32 WHAT IS LIFE 



any real scientific treatment of physical phenom- 

 ena is possible. Indeed, from the point of view of 

 that ancient philosopher, the problem of all 

 natural philosophy is to drive out qualitative 

 conceptions and to replace them by quantitative 

 relations. And this point of view has been em- 

 phasized by the far-seeing throughout all the 

 history of physics clear down to the present. 

 One of the greatest of modern physicists. Lord 

 Kelvin, writes: 'When you can measure what 

 you are speaking about and express it in num- 

 bers, you know something about it, and when 

 you cannot measure it, when you cannot express 

 it in numbers, your knowledge is of a meagre 

 and unsatisfactory kind. It may be the beginning 

 of knowledge, but you have scarcely in your 

 thought advanced to the stage of a science.' "^ 

 Millikan points out: 



"There is an interesting and instructive paral- 

 lelism between the histories of the atomic con- 

 ception of matter and the atomic theory of 

 electricity, for in both cases the ideas themselves 

 go back to the very beginnings of the subject. In 

 both cases, too, these ideas remained absolutely 

 sterile until the development of precise quan- 

 titative methods of measurement touched them 



* The Electron, second edition, 4. 



