322 THE GRAMMAR OF SCIENCE 



Law I. — Every body continues in its state of rest or of 

 uniform motion in a straight line, except in so far as it may 

 be compelled by force to change that state. 



Now the reader who is acquainted with treatises on 

 dynamics will remember that one of the most difficult 

 chapters is frequently entitled, Motion of a Body under 

 the Action of no Forces. The motion described is of an 

 extremely complex kind. For example, the body may 

 not only be spinning about an axis, but may be, and as 

 a general rule is, conceived as continually changing the 

 axis about which it spins. The "state of rest or of 

 uniform motion in a straight line " is thus not that which 

 the physicist postulates to describe the motion of a body 

 under the action of no forces. It is quite true that we 

 conceive a certain point termed the centre of mass of such 

 a body to be either at rest or moving uniformly in a 

 straight line ; this, however, is not a conception which is 

 itself axiomatic, but arises from an application of the 

 principle of the equality of action and reaction to the 

 particles by which we conceptually construct the body. 

 In the first place, therefore, the use of the word body does 

 not really give generality to the law, but introduces 

 obscurity ; we ought at least to replace it by the word 

 particle. In the next place, the law is very wanting in 

 explicitness as to what we are to understand by state of 

 rest or of uniform motion in a straight line. All motion 

 must be relative to something, but Newton does not in- 

 dicate with regard to what, for example, the relative path 

 is a straight line. Force is also a relative term (p. 304), 

 but Newton nowhere tells us what the force on the body 

 is related to. Thus, until a second body (or a definite 

 "frame," p. 208) be introduced (p. 287), the law remains 

 meaningless. In the last place, what are we to understand 

 by the words " compelled by force to change that state " ? 

 We take force to be a certain measure of motion, namely, 



or in his respect for the authors of the above classical Treatise. Yet he cannot 

 believe that the two centuries which have elapsed since Newton stated his 

 Leges Alotth "have not shown a necessity for any addition or modification " ! 

 Old words grow as men are compelled to express new ideas in terms of them, 

 and few definitions have a virile life of even a score years. 



