SENSE-DATA AND PHYSICS 169 



XI. THE PERSISTENCE OF THINGS AND MATTER 



Apart from any of the fluctuating hypotheses of 

 physics, three main problems arise in connecting the 

 world of physics with the world of sense, namely : 



1. the construction of a single space ; 



2. the construction of a single time ; 



3. the construction of permanent things or matter. 



We have already considered the first and second of 

 these problems ; it remains to consider the third. 



We have seen how correlated appearances in different 

 perspectives are combined to form one " thing " at one 

 moment in the all-embracing time of physics. We have 

 now to consider how appearances at different times are 

 combined as belonging to one " thing," and how we 

 arrive at the persistent " matter " of physics. The 

 assumption of permanent substance, which technically 

 underlies the procedure of physics, cannot of course be 

 regarded as metaphysically legitimate : just as the one 

 thing simultaneously seen by many people is a con- 

 struction, so the one thing seen at different times by the 

 same or different people must be a construction, being in 

 fact nothing but a certain grouping of certain " sensibilia." 



We have seen that the momentary state of a *' thing " 

 is an assemblage of " sensibilia," in different perspectives, 

 not all simultaneous in the one constructed time, but 

 spreading out from " the place where the thing is " with 

 velocities depending upon the nature of the " sensibilia." 

 The time at which the " thing " is in this state is the lower 

 limit of the times at which these appearances occur. We 

 have now to consider what leads us tok^speak of another 

 set of appearances as belonging to the same^* thing " at 

 a different time. 



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