PliEFACE TO THE SECOND EDITION. xxi 



concerniijg the history of logic, and in his well-known 

 Sustera of Logic and History of Logical Doctrines,^ he gives 

 some account of tlie principle of substitution, especially 

 as it is implicitly stated in the Port Royal Logic. Jkit he 

 omits all reference to Leibnitz in this connection, nor does 

 he elsewhere, so far as I can find, supply the omission. 

 His English editor, Professor T. M. Lindsay, in referring to 

 my Substitution of Similars, points out how I was antici- 

 pated by Beneke ; but he also ignores Leibnitz. It is thus 

 apparent that the most learned logicians, even when writing 

 fspc^cially on the history of logic, displayed ignorance of 

 Leibnitz' most valuable logical writings. 



It has been recently pointed out to me, however, that 

 the Eev. liobert Hurley did draw attention, at the Not- 

 tingham Meeting of the British Association, in iS66, to 

 Leiljnitz' anticipations of Boole's laws of logical notation,''^ 

 and I am inibrmed that Boole, about a year after the pub- 

 lication of his Laws of Thought, was made acquainted with 

 the.se anticipations by E. Leslie Ellis. 



There seems to have been at least one otlier German 

 logician who discovered, or adopted, the principle of sub- 

 stitution. Iteusch, in his Systema Logician, published in 

 1734, laboured to give a broader basis to the Dictum dc 

 Omni et Nullo. He argues, that •' the whole business of 

 ordinary reasoning is accomplished by the substitution of 

 ideas in place of the subject or predicate of the fumla- 

 inental proposition. Tliis some call the equation of thoughts." 

 But. in the hands of Keusch, substitution does not seem to 

 lead to simplicity, since it has to be carried on according 

 to the vule.s of ICquipoUence, Keciprocation, Subordination, 

 and Co-ordination.^ lleusch is elsewhere spoken of ^ as the 

 " celebrated Keusch " ; nevertheless, I have not been able to 



' Section 120. 



- See his " R«niark.s on Boole's Mathematical .\iialysis of Logi(\'' 

 Jlrport of the 36^^ Meeting 0/ the British Assvciatiun, TranMdioiis of the 

 Se:iions, pp. 3—6. 



•• Hamilton's l.ecl'jres, vol. iv. {>. 319. 



* JImJ. p. jjo. 



