THE LAWS OF MOTION 305 



a second corpuscle, this measure depending partly on the 

 individual character of the first corpuscle (its mass) and 

 partly on the attention it is paying to the presence of a 

 second corpuscle (its acceleration due to the second 

 corpuscle). That this measure is scientifically a convenient 

 one is proven by its general use, and may be almost fore- 

 seen by comparing the simplicity of the statement (e) with 

 the complexity of (7). The definition of force we have 

 reached is a perfectly intelligible one ; it is completely 

 freed from any notion of matter as " the moving thing," or 

 from any notion of a metaphysical " cause of motion." 

 We have only to take the step which represents the 

 acceleration of A due to B's presence and to stretch or 

 magnify its length in the ratio of A's mass to the mass of 

 the standard body Q, and we have a new step which 

 represents B's force on A. Force is accordingly an 

 arbitrary conceptual measure of motion without any 

 perceptual equivalent. 



The distinction between the definition of force thus 

 given and that to be found in the ordinary text-books ^ 

 may at first sight seem slight to the reader, but the writer 

 ventures to think that the distinction makes all the differ- 

 ence between an intelligible and an unintelligible theory 

 of life, between sound physical science and crude meta- 

 physical materialism. Causation, as we have had occasion 

 more than once to point out, is only intelligible in the 

 perceptual sphere as antecedence in a routine of sense- 

 impressions. In the conceptual sphere, on the other hand, 

 the cause of change in the motion of our corpuscles lies 

 solely in our desire to form an accurate mechanical model 

 of the world of phenomena. For every definite configura- 

 tion of the corpuscles we postulate certain mutual accelera- 

 tions as a mode of bringing our mechanism into tune with 

 our sense-impressions of change. Force as an arbitrary 

 measure of these conceptual changes in motion is in- 



1 " Force is any cause which tends to alter a body's natural (szc!) state of 

 rest, or of uniform motion in a straight line " (Tail's Dynamics of a Particle, 

 art. 53). It is perhaps unnecessary to remark that we cannot conceive any 

 body to be naturally at rest or moving in a straight line unless the word 

 natural be re-defined in some novel sense, say, as artificial. 



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