354 THE POPULAR SCIENCE MONTHLY. 



ceived theories of physical science respecting the nature of force, it is 

 manifest, irrespective of the considerations which I have presented in 

 this and the preceding essays, that force is not an individual thing or 

 distinct entity which presents itself directly either to observation or 

 to thought, but that, so far as it is taken as a definite and unital term 

 in the operations of thought, it is purely a fiction of the intellect. 

 The cause of motion, or change of motion, in a body is simply the 

 condition or group of conditions upon which this motion depends ; 

 and this condition, or group of conditions, as we have already seen, is 

 always a corresponding motion, or change of motion, in the bodies, 

 outside of the moving body, which are its correlates. Otherwise ex- 

 pressed, force is a mere inference from the motion itself under the 

 universal conditions of reality, and its measure, therefore, is simply 

 the effect for which it is postulated as a cause ; it has no other exist- 

 ence. The only reality of force and of its action is the correspond- 

 ence between physical phenomena in conformity to the principle of 

 the essential relativity of all material existence. 



That force has no independent reality is so plain and obvious that 

 it has been proposed by some thinkers to abolish the term force, like 

 the term cause, altogether. However desirable this might be in 

 some respects, it is impossible, for the reason that the concept " force," 

 when properly interpreted in terms of experience, is valid, and, if its 

 name were abolished, it would instantly reappear under another name. 

 There is hardly any concept which has not, in science as well as in 

 metaphysics, given rise to the same confusion which prevails in 

 regard to " force " and " cause ; " and the blow leveled at these would 

 demolish all concepts whatever. Nevertheless, it is of the greatest 

 moment, in all speculations concerning the interdependence of physi- 

 cal phenomena, never to lose sight of the fact that the reality of force 

 is purely conceptual, and that it is not a distinct and individual tan- 

 gible or intangible entity. 



How imperfectly all this is understood by the physicists of our 

 time appears at once upon an examination of elementary treatises 

 as well as original disquisitions on physical science. Thus the re- 

 lation of force to mechanical motion is constantly spoken of as a 

 " fact " ascertained by experience and verified experimentally beyond 

 the possibility of question. In a learned article by J. Croll, published 

 in the July number, 1872, of the Philosophical Magazine (" What 

 determines Molecular Motion," etc., Phil. Mag., fourth series, vol. xl., p. 

 37), it is said : " In regard to the first question (what produces motion) 

 there is no diversity of opinion. All agree that what produces 

 change or causes motion is force." The obvious meaning of this is 

 that it might possibly admit of question whether material change or 

 motion is produced by force or something else, and that physicists, on 

 the whole, have come to the conclusion that it is produced by force. 

 Such a question ought, indeed, to be solemnly pondered by grave 



