PREFACE TO THE THIRD EDITION xi 



tions about the nature of reality deserve, I think, 

 more attention than has yet been given to them. 

 That they exist is certain ; that they modify the 

 indifferent impartiality of pure empiricism can 

 hardly be denied." 



The principle of simplicity lies at the base 

 of all our explanations of phenomena, and 

 Mr Balfour's address will do much to lead to 

 a clearer recognition of its importance. 



Advantage has been taken of this opportunity 

 to correct a few verbal errors which appeared 

 in the first edition of the book. The writer's 

 thanks are due to several correspondents, some 

 of them known to him personally and some not, 

 who were good enough to send notes of these 

 errors. 



Certain additions, descriptive ofwork published 

 within the last few months, have been made ; and 

 in places the treatment has been modified in order 

 to make the meaning clearer. In this task the 

 writer acknowledges gratefully the help of his 

 friend, Mr Stanley Leathes. 



22nd September 1904. 



THIRD EDITION 



Little more than verbal changes have been 

 made in transforming the second into the third 

 edition. 



xoth November 1904. 



