MATTER IN MOTION 99 



velocity, and that the two diagonals have the 

 same length means that the sum of the masses 

 is constant. 



If I had the time, and you the patience, I 

 might show you more fully how simple is the 

 description of mechanics in terms of the new 

 geometry; how fundamental are the little ar- 

 rows that I have drawn in Figure 18, and how 

 shadowlike are those projections (upon arbi- 

 trarily chosen space and time) which we call 

 velocity, momentum, mass and energy. 



If in my ver^^ hasty survey of the science of 

 mechanics I say nothing of the concept of force, 

 it is because this concept has fallen a little into 

 the background in modern physics, and not be- 

 cause I share to any great extent the feeling 

 that force is a bad physical concept because it 

 emphasizes cause and effect. Force can be de- 

 fined so as to have no causal connotation, and, 

 on the other hand, one who is looking for cause 

 and effect can alwavs find it. Thus, such a one 

 would say, with reference to Figure 18, that the 

 neighborhood of one locus causes the curvature 

 of the other. 



It is indeed a serious question whether the 

 concept of causality can be regarded as a scien- 

 tific concept, and therefore within the scope of 



