140 THE' WORLD-ENERGY 



just seen that the second law, rightly understood,, already 

 anticipates the mutual actions of force affirmed in the 

 third law, and that it points out, through the illustrative 

 parallelogram of forces, this further vitally important 

 point : that the whole truth of the motion of any body, 

 whether mote or star, is to be known only by compounding 

 into one all the forces impressed upon such body. 



Let us now further recall the fact that in the very 

 nature of force, as the essence of matter, there can be no 

 such thing as an isolated body in all the universe, but, 

 rather, that every body, or force-center, is necessarily 

 related concretely with every other body or force-center 

 "bodies" being but the discrete phases of "force" or 

 ' ' energy," which again is the name given to the physically 

 continuous ; that is, to the reality which occupies space. 

 Then, holding these several points together in our minds, 

 it must become evident that the third law of motion is 

 applicable equally to any and every group of bodies, to 

 the most complex as well as to the simplest case of 

 physical relations manifested in the mutual actions of 

 bodies. That is, the third law of motion is applicable to 

 the total sum of actions and reactions, or of mutual 

 actions constituting the physical universe as a whole. 



Here, indeed, we come upon that universal relation of 

 every body to every other body, to which Newton gave 

 definition in the law of gravity, and which we shall have 

 occasion to consider more fully in a succeeding chapter. 



Glancing now once more at the three laws of motion, 

 their organic relation to each other becomes strikingly 

 manifest. The first law expresses negatively the funda- 

 mental characteristic of the external world, declaring it 

 to be a world of inertia a world in which there is no 



