SENSE-DATA AND PHYSICS 149 



entering into the relation of marriage, and similarly a 

 sensibile becomes a sense-datum by entering into the 

 relation of acquaintance. It is important to have both 

 terms ; for we wish to discuss whether an object which 

 is at one time a sense-datum can still exist at a time 

 when it is not a sense-datum. We cannot ask ' Can 

 sense-data exist without being given ? " for that is like 

 asking " Can husbands exist without being married ? ' 

 We must ask " Can sensibilia exist without being given ? ' 

 and also "Can a particular sensibile be at one time a 

 sense-datum, and at another not ? ' Unless we have the 

 word sensibile as well as the word "sense-datum," such 

 questions are apt to entangle us in trivial logical puzzles. 



It will be seen that all sense-data are sensibilia. It is 

 a metaphysical question whether all sensibilia are sense- 

 data, and an epistemological question whether there 

 exist means of inferring sensibilia which are not data 

 from those that are. 



A few preliminary remarks, to be amplified as we pro- 

 ceed, will serve to elucidate the use which I propose to 

 make of sensibilia. 



I regard sense-data as not mental, and as being, in 

 fact, part of the actual subject-matter of physics. There 

 are arguments, shortly to be examined, for their sub- 

 jectivity, but these arguments seem to me only to prove 

 physiological subjectivity, i.e. causal dependence on the 

 sense-organs, nerves, and brain. The appearance which 

 a thing presents to us is causally dependent upon these, 

 in exactly the same way as it is dependent upon inter- 

 vening fog or smoke or coloured glass. Both dependences 

 are contained in the statement that the appearance 

 which a piece of matter presents when viewed from a 

 given place is a function not only of the piece of matter, 

 but also of the intervening medium. (The terms used in 



