WORLDS OF PHYSICS AND OF SENSE in 



it should be true, but it must also be such as we can 

 discover to be true. Thus verifiability depends upon our 

 capacity for acquiring knowledge, and not only upon 

 the objective truth. In physics, as ordinarily set forth, 

 there is much that is unveriflable : there are hypotheses 

 as to (a) how things would appear to a spectator in a 

 place where, as it happens, there is no spectator ; (J3) how 

 things would appear at times when, in fact, they are not 

 appearing to anyone ; (7) things which never appear at 

 all. All these are introduced to simplify the statement of 

 the causal laws, but none of them form an integral part 

 of what is known to be true in physics. This brings us 

 to our second answer. 



(J?) If physics is to consist wholly of propositions 

 known to be true, or at least capable of being proved or 

 disproved, the three kinds of hypothetical entities we 

 have just enumerated must all be capable of being 

 exhibited as logical functions of sense-data. In order to 

 show how this might possibly be done, let us recall the 

 hypothetical Leibnizian universe of Lecture III. In that 

 universe, we had a number of perspectives, two of which 

 never had any entity in common, but often contained 

 entities which could be sufficiently correlated to be 

 regarded as belonging to the same thing. We will call 

 one of these an " actual ' ' private world when there is an 

 actual spectator to which it appears, and " ideal ' when 

 it is merely constructed on principles of continuity. A 

 physical thing consists, at each instant, of the whole set 

 of its aspects at that instant, in all the different worlds ; 

 thus a momentary state of a thing is a whole set of 

 aspects. An " ideal ' appearance will be an aspect 

 merely calculated, but not actually perceived by any 

 spectator. An " ideal ' ' state of a thing will be a state at 

 a moment when all its appearances are ideal. An ideal 



