I should like to explain how it happened that these logical writings of Leibnitz were unknown to me, until within the last twelve months. I am so slow a reader of Latin books, indeed, that my overlooking a few pages of Leibnitz’ works would not have been in any case surprising. But the fact is that the copy of Leibnitz’ works of which I made occasional use, was one of the edition of Dutens, contained in Owens College Library. The logical tracts in question were not printed in that edition, and with one exception, they remained in manuscript in the Royal Library at Hanover, until edited by Erdmann, in 1839–40. The tract “Difficultates Quædam Logicæ,” though not known to Dutens, was published by Raspe in 1765, in his collection called *Œuvres Philosophiques 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 contents 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 *B*, and *C* 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,[6] he omits all mention of the principle of substitution. [6] Fifth Edition, 1860, p. 158.