172 MYSTICISM AND LOGIC 



or construction of something persisting throughout the 

 time of the motion. The motions which are regarded as 

 occurring, during a period in which all the " sensibiha " 

 and the times of their appearance are given, will be 

 different according to the manner in which we combine 

 *' sensibilia " at different times as belonging to the same 

 piece of matter. Thus even when the whole history of 

 the world is given in every particular, the question what 

 motions take place is still to a certain extent arbitrary 

 even after the assumption of continuity. Experience 

 shows that it is possible to determine motions in such a 

 way as to satisfy the laws of dynamics, and that this 

 determination, roughly and on the whole, is fairly in 

 agreement with the common-sense opinions about per- 

 sistent things. This determination, therefore, is adopted, 

 and leads to a criterion by which we can determine, some- 

 times practically, sometimes only theoretically, whether 

 two appearances at different times are to be regarded as 

 belonging to the same piece of matter. The persistence 

 of all matter throughout all time can, I imagine, be 

 secured by definition. 



To recommend this conclusion, we must consider what 

 it is that is proved by the empirical success of physics. 

 What is proved is that its hypotheses, though unverifiable 

 where they go beyond sense-data, are at no point in 

 contradiction with sense-data, but, on the contrary, are 

 ideally such as to render all sense-data calculable when a 

 sufficient collection of " sensibilia " is given. Now 

 physics has found it empirically possible to collect sense- 

 data into series, each series being regarded as belonging 

 to one "thing," and behaving, with regard to the laws 

 of physics, in a way in which series not belonging to one 

 thing would in general not behave. If it is to be un- 

 ambiguous whether two appearances belong to the same 



