38 SCIENTIST 



shape and roughness of its surface, and so on. What 

 Newton (with the help of GaHleo) managed to do was 

 to stand back from the entire situation and say that most 

 of the particular properties of bodies — their size, shape, 

 color, and roughness — are relatively unimportant insofar 

 as motion is concerned. Furthermore, motion is not an 

 intrinsic property of objects but is imparted to them by 

 outside influences. Finally, Newton invented two ideas 

 which he then pointed to as the really essential elements 

 in the motion situation. The first of these is the idea of 

 mass, which he regarded as a property of all real objects. 

 The second is the idea of force, which acts on the masses. 

 As we saw in Chapter 1, these two abstractions could be 

 combined in the equations which formed the basis for 

 understanding all the types of motion known until the time 

 of Einstein. 



The odd and somewhat upsetting part of the whole 

 situation is that it is extremely difficult to be entirely clear 

 about just what mass and force really are. In point of fact, 

 they can be successfully defined only in terms of each 

 other. The only way we can know what a given mass is 

 is by applying a given force to it and vice versa. 



In any case, they didn't exist before Newton thought 

 of them. When we ask ourselves how he managed to think 

 of them, we find that he essentially "pulled them" out of 

 a large number of instances of particular motions or move- 

 ments of particular bodies. In much the same way, the 

 idea of "birdness" was pulled out of experiencing large 

 numbers of individual birds. We therefore call such ideas 

 "abstractions," a word derived from the Latin for "pulling 

 out." The weight of opinion seems to be that abstractions 

 are best understood as inventions of the human mind and 

 that their validity depends primarily on their usefulness 



