INTRODUCTORY REMARKS, n 



this experiment alone, the use of the term force would be 

 superfluous, and would not add to our knowledge on the sub- 

 ject. All the information which our minds could get would 

 be as sufficiently obtained from the expression, when the 

 string is cut the bow becomes straight, as from the expression, 

 the bow becomes straight by its elastic force. Do we know 

 more of the phenomenon, viewed without reference to other 

 phenomena, by saying it is produced by force ? Certainly 

 not. All we know or see is the effect ; we do not see force 

 we see motion or moving matter. 



If now we take a piece of caoutchouc and stretch it, when 

 released it returns to its original length. Here, though the 

 subject-matter is very different, we see some analogy in the 

 effect or phenomenon to that of the strung bow. If, again, we 

 suspend an apple by a string, cut the string, the apple falls. 

 Here, though it is less striking, there is still an analogy to the 

 strung bow and the caoutchouc. 



Now, when the word force is employed as comprehending 

 these three different phenomena, we find some use in the term, 

 not by its explaining or rendering more intelligible the modus 

 agendi of matter, but as conveying to the mind something 

 which is alike in the three phenomena, however distinct they 

 may be in other respects : the word becomes an abstract or 

 generalised expression, and regarded in this light is of high 

 utility. Although I have given only three examples, it is 

 obvious that the term would equally apply to 300 or 3,000 

 cases. 



But it will be said, the term force is used not as expressing 

 the effect, but as that which produces the effect. This is true, 

 and in this its ordinary sense I shall use it in these pages. 

 But though the term has thus a potential meaning, to depart 

 from which would render language unintelligible, we must 

 guard against supposing that we know essentially more of the 

 phenomena by saying they are produced by something, which 

 something is only a word derived from the constancy and 

 similarity of the phenomena we seek to explain by it. The 

 relations of the phenomena to which the terms force or forces 

 are applied give us real knowledge ; these relations may be 

 called relations of forces ; our knowledge of them is not 



