SENSE-DATA AND PHYSICS 171 



any one thing continuously, and it is merely a hypo- 

 thesis to assume that, while we are not observing it, it 

 passes through conditions intermediate between those in 

 which it is perceived. During uninterrupted observa- 

 tion, it is true, continuity is nearly verified ; but even 

 here, when motions are very rapid, as in the case of 

 explosions, the continuity is not actually capable of 

 direct verification. Thus we can only say that the sense- 

 data are found to permit a hypothetical complement of 

 " sensibilia ' such as will preserve continuity, and that 

 therefore there may be such a complement. Since, how- 

 ever, we have already made such use of hypothetical 

 " sensibilia," we will let this point pass, and admit such 

 " sensibilia/' as are required to preserve continuity. 



Secondly, continuity is not a sufficient criterion of 

 material identity. It is true that in many cases, such as 

 rocks, mountains, tables, chairs, etc., where the appear- 

 ances change slowly, continuity is sufficient, but in other 

 cases, such as the parts of an approximately homogeneous 

 fluid, it fails us utterly. We can travel by sensibly 

 continuous gradations from any one drop of the sea at 

 any one time to any other drop at any other time. We 

 infer the motions of sea-water from the effects of the 

 current, but they cannot be inferred from direct sensible 

 observation together with the assumption of continuity. 



The characteristic required in addition to continuity is 

 conformity with the laws of dynamics Starting from 

 what common sense regards as persistent things, and 

 making only such modifications as from time to time 

 seem reasonable, we arrive at assemblages of " sensibilia ' 

 which are found to obey certain simple laws, namely those 

 of dynamics. By regarding " sensibilia ' at different 

 times as belonging to the same piece of matter, we are 

 able to define motion, which presupposes the assumption 



