PEEFACE TO THE SECOND EDITION. xxi 



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

 System of Logic and History of Logical Doctrines, 1 he gives 

 some account of the principle of substitution, especially 

 as it is implicitly stated in the Port Eoyal Logic. But ho 

 omits all reference to Leibnitz in this connection, nor does 

 he elsewhere, so far as I can rind, 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 

 especially on the history of logic, displayed ignorance of 

 Leibnitz' most valuable logical writings. 



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

 the Kev. Eobert Harley did draw attention, at the Not- 

 tingham Meeting of the British Association, in 1866, to 

 Leibnitz' anticipations of Boole's laws of logical notation, 3 

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

 lication of his Laws of Thought, was made acquainted with 

 these anticipations by E. Leslie Ellis. 



There seems to have been at least one other German 

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

 stitution. Keusch, in his Systema Logicum, published in 

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

 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 funda- 

 mental proposition. This some call the equation of thoughts" 

 But, in the hands of Eeusch, substitution does not seem to 

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

 to the rules of Equipollence, Eeciprocation, Subordination, 

 and Co-ordination. 3 Eeusch is elsewhere spoken of 4 as the 

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



1 Section 120. 



2 See his "Remarks on Boole's Mathematical Analysis of Logic." 

 Report of the $6th Meeting of the British Association, Transactions of the 



Sections, pp. 36. 

 3 Hamilton's I 



Lectures, vol. ir. p. 319. 

 Ibid. p. 326. 



