PREFACE. 



for controversy, but because the opportunity was 

 favourable for placing my own conclusions, and the 

 grounds of them, more clearly and completely before 

 the reader. Truth, on these subjects, is militant, 

 and can only establish itself by means of conflict. 

 The most opposite opinions can make a plausible 

 show of evidence while each has the statement of its 

 own case ; and it is only possible to ascertain which 

 of them is in the right, after hearing and comparing 

 what each can say against the other, and what the 

 other can urge in its defence. 



Even the criticisms from which I most dissent 

 have been of great service to me, by showing in what 

 places the exposition most needed to be improved, or 

 the argument strengthened. And I should have 

 been well pleased if the book had undergone a much 

 greater amount of attack ; as in that case I should 

 probably have been enabled to improve it still more 

 than I believe I have now done. 



In the subsequent editions, the attempt to improve 

 the work by additions and corrections, suggested by 

 criticism or by thought, has been continued. In the 

 present (seventh) edition, a few further corrections 

 have been made, but no material additions. 



