SENSE-DATA AND PHYSICS 151 



It will be seen that the mental and the physical are 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 

 &quot; mental &quot; 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 npjt hold, with 

 Mach and James and the &quot;new realists,&quot; 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 sensibilia which are data at a 

 certain time some times 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, 1 

 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 implication 

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

 subject-matter of physics. 



