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 &quot; Can 

 sense-data exist without being given ? &quot; for that is like 

 asking &quot; Can husbands exist without being married ? &quot; 

 We must ask &quot; Can sensibilia exist without being given ? &quot; 

 and also &quot;Can a particular sensibile be at one time a 

 sense-datum, and at another not ? &quot; Unless we have the 

 word sensibile as well as the word &quot;sense-datum,&quot; 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 

 physiologicaj_s\^]e.ciivity, 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 



