Ueberweg is probably considered the best authority concerning the history of logic, and in his well-known *System of Logic and History of Logical Doctrines*,[7] he gives some account of the principle of substitution, especially as it is implicitly stated in the *Port Royal Logic*. But 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 anticipated 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. [7] Section 120. It has been recently pointed out to me, however, that the Rev. Robert Harley did draw attention, at the Nottingham Meeting of the British Association, in 1866, to Leibnitz’ anticipations of Boole’s laws of logical notation,[8] and I am informed that Boole, about a year after the publication of his *Laws of Thought*, was made acquainted with these anticipations by R. Leslie Ellis. [8] See his “Remarks on Boole’s Mathematical Analysis of Logic.” *Report of the 36th Meeting of the British Association, Transactions of the Sections*, pp. 3–6. There seems to have been at least one other German logician who discovered, or adopted, the principle of substitution. Reusch, 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 fundamental proposition. This some call the *equation of thoughts*.” But, in the hands of Reusch, substitution does not seem to lead to simplicity, since it has to be carried on according to the rules of Equipollence, Reciprocation, Subordination, and Co-ordination.[9] Reusch is elsewhere spoken of[10] as the “celebrated Reusch”; nevertheless, I have not been able to find a copy of his book in London, even in the British Museum Library; it is not mentioned in the printed catalogue of the Bodleian Library; Messrs. Asher have failed to obtain it for me by advertisement in Germany; and Professor Adamson has been equally unsuccessful. From the way in which the principle of substitution is mentioned by Reusch, it would seem likely that other logicians of the early part of the eighteenth century were acquainted with it; but, if so, it is still more curious that recent historians of logical science have overlooked the doctrine. [9] Hamilton’s Lectures, vol. iv. p. 319. [10] Ibid. p. 326.