CONSTITUENTS OF MATTER 143 



confusions derived from unduly simple notions as to 

 space, from the causal correlation of sense-data with 

 sense-organs, and from failure to distinguish between 

 sense-data and sensations. If what we have said on 

 these subjects is valid, the existence of sense-data is 

 logically independent of the existence of mind, and is 

 causally dependent upon the body of the percipient rather 

 than upon his mind. The causal dependence upon the 

 body of the percipient, we found, is a more complicated 

 matter than it appears to be, and, like all causal depend- 

 ence, is apt to give rise to erroneous beliefs through mis- 

 conceptions as to the nature of causal correlation. If we 

 have been right in our contentions, sense-data are merely 

 those among the ultimate constituents of the physical 

 world, of which we happen to be immediately aware ; 

 they themselves are purely physical, and all that is mental 

 in connection with them is our awareness of them, which 

 is irrelevant to their nature and to their place in physics. 

 Unduly simple notions as to space have been a great 

 stumbling-block to realists. When two men look at the 

 same table, it is supposed that what the one sees and 

 what the other sees are in the same place. Since the 

 shape and colour are not quite the same for the two men, 

 this raises a difficulty, hastily solved, or rather covered 

 up, by declaring what each sees to be purely 'sub- 

 jective " though it would puzzle those who use this glib 

 word to say what they mean by it. The truth seems to 

 be that space and time also is much more complicated 

 than it would appear to be from the finished structure of 

 physics, and that the one all-embracing three-dimensional 

 space is a logical construction, obtained by means of 

 correlations from a crude space of six dimensions. The 

 particulars occupying this six-dimensional space, classi- 

 fied in one way, form " things/' from which with certain 

 further manipulations we can obtain what physics can 



