NATURE OF EXACT SCIENCE 255 



but when it came to an actual determination of mass, an 

 experimental method was prescribed which had no 

 bearing on this definition. The belief that the quantity 

 determined by the accepted method of measurement 

 represented the quantity of matter in the object was 

 merely a pious opinion. At the present day there is no 

 sense in which the quantity of matter in a pound of lead 

 can be said to be equal to the quantity in a pound of 

 sugar. Einstein's theory makes a clean sweep of these 

 pious opinions, and insists that each physical quantity 

 should be defined as the result of certain operations of 

 measurement and calculation. You may if you like 

 think of mass as something of inscrutable nature to 

 which the pointer reading has a kind of relevance. But 

 in physics at least there is nothing much to be gained 

 by this mystification, because it is the pointer reading 

 itself which is handled in exact science; and if you 

 embed it in something of a more transcendental nature, 

 you have only the extra trouble of digging it out 

 again. 



It is quite true that when we say the mass is two tons 

 we have not specially in mind the reading of the particu- 

 lar machine on which the weighing was carried out. 

 That is because we do not start to tackle the problem of 

 the elephant's escapade ab initio as though it were the 

 first inquiry we had ever made into the phenomena of 

 the external world. The examiner would have had to be 

 much more explicit if he had not presumed a general 

 acquaintance with the elementary laws of physics, i.e. 

 laws which permit us to deduce the readings of other 

 indicators from the reading of one. // is this connec- 

 tivity of pointer readings, expressed by physical laws, 

 which supplies the continuous background that any realis- 

 tic problem demands. 



