134 MYSTICISM AND LOGIC 



causal dependence of objects of sense upon the percipient. 

 Now the notion of causal dependence is very obscure and 

 difficult, much more so in fact than is generally realised 

 by philosophers. I shall return to this point in a moment. 

 For the present, however, accepting the notion of causal 

 dependence without criticism, I wish to urge that the 

 dependence in question is rather upon our bodies than 

 upon our minds. The visual appearance of an object is 

 altered if we shut one eye, or squint, or look previously 

 at something dazzling ; but all these are bodily acts, and 

 the alterations which they effect are to be explained by 

 physiology and optics, not by psychology. 1 They are in 

 fact of exactly the same kind as the alterations effected 

 by spectacles or a microscope. They belong therefore to 

 the theory of the physical world, and can have no bearing 

 upon the question whether what we see is causally 

 dependent upon the mind. What they do tend to prove, 

 and what I for my part have no wish to deny, is that what 

 we see is causally dependent upon our body and is not, 

 as crude common sense would suppose, something which 

 would exist equally if our eyes and nerves and brain 

 were absent, any more than the visual appearance pre- 

 sented by an object seen through a microscope would re- 

 main if the microscope were removed. So long as it is 

 supposed that the physical world is composed of stable and 

 more or less permanent constituents, the fact that what we 

 see is changed by changes in our body appears to afford 

 reason for regarding what we see as not an ultimate con- 

 stituent of matter. But if it is recognised that the ultimate 

 constituents of matter are as circumscribed in duration as 

 in spatial extent, the whole of this difficulty vanishes. 



There remains, however, another difficulty, connected 

 with space. When we look at the sun we wish to know 



1 This point has been well urged by the American realists. 



