170 MYSTICISM AND LOGIC 



For this purpose, we may, at least to begin with, 

 confine ourselves within a single biography. If we can 

 always say when two " sensibilia " in a given biography 

 are appearances of one thing, then, since we have seen 

 how to connect " sensibilia " in different biographies as 

 appearances of the same momentary state of a thing, we 

 shall have all that is necessary for the complete con- 

 struction of the history of a thing. 



It is to be observed, to begin with, that the identity of 

 a thing for common sense is not always correlated with 

 the identity of matter for physics. A human body is one 

 persisting thing for common sense, but for physics its 

 matter is constantly changing. We may say, broadly, 

 that the common-sense conception is based upon con- 

 tinuity in appearances at the ordinary distances of sense- 

 data, while the physical conception is based upon the 

 continuity of appearances at very small distances from 

 the thing. It is probable that the common-sense con- 

 ception is not capable of complete precision. Let us there- 

 fore concentrate our attention upon the conception of the 

 persistence of matter in physics. 



The first characteristic of two appearances of the same 

 piece of matter at different times is continuity. The two 

 appearances must be connected by a series of inter- 

 mediaries, which, if time and space form compact series, 

 must themselves form a compact series. The colour of 

 the leaves is different in autumn from what it is in summer; 

 but we believe that the change occurs gradually, and that, 

 if the colours are different at two given times, there are 

 intermediate times at which the colours are intermediate 

 between those at the given times. 



But there are two considerations that are important as 

 regards continuity. 



First, it is largely hypothetical. We do not observe 



