324 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



other hand, the term "force' is applied to so wide a range 

 of experiences — weight, inertia, energy, mass, momentum, 

 and work — that one wonders whether any definition of the 

 term which would include such a variety of connotations 

 would be possible. On the side of science itself, the confusion 

 is of another kind. There seems to be little doubt but that 

 Newton's laws of motion constitute the essential postulate 

 system for the definition of 'force" as it is used in science. 

 But the task of determining which among these postulates 

 is to be considered as definitive of the nature of force, and 

 which merely as making assertions "about force," is not at 

 all easy. Furthermore, dispute arises as to whether force is 

 itself a basic notion for a system of mechanics, or must be 

 defined in terms of a more fundamental notion, e.g., mass, 

 or momentum. Hence one cannot be sure what is meant by 

 force, even for contemporary science. As a result, the prob- 

 lem of logical derivation from its empirical foundation be- 

 comes essentially insoluble; logical techniques cannot be 

 made precise if there is no clear-cut knowledge of that with 

 which they start and that with which they end. At this 

 level the problem is very much complicated by the ideal 

 character of the laws of mechanics; they describe not actual 

 but theoretical situations, hence their derivations from em- 

 pirical situations must always be by involved routes. In the 

 face of these difficulties the more or less systematic analysis 

 of the preceding pages must be abandoned, and a somewhat 

 random series of comments must take its place. 



Attempting to ascertain the empirical foundation of the 

 concept of force, one is compelled to recognize that forces 

 have their loci in situations provoking in us feelings of effort. 

 "Unquestionably the sensational basis of the scientific con- 

 cept of force is the feelings of strain that we experience when 

 we drag a heavy body along, or throw a stone, or bend a 

 bow." x It seems hard to deny that there is a wide range of 

 situations which give rise in us to feelings of a similar kind — 

 stretching an elastic band, stopping a swiftly thrown ball, 



1 C. D. Broad, Scientific Thought, p. 162. 



