CLASSICAL AND NEW MECHANICS 157 



supposed for the time being to be at rest, or that posi- 

 tion and motion were relative and not absolute; yet it 

 was not explicitly stated that there could not be absolute 

 rest or motion, such as would occur if the motion of a 

 body were referred to an absolutely fixed center of the 

 universe or to an ether which was incapable of motion. 

 For all practical problems, Newton's third law of mo- 

 tion, which states that to every action there is an 

 equal and oppositely directed reaction, announces the 

 universality of relativity. 



We should finally note, that mass, dimensions, and 

 time were held to be unaffected by the motion of a 

 body. Newton expresses this by saying that force 

 actions of matter, or the science of dynamics, are inde- 

 pendent of its initial state of rest or motion. 



The first serious criticism of these postulates was 

 made by a number of physicists, forming what is often 

 called the school of energetics, who proposed to sub- 

 stitute energy for mass as the fundamental attribute 

 of matter. At the time, the change was rather imma- 

 terial as we were accustomed to think that mass and 

 energy were coexistent and that either one was unin- 

 telligible without the other. As I have said before, it 

 was the same kind of a problem as deciding which came 

 first, the owl or the egg; the answer to which is we 

 know nothing about the matter. 



The discrepancies between electricity and mechanics 

 did not prove to be embarrassing until certain problems 



