PROF. TAIT need not wonder if an attack that is “totally unexpected” should seem “appallingly sudden.” In the absence of a statute of limitations restricting to two years and a half the right to take up a gage, there can be no reason why an attack should not be made, save its personal bearings; and the circumstances of the challenge might be cited in bar of any exception taken on that ground. I thank the Professor for his explanations. I could not have guessed that under cover of his challenge to produce a metaphysician who was also a mathematician, lurked the assumptions, that every mathematician was a metaphysician, and that every metaphysician was either a mathematician or (in the old sense) a physician. Well, he has a perfect right, for his own private convenience or pleasure, to identify two names which he had from the first asserted to be eternally distinct. Accepting his classification, then, for the sake of argument—certainly not for fruitless controversy—to wit, that everyone is either a mathematician or a non-mathematician, and that every true metaphysician must be either mathematician or physician (Faraday did not hate the term “physicist” worse than I do) we are confronted with some surprising results. Leibnitz, the author of the Monadologie and the Theodicee, works that are known to contain the germs of the Kritik der reinen Vernunft, was a spurious metaphysician. Why, in the name of common sense? “Because,” says Prof. Tait, “he was a non-mathematician; there is no medium, you know; he must have been either a non-mathematician or a mathematician, and a mathematician he was not” What! Leibnitz not a mathematician? “Not a bit of it,” says Prof. Tait; “ for he was, I fear, simply a thief as regards mathematics, and in physics he did not allow the truth of Newton's discoveries.” I do not object to the Professor calling a spade a spade; but I assure him that this charge is made just twenty years too late. It is exactly that time since the last vestige of presumption against the fair fame of the great German was obliterated. If Prof. Tait does not understand me, or, understanding me, disputes the unqualified truth of my statement, I promise to be more explicit in a future letter. But I incline to think the question is not susceptible of proof until the Council of the Royal Society, who so grossly disgraced themselves in 1712, shall do the simple act of justice and reparation required of them, viz., publish the letters and papers relating to this controversy, which since that date have slumbered in the secret archives. I advise Prof. Tait to utilise the meantime by reconsidering some of his utterances on the Principia, lib. ii. lem. 2.