us ; of which the one which remains when all the others are 

 absent, is resistance to our efforts a resistance Xve are obliged tc symbolize 

 as the equivalent of the muscular force it opposes. In imagining a unit of 

 matter we may not ignore this symbol, by which alone a unit of matter can 

 be figured in thought as an existence. It is not allowable to speak as though 

 there remained a conception of an existence when that conception has been 

 eviscerated deprived of the element of thought by which it is distinguished 



