108 SCIENTIFIC METHOD IN PHILOSOPHY 



science the matter composing it is continually changing. 

 This conflict, however, is not very serious, and may, for 

 our rough preliminary purpose, be largely ignored. The 

 problem is : by what principles shall we select certain 

 data from the chaos, and call them all appearances of the 

 same thing ? 



A rough and approximate answer to this question is 

 not very difficult. There are certain fairly stable collec- 

 tions of appearances, such as landscapes, the furniture of 

 rooms, the faces of acquaintances. In these cases, we 

 have little hesitation in regarding them on successive 

 occasions as appearances of one thing or collection of 

 things. But, as the Comedy of Errors illustrates, we may 

 be led astray if we judge by mere resemblance. This 

 shows that something more is involved, for two different 

 things may have any degree of likeness up to exact 

 similarity. 



Another insufficient criterion of one thing is continuity. 

 As we have already seen, if we watch what we regard as 

 one changing thing, we usually find its changes to be con- 

 tinuous so far as our senses can perceive. We are thus 

 led to assume that, if we see two finitely different appear- 

 ances at two different times, and if we have reason to 

 regard them as belonging to the same thing, then there 

 was a continuous series of intermediate states of that 

 thing during the time when we were not observing it. 

 And so it comes to be thought that continuity of change 

 is necessary and sufficient to constitute one thing. But 

 in fact it is neither. It is not necessary, because the 

 unobserved states, in the case where our attention has 

 not been concentrated on the thing throughout, are 

 purely hypothetical, and cannot possibly be our ground 

 for supposing the earlier and later appearances to belong 

 to the same thing ; on the contrary, it is because we sup- 



