40 PHYSICAL SCIENCE 



ideas are very different from those in which they 

 took their origin. 



While Natural Science is not committed to 

 any particular philosophical system, while in its 

 essence it is independent of all such systems, the 

 language it uses habitually is based on the 

 common-sense realism, which is the philosophic 

 creed of most men of science — indeed, of the 

 great bulk of mankind, or at all events, of that 

 part of mankind belonging to the races of Western 

 Europe. The mass and energy with which we 

 deal in physical experiments, and in the mathe- 

 matical reasoning based on inductions from the 

 experiments, are purely conceptual quantities, 

 introduced to bring order and simplicity into our 

 perceptions of phenomena. Perhaps they are 

 not absolute quantities at all, but merely relations 

 between ourselves and the systems we describe 

 in terms of them. They may be replaced by other 

 concepts as our changing knowledge requires. 



Possibly the quantum, or unit of ''action," 

 which we are forced to accept though it accords 

 ill with previous ideas, may be nearer reality. 

 But science still talks of matter and energy as 

 though it knew of the existence of realities corre- 

 sponding with the mental Images to which alone 

 these names strictly apply. In the laboratory, as 

 in practical life, there Is neither room nor time 

 for philosophic doubt. In periods of reflection, 

 however, when considering the theoretical bearing 

 of the results of our experiments, it is sometimes 

 well to remember the limitation of our present 

 certain knowledge, and the purely conceptual 

 nature of our scheme of Natural Science when 

 based merely on Its own inductions. 



