THE LAWS OF MOTION 323 



the product of mass into acceleration ; then to assert the 

 absence of force is to assert the absence of acceleration, or 

 the law would merely contain the platitude that without 

 change of motion a particle moves uniformly. But Newton 

 certainly meant something more than this, for he was 

 thinking of force in the sense of mediaeval metaphysics as 

 " a cause of change in motion." Now the nearest approach 

 we can get to his idea is that position relative to surround- 

 ing particles determines a given particle's acceleration, 

 and thus the first law is seen, liberally interpreted, to 

 amount to the statement that surrounding circumstances 

 determine acceleration — that without the presence of other 

 particles there is no acceleration. This is the important 

 principle of inertia to which we have already referred (p. 

 286), but it certainly appears to be stated with very great 

 obscurity in Newton's first law of motion. Further, even 

 in this law, as I have restated it, no hint is given as to 

 what application the principle may have to other cor- 

 puscles than particles of gross "matter" (p. 289). 



Law II. — Cha7ige of Diotion is proportional to force 

 applied, and takes place in tJie direction of t/ie straight line 

 in ivJiicJi force acts. 



This is a veritable metaphysical somersault. How the 

 imperceptible cause of change in motion can be applied in 

 a straight line surpasses comprehension ; the only straight 

 line that can be conceived, or, as some physicists would 

 have it, perceived, is the direction of change of motion. 

 We may assert that the imperceptible has this direction, 

 but to postulate that the imperceptible will determine this 

 direction for us seems to be pure metaphysics. We come 

 down on our feet again, however, when we interpret this 

 law as simply indicating that physically force is going to 

 be taken as a measure for some change in motion (p. 304). 

 As to the exact meaning of change of motion taking place 

 in a straight line, all the real difficulties as to what thing 

 we are to suppose changing its motion, and what is the 

 presence associated with this change of motion, i.e. the 

 difficulties about the line joining two corpuscles (p. 310), 

 are concealed by talking vaguely about force as an entity 



