104 MYSTICISM AND LOGIC 



This paragraph illustrates the kind of way in which 

 the philosopher is tempted to give an air of absoluteness 

 and necessity to empirical generalisations, of which only 

 the approximate truth in the regions hitherto investi- 

 gated can be guaranteed by the unaided methods of 

 science. It is very often said that the persistence of 

 something or other is a necessary presupposition of all 

 scientific investigation, and this presupposition is then 

 thought to be exemplified in some quantity which 

 physics declares to be constant. There are here, as it 

 seems to me, three distinct errors. First, the detailed 

 scientific investigation of nature does not presuppose an} 7 

 such general laws as its results are found to verify. 

 Apart from particular observations, science need pre- 

 suppose nothing except the general principles of logic, 

 and these principles are not laws of nature, for they are 

 merely hypothetical, and apply not only to the actual 

 world but to whatever is possible. The second error 

 consists in the identification of a constant quantity with 

 a persistent entity. Energy is a certain function of 

 a physical system, but is not a thing or substance per- 

 sisting throughout the changes of the system. The same 

 is true of mass, in spite of the fact that mass has often 

 been defined as quantity of matter. The whole conception, 

 of quantity, involving, as it does, numerical measurement 

 based largely upon conventions, is far more artificial, 

 far more an embodiment of mathematical convenience, 

 than is commonly believed by those who philosophise 

 on physics. Thus even if (which I cannot for a moment 

 admit) the persistence of some entity were among the 

 necessary postulates of science, it would be a sheer error 

 to infer from this the constancy of any physical quantity, 

 or the a priori necessity of any such constancy which 

 may be empirically discovered. In the third place, it 



