“The last winter, by sleeping too often by my fire, I got an ill habit of sleeping; and a distemper, which this summer has been epidemical, put me farther out of order, so that when I wrote to you, I had not slept an hour a night for a fortnight together, and for five days together not a wink. I remember I wrote to you, but what I said of your book I remember not. If you please to send me a transcript of that passage, I will give you an account of it if I can.—I am your most humble servant, “IS. NEWTON. “_Cambridge, Oct. 5th, 1693._” Although the first of these letters evinces the existence of a nervous irritability which could not fail to arise from want of appetite and of rest, yet it is obvious that its author was in the full possession of his mental powers. The answer of Mr. Locke, indeed, is written upon that supposition; and it deserves to be remarked, that Mr. Dugald Stewart, who first published a portion of these letters, never imagines for a moment that Newton was labouring under any mental alienation. The opinion entertained by Laplace, that Newton devoted his attention to theology only in the latter part of his life, may be considered as deriving some countenance from the fact, that the celebrated general scholium at the end of the second edition of the Principia, published in 1713, did not appear in the first edition of that work. This argument has been ably controverted by Dr. J. C. Gregory of Edinburgh, on the authority of a manuscript of Newton, which seems to have been transmitted to his ancestor, Dr. David Gregory, between the years 1687 and 1698. This manuscript, which consists of twelve folio pages in Newton’s handwriting, contains, in the form of additions and scholia to some propositions in the third book of the Principia, an account of the opinions of the ancient philosophers on gravitation and motion, and on natural theology, with various quotations from their works. Attached to this manuscript are three very curious paragraphs. The first two appear to have been the original draught of the general scholium already referred to; and the third relates to the subject of an ethereal medium, respecting which he maintains an opinion diametrically opposite to that which he afterward published at the end of his Optics.[84] The first paragraph expresses nearly the same ideas as some sentences in the scholium beginning “Deus summus est ens æternum, infinitum, absolute perfectum;”[85] and it is remarkable that the second paragraph is found only in the third edition of the Principia, which appeared in 1726, the year before Newton’s death.