APEn, 8, 1910] 



SCIENCE 



525 



The postulate which the philosopher here 

 shows us to be one of our presuppositions 

 is as follows : events in physical science de- 

 pend upon a few antecedents, knowing 

 which we may successfully predict the im- 

 mediate consequence, and may safely dis- 

 regard all other circumstances. The brev- 

 ity of the sequence which really deter- 

 mines phenomena in physics is a matter of 

 continual surprise — while the length and 

 complexity of the sequence in the case of 

 ordinary human actions is a matter of 

 equal astonishment. 



But it is very easy to forget what a pow- 

 erful influence this postulate has at times 

 exerted in almost all departments of sci- 

 ence. Few physicists, and still fewer engi- 

 neers, of the present seem to realize that 

 some of the most fundamental conceptions 

 of our science have been introduced directly 

 through the adoption of this postulate. 



Take, for instance, what is perhaps the 

 central idea of modern dynamics— the 

 idea of force — an idea which is older than 

 either that of mass or of energy. When 

 viewed in the light of the causal postulate, 

 i. e., in the light of history, the definition 

 of force becomes a matter of the utmost 

 simplicity and perfect clarity. From 

 many other points of view it is one of the 

 most complex and puzzling of physical 

 quantities. Sir Oliver Lodge says: 



We are chiefly familiar, from our youth up, 

 with two apparently simple things, motion and 

 force. We have a direct sense for both of these 

 things. We do not understand them in any deep 

 way, probably do not understand them at all, but 

 we are accustomed to them. Motion and force are 

 our primary objects of experience and conscious- 

 ness; and, in terms of them, all other less familiar 

 occurrences may be stated and grasped. 



To identify "force" in this manner with 

 the "muscular sensation" of tension or 

 pressure, which we feel when giving an 

 accelerated motion to a body or when 

 equilibrating by muscular effort the pull 



of the earth upon a body, seems to me dan- 

 gerously near darkening counsel with 

 words, and quite contrary to the spirit of 

 the modern mathematician and physicist 

 who are mending their fences at every pos- 

 sible point to keep out ideas which are not 

 clear, sharp and definite. 



The standard definition of the engineer, 

 and, I fear, of not a few students of phys- 

 ics, is set forth by Professor William Kent 

 in his article on the teaching of dynamics 

 which appeared in Science^ a few weeks 

 ago, namely, ' ' Force is defined as a pull or 

 push, something that causes or tends to 

 cause either motion or a change in the 

 velocity or direction of motion." 



Now considering both of these points of 

 view, which I believe are widespread, every 

 one is willing to admit at once the existence 

 of certain elastic, and gravitational, and 

 muscular, and electric, and cohesive, 

 stresses which none of us understand: but 

 the historical, or, if you please, the meta- 

 physical, point of view would appear to be 

 something like the following. 



So far from our possessing any direct 

 muscular sense of force, in the physical 

 meaning of the word as distinguished from 

 muscular tension, with which we are all 

 familiar, the idea is one which was intro- 

 duced by an Italian professor of mathe- 

 matics, but a comparatively short time ago. 

 How short may be illustrated by the fol- 

 lowing circumstances: 



My grandmother, who lived in my own 

 home for a number of years, was born on 

 the banks of the Brandywine in 1789. She 

 was therefore a contemporary as well as 

 a neighbor of Benjamhi Franklin. When 

 Franklin was a printer's lad in London he 

 had a promise from a friend that he should 

 be taken to visit Sir Isaac Newton. Sir 

 Isaac Newton was bom within the same 

 week in which Galileo died. Two human 



° Science, Vol. 30, p. 919, 1909. 



