After the substance of this treatise had been thought out, and while the author was committing it to paper, the essays on "Space and Time," and on "The Philosophy of the Unconditioned," in the numbers of the "North American Review" for July and October, 1864, happened to fall under his notice. Some persons will appreciate the delight and avidity with which he read them; and how grateful it was to an obscure student, almost wholly isolated in the world, to find the views which he had wrought out in his secluded chamber, so ably advocated in the leading review of his country. Not that he had gone as far, or examined the subjects in hand as thoroughly as has been there done. By no means. Rather what results he had attained accord with some of those therein laid down. Of those essays it is not too much to say, that, if they have not exhausted the topics of which they treat, they have settled forever the conclusions to be reached, and leave for other writers only illustration and comment. If the author shall seem to differ from them on a minor question,--that of quantitative infinity,--the difference will, it is believed, be found to be one of the form of expression only. And the difference is maintained from the conviction that no term in science should have more than one signification. It is better to adopt illimitable and indivisible, as the technical epithets of Space, in place of the commonly used terms infinite and absolute. A metaphysical distinction has been incidentally touched upon in the following discussion, which deserves a more extensive consideration than the scope and plan of this work would permit to it here; and which, so far as the author's limited reading goes, has received very little attention from modern writers on metaphysics. He refers to the distinction between the animal nature and spiritual person, so repeatedly enounced by that profound metaphysical theologian, the apostle Paul, and by that pure spiritual pastor, the apostle John, in the terms "flesh" and "spirit." The thinkers of the world, even the best Christian philosophers, seem to have esteemed this a moral and religious distinction, and no more, when in fact it cleaves down through the whole human being, and forms the first great radical division in any proper analysis of man's soul, and classification of his constituent elements. _This is a purely natural division._ It is organic in man. It belonged as much to Adam in his purity, as it does to the most degraded wretch on the globe now. It is of such a character that, had it been properly understood and developed, the Hamiltonian system of philosophy could never have been constructed.