PHILOSOPHICAL AND PSYCHOLOGICAL ASPECTS 225 



to say that light reflected from the book affects the retina 

 of your eye, which in turn stimulates your optic nerve, which, 

 in its turn, stimulates the centers of your brain. You know 

 only the sensations which are thus received; of the nature of 

 books you know nothing; for you perceive nothing beyond 

 the group of sense-impressions which you call a book. If 

 the book is a thing in itself other than these impressions, you 

 do not know it; for your ultimate objective reality consists 

 solely of sense-impressions. 1 



The case is not otherwise with your knowledge of your 

 fellow men, and even of your own body sense-organs 

 included. A friend before you is a group of sense-impres- 

 sions, like the book, save that among the many sense- 

 impressions which constitute your friend there are those 

 leading you to infer the existence of another personality 

 like your own. These three cases, the book, your friend, and 

 your body, are typical of the whole external world of persons 

 and things; and sense-impressions are thus the ultimate 

 external reality that is perceived by the human mind. Upon 

 these impressions we build, within our minds, a so-called 

 external world. The problem of what lies behind the sense- 

 impression, what is the nature of the thing in itself, science 

 leaves to philosophy, believing that the nature of this ul- 

 timate philosophical reality is not open to investigation by 

 any of the scientific methods now available. 



But one's knowledge of a book could never be so simple 

 a matter as above described, unless indeed he were a savage 

 who had never before seen a book. No sooner do the sense- 

 impressions reach his consciousness than he remembers other 

 books. The title attracts his attention; he remembers books 

 with similar subject-matter; and as he reads the pages his 

 mind may call up a multitude of earlier sense-impressions. 

 He may remember how he previously correlated impressions 

 derived from books, and conceived of books in general, or 



1 An exposition of the facts of science, which is similar to the one here given, 

 appears in Part I, Chapter II of the "Grammar of Science," by Karl Pearson. 



