PREFACE TO THE SECOND EDITION. 



edition, and with one exception, they remained in manu- 

 script in the Royal Library at Hanover, until edited by 

 Erdmann,in 1839-40. The tract '< Difficultates Quaedam 

 Logicffi," though not known to Dutens, was published by 

 Baspe in 1765, in his collection caUed (Euvres Philo- 

 sophiques de feu M r Leibnitz; but this work had not 

 come to my notice, nor does the tract in question seem 

 to contain any explicit statement of the principle of 

 substitution. 



It is, I presume, the comparatively recent publication of 

 Leibnitz' most remarkable logical tracts which explains 

 the apparent ignorance of logicians as regards their con- 

 tents and importance. The most learned logicians, such 

 as Hamilton and Ueberweg, ignore Leibnitz' principle of 

 substitution. In the Appendix to the fourth volume of 

 Hamilton's Lectures on Metaphysics and Logic, is given 

 an elaborate compendium of the views of logical writers 

 concerning the ultimate basis of deductive reasoning. 

 Leibnitz is briefly noticed on p. 319, but without any 

 hint of substitution. He is here quoted as saying, " What 

 are the same with the same third, are the same with each 

 other ; that is, if A be the same with J5, and G be the 

 same with B, it is necessary that A and C should also 

 be the same with one another. For this principle flows 

 immediately from the principle of contradiction, and is 

 the ground and basis of all logic ; if that fail, there is no 

 longer any way of reasoning with certainty." This view 

 of the matter seems to be inconsistent with that which he 

 adopted in his posthumous tract. 



Dr. Thomson, indeed, was acquainted with Leibnitz' 

 tracts, and refers to them in his Outline of the Necessary 

 Laws of Thought. He calls them valuable ; nevertheless, 

 he seems to have missed the really valuable point ; for in 

 making two brief quotations, 1 he omits all mention of the 

 principle of substitution. 



Ueberweg is probably considered the best authority 



1 Fifth Edition, 1860, p. 158. 



