XX PREFACE TO THE SECOND EDITION. 



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

 script in the Royal Library at tlanover, \mtil edited by 

 Erdmann, in 1839-40. The tract " Dillicultates Qusedam 

 Logicije," thout!;li not known to Dutens, was published by 

 liaspe in 1765, in his collection called CEuvres Philo- 

 sophiques tie Jcu M^' Ldbyiitz ; but tliis 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, 1 piesume, 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 Llamilton 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 

 concerninjT 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 tlurd, are the same with each 

 other ; that is, if A be the same with B, 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 flov/s 

 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 OiUline of the Necessary 

 Lavjs of Thought. He calls them valuable ; nevertheless, 

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

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

 principle of substitution. 



Ueberweg is probably considered the best authority 



' Fifth Edition, 1S60, p. 158., 



