SENSE-DATA AND PHYSICS 173 



thing or not, there must be only one way of grouping 

 appearances so that the resulting things obey the laws of 

 physics. It would be very difficult to prove that this is 

 the case, but for our present purposes we may let this 

 point pass, and assume that there is only one way. Thus 

 we may lay down the following definition : Physical 

 things are those series of appearances whose matter obeys 

 the laws of physics. That such series exist is an empirical 

 fact, which constitutes the verifiability of physics. 



XII. ILLUSIONS, HALLUCINATIONS, AND DREAMS 



It remains to ask how, in our system, we are to find a 

 place for sense-data which apparently fail to have the 

 usual connection with the world of physics. Such sense- 

 data are of various kinds, requiring somewhat different 

 treatment. But all are of the sort that would be called 

 " unreal/' and therefore, before embarking upon the dis- 

 cussion, certain logical remarks must be made upon the 

 conceptions of reality and unreality. 



Mr. A. Wolf 1 says : 



" The conception of mind as a system of transparent 

 activities is, I think, also untenable because of its failure 

 to account for the very possibility of dreams and hallu- 

 cinations. It seems impossible to realise how a bare, 

 transparent activity can be directed to what is not there, 

 to apprehend what is not given." 



This statement is one which, probably, most people 

 would endorse. But it is open to two objections. First 

 it is difficult to see how an activity, however un- " trans- 

 parent," can be directed towards a nothing : a term of a 

 relation cannot be a mere nonentity. Secondly, no reason 



t " Natural Realism and Present Tendencies in Philosophy," Proc, 

 Arist. Sac., 1908-1909, p. 165. 



