THE LAWS OF MOTION 287 



if it be not relative to something. The moment, however, 

 we introduce other corpuscles into the field in order to 

 measure the motion of the first, they begin to pay regard 

 to each other's presence, and we are no longer dealing 

 with the motion of an isolated corpuscle. But we have 

 seen that the greater the distance between the corpuscles, 

 the less this influence must be conceived to be ; hence we 

 may take the conceptual limit by supposing that the 

 corpuscles are so far off each other that their mutual 

 influence is negligible, while their mutual presence will 

 still suffice to provide the "frame" (see p. 208) necessary 

 for describing a relative motion.^ Now in order that the 

 laws which govern the motion of corpuscles shall lead to 

 the construction of complex motions, fully describing the 

 phases of our perceptual experience, we are compelled to 

 suppose that the more and more completely we separate 

 one corpuscle from the influence of other corpuscles, the 

 more and more nearly does its motion relative to a suit- 

 able frame determined by these corpuscles cease to vary. 

 The first corpuscle either remains at rest relatively to this 

 frame or continues to move with the same speed — the 

 same number of miles per minute — in the same direction. 

 But this is what we term uniform motion, or motion 

 without acceleration (pp. 231-2), and we are thus endowing 

 our corpuscles with a very important property, namely, 

 we assert that they will not dance, that is, alter their 

 motion, unless they have partners to dance with. This 

 characteristic which we attribute to corpuscles, namely, that 

 their uniform motion is not altered except in the presence 

 of other corpuscles, is scientifically termed their inertia. 



Now the reader must be very careful to note the 

 essential features of this principle of inertia. In the first 

 place we consider that all corpuscles are going to in- 

 fluence each other's motion, and in the second place we 

 find it necessary, owing to the relativity of all motion, to 



1 The reader must remember that relative position is conceptualised by a 

 directed step, and that it is a series of directed steps which form the path of 

 the relative motion (p. 210). Each directed step is to be conceived as "fixed" 

 in direction by a " frame," and the points of this frame are to be considered 

 .as having no accelerations relative to each other. See Appendix, Note I. 



