108 WHAT IS SCIENCE? 



tales that one which nature is prepared to accept and can 

 therefore be transferred from the realm of fantasy .to 

 that of solid fact. And when a theory has been so trans- 

 ferred, when it has gained universal acceptance because, 

 alone of all possible alternatives, it will predict true laws, 

 then, although it has purpose and value for us because 

 it renders the world intelligible, it is so clearly distin- 

 guished from all other attempts to achieve the same pur- 

 pose and to attain the same value that the ideas involved 

 in it, like the ideas involved in laws, have the certainty 

 and the universality that is characteristic of real objects. 

 A molecule is as real, and real in the same way, as the gases 

 the laws of which it explains. It is an idea essential to 

 the intelligibility of the world not to one mind, but to 

 all ; it is an idea which nature as well as mankind accepts. 

 That, I maintain, is the test and the very meaning of 

 reality. 



