SENSE-DATA AND PHYSICS 151 



It will be seen that the mental and the physical arc not / 

 necessarily mutually exclusive, although I know of no 

 reason to suppose that they overlap. 



The doubt as to the correctness of our definition of the 

 " mental " is of little importance in our present dis- 

 cussion. For what I am concerned to maintain is that 

 sense-data are physical, and this being granted it is a 

 matter of indifference in our present inquiry whether or 

 not they are also mental. Although I do not hold, with 

 Mach and James and the " new realists," that the 

 difference between the mental and the physical is merely 

 one of arrangement, yet what I have to say in the present 

 paper is compatible with their doctrine and might have 

 been reached from their standpoint. 



In discussions on sense-data, two questions are com- . 

 monly confused, namely : 



(i) Do sensible objects persist when we are not sensible 

 of them ? in other words, do sensihilia which are data at a 

 certain time sometimes continue to exist at times when they 

 are not data ? And (2) are sense-data mental or physical ? 



I propose to assert that sense-data are physical, while 

 yet maintaining that they probably never persist un- 

 changed after ceasing to be data. The view that they do 

 not persist is often thought, quite erroneously in my 

 opinion, to imply that they are mental ; and this has, I 

 believe, been a potent source of confusion in regard to 

 our present problem. If there were, as some have held, 

 a logical impossibility in sense-data persisting after ceasing 

 to be data, that certainly would tend to show that they 

 were mental ; but if, as I contend, their non-persistence 

 is merely a probable inference from empirically ascer- 

 tained causal laws, then it carries no such imphcation 

 with it, and we are quite free to treat them as part of the 

 subject-matter of physics. 



