SENSE-DATA AND PHYSICS 159 



once. The things seen by two different people are often 

 closely similar, so similar that the same words can be used 

 to denote them, without which communication with 

 others concerning sensible objects would be impossible. 

 But, in spite of this similarity, it would seem that some 

 difference always arises from difference in the point of 

 view. Thus each person, so far as his sense-data are con- 

 cerned, lives in a private world. This private world 

 contains its own space, or rather spaces, for it would 

 seem that only experience teaches us to correlate the 

 space of sight with the space of touch and with the 

 various other spaces of other senses. This multiplicity 

 of private spaces, however, though interesting to the 

 psychologist, is of no great importance in regard to our 

 present problem, since a merely solipsistic experience 

 enables us to correlate them into the one private space 

 which embraces all our own sense-data. The place at 

 which a sense-datum is, is a place in private space. This 

 place therefore is different from any place in the private 

 space of another percipient. For if we assume, as logical 

 economy demands, that all position is relative, a place is 

 only definable by the things in or around it, and therefore 

 the same place cannot occur in two private worlds which 

 have no common constituent. The question, therefore, 

 of combining what we call different appearances of the 

 same thing in the same place does not arise, and the fact 

 that a given object appears to different spectators to 

 have different shapes and colours affords no argument 

 against the physical reality of all these shapes and 

 colours. 



In addition to the private spaces belonging to the 

 private worlds of different percipients, there is, however, 

 another space, in which one whole private world counts 

 as a point, or at least as a spatial unit. This might be 



