MATTER IN MOTION 103 



While we are indulging in this philosophic 

 discussion, I may point out another set of ideas 

 of which we are reminded by the geometrical 

 treatment of mechanics. I refer to the problem 

 of individuality and identity. When a cater- 

 pillar emerges from the egg, later to hide him- 

 self in his cocoon, and again to appear as a 

 butterfly, what is there throughout these kalei- 

 doscopic metamorphoses that justifies us in 

 speaking of the continuous existence of a single 

 individual.? I shall not hazard an answer to this 

 question, but only point out that an entirely 

 similar problem is presented in physics. Our 

 space-time map represents a material object as 

 a sort of endless filament with an infinite exten- 

 sion in time, but not in space, and the same 

 thing is true if we consider our protons and 

 electrons which are at the present time believed 

 to maintain their individuality through the 

 ages. It is perhaps not quite true to state that 

 an electron is assumed to occupy a sharply cir- 

 cumscribed region of space, for we ascribe to 

 the electron a field which extends to infinite dis- 

 tance, and we sometimes think of the electron 

 as nothing but this field. Nevertheless, this ex- 

 tension in space is quite different from the as- 

 sumed extension in time, and does not prevent 



