VI PREFACE. 



from any taste 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 sub- 

 jects 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. 



PREFACE TO THE EIGHTH EDITION. 



In the subsequent editions, the attempt to improve the work by 

 additions and corrections, suggested by criticism or by thought, has 

 been continued. The additions and corrections in tlie present (eighth) 

 edition, which are not very considerable, are chiefly such as have 

 been suggested by Professor Bain's Logic, a book of great merit and 

 value. Mr. Bain's view of the science is essentially the same with 

 that taken in the present treatise, the differences of opinion being few 

 and unimportant compared with the agreements ; and he has not 

 only enriched the exposition by many applications and illustrative 

 details, but has appended to it a minute and very valuable discussion 

 of the logical principles specially applicable to each of the sciences ; a 

 task for which the encyclopedical character of his knowledge peculiarly 

 qualified him. I have in several instances made use of his exposition 

 to improve my ow^n, by adopting, and occasionally by controverting, 

 matter contained in his treatise. 



The longest of tlie additions belongs to the chapter on Causation, 

 and is a discussion of the question, hoAV far, if at all, tlie ordinaiy 

 mode of stating the law of Cause and Effect requires modification to 

 adapt it to the new doctrine of the Conservation of Force : a point 

 still more fully and elaborately treated in Mr. Bain's work. 



