MOTION, FORCE, MATTER 333 



tildes is continuous." * But where a recognized physical 

 technique is available it is always to be preferred to a mental 

 technique, even in those rare cases where the latter is to be 

 found. Hence the attempt to define the scientific concept of 

 force is the attempt to find a technique for the measurement 

 of force. 



The scientific notion of force presumes to include in its 

 definition precisely such a technique ; force is measured by the 

 product of the mass and the acceleration. The necessity for 

 this conception resides in the fact that there seems to be no 

 way of measuring force directly. One cannot detach forces 

 from objects and move them about as one can clocks and 

 meter rods. One cannot even add and subtract forces in 

 quite the same way that one can spaces and times. Force is 

 an intensive quantity, like motion. Hence, like motion, it is 

 measured not directly but through correlation with events of 

 another kind which, in turn, are susceptible of more direct 

 measurement. Motion, it was noted, is measured in terms 

 of space and time, both of which are extensive quantities and 

 readily measurable. Acceleration is measurable in terms of 

 motion and time. But what may be said of mass? Is this 

 also directly or indirectly measurable? If it is directly 

 measurable, what is the qualitative experience of which it is 

 the measure? If it is not directly measurable, with what 

 qualitative experience is it correlated? 



The answers to these questions are crucial. For if the 

 meaning of the scientific concept of force is identified with 

 its measured value, no analysis of this meaning is possible 

 without a recognition of the techniques of measurement. 

 What, then, is the empirical foundation of the concept of 

 mass? 



There are two main attempts to solve this problem. The 

 first involves the search for a clearly given qualitative datum 

 to which the term refers. The presumption is that mass is 

 one of the psychological simples of experience, and if one can 

 only locate it and measure it he will have the information 



1 P. W. Bridgman, Logic of Modern Physics, p. 5. 



