334 INTRODUCTION TO PHILOSOPHY OF SCIENCE 



through which the concept of force may be clarified. Un- 

 fortunately, this attempt seems to fail. To describe mass, as 

 Newton did, by the phrase "quantity of matter'' does not 

 help one to locate it unless the meaning of this phrase is made 

 clear. To describe it in terms of inertia is inadequate, as 

 Lenzen points out, for "the status of inertia as a property 

 is very dubious ; it seems evident that we are not acquainted 

 with inertia in the sense in which we are acquainted with 

 redness, extension, etc." l Furthermore, even if there is 

 something empirically given which can be called "inertia," 

 this datum is one of the general kind called "force," and 

 unless the features which distinguish it from all other 

 "forces" can be made clear, one has made no progress. What 

 is worse, unless there is some technique for measuring 

 inertia, one has not advanced the problem. To suggest, as 

 Newton did, that mass is measured by the product of the 

 density by the volume is of no help, for density is measurable 

 only as the mass per unit volume. Similar difficulties arise 

 in the attempt to define mass through gravitation. For 

 example, a recent writer 2 asserts that "the mass of a body 

 cannot be more definitely described than to say that it is the 

 quality to which are due both its inertia and its gravitational 

 action." Apart from the difficulty associated with the notion 

 of inertia and gravitation being due to mass, there is a further 

 obscurity associated with the notion of gravitational action. 

 The presumption is that this term is to be employed as a 

 designation for the fact of weight; but if so the circular char- 

 acter of the designation becomes apparent, for weight, like 

 inertia, is a datum of the general kind called 'force," and 

 unless the features which distinguish it from all other 

 "forces " can be made clear one has made no progress. Hence 

 the attempt to locate a qualitative datum to which "mass" 

 directly refers, seems to lead up a blind alley. All such data 

 appear to be merely force-experiences of certain kinds. 

 Hence one finds that in the attempt to measure force by 



1 Physical Theory, p. 111. 



2 F. H. Saunders, Survey of Physics (New York: Holt, 1930), p. 61. 



