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It has been generally and justly remarked, that the Treatise is among the least systematic of philosophical works—that it has neither a definite and comprehensive plan, nor a logical arrangement. It was, indeed, so utterly deficient in the former—there was so complete a want of any projected scope of subject which the author was bound to exhaust in what he wrote—that an attempt to divide and subdivide the matter after it had been written, according to a logical arrangement, would only, as a sort of experimentum crucis, have exposed the imperfect character of the original plan. The author, therefore, [67]very discreetly allowed his matter to be arranged as the subjects of which he treated had respectively suggested themselves, and bestowed on his work a title rather general than comprehensive,—a title, of which all that can be said of its aptness to the subject is, that no part of his book can be said to be wholly without it, while he might have included an almost incalculable multitude of other subjects within it. He called it simply "A Treatise of Human Nature;" and by a subsidiary title, explanatory rather of his method than definitive of his matter, he called it "an attempt to introduce the experimental method of reasoning into moral subjects."

The purely metaphysical,[67:1] and, at the same time, the most original portion of the work, and that which has most conspicuously rendered itself a constituent part of the literature of intellectual philosophy, is "Book I. Of the Understanding." "Book II. Of the Passions," contains mixed metaphysics and ethics, with occasional notices of phenomena, which, though Hume does not, other writers would be likely to connect with physiological inquiries. The third book, "Of Virtue and Vice in General," published a year later, is of an ethical character, being an inquiry into the origin and proper system of morals, and an application of the system to government and politics.

[68]The "Treatise of Human Nature" afforded materials for the criticism of two very distinct classes of writers. The one consisted of men imbued with a spirit of inquiry kindred to that of Hume, and a genius capable of appreciating his services in the cause of truth; who, as the teachers of systems of which they were themselves the architects, had to attack or to defend the principles promulgated in the Treatise, according as these differed from or corresponded with their own. It is in the writings of these men that the true immortality of Hume as a philosopher consists. Whether they find in him great truths to acknowledge, or subtle and plausible errors to attack, they are the vital evidence of the originality of his work, of the genius that inspired it, and of its great influence on human thought and action. The other class of critics are those who, in pamphlets, or works more ambitious but not rising in real solidity above that fugitive class, or in occasional digressions from other topics, have endeavoured to prejudice the minds of their readers against the principles of the Treatise, by exaggeration, or by the misapplication of their metaphysical doctrines to the proceedings of every-day life,—a set of literary efforts of quick production and as quick decay.

To the former class of authors, it is of course not within the scope of the present writer's ambition to belong, and he sees no occasion to attempt to imitate the latter. In a work, however, which professes to give a life of David Hume, it is necessary to say something about the "Treatise of Human Nature;" and as a preliminary to such an attempt, it may be well to mark the boundaries within which the writer conceives that the duty he has assumed calls on him for a description of the work, neither impugning nor defending any of the opinions it sets forth.

[69]It seems to be right that some attempt should be made to describe the character and strength of the author's intellect, and the method of its operations; and to give a view of the fundamental characteristic principles by which he professes to distinguish his own philosophy from that of other writers on metaphysical subjects. An attempt should also be made to tell in what respect Hume has made incidental suggestions which have either been admitted as new truths in metaphysics, or have, as original but perhaps fallacious suggestions, afforded to other thinkers the means of establishing truths. These being the general objects to be kept in view, there is no intention to take them in any precise order, or to exhaust them in remarks on this one work. To attempt an analysis of the work would be out of place. There can be no more repulsive matter for reading than condensed metaphysics; and probably there is nothing less instructive than those abridgments, which, necessarily suppressing the author's discursive arguments, appeal almost entirely to the memory. To seize on and give a descriptive rather than an analytical account of the prominent features of the system, will be the chief aim of these remarks. Moreover, the Treatise bears on subjects which are nearly all recalled in its author's subsequent works; and while there are some things in the critical history of Hume's opinions which may be appropriately viewed in connexion with his first publication, there are others which it may be more expedient to examine when he is found reconsidering the subjects in his later works; and again, others which may be viewed in a general attempt to describe the extent of his literary achievements.

The Treatise has been already spoken of as embracing two great objects, metaphysics and ethics; or three, if [70]politics be considered as distinct from ethics. The great leading principle of the metaphysical department, and a principle which is never lost sight of in any part of the book, is, that the materials on which intellect works are the impressions which represent immediate sensation, whether externally as by the senses, or internally as by the passions, and ideas which are the faint reflections of these impressions. Thus to speak colloquially, when I see a picture, or when I am angry with some one, there is an impression; but when I think about this picture in its absence, or call to recollection my subsided anger, what exists in either case is an idea. Hume looked from words to that which they signified, and he found that where they signified any thing, it must be found among the things that either are or have been impressions. The whole varied and complex system of intellectual machinery he found occupied in the representation, the combination, or the arrangement of these raw materials of intellectual matter. If I say I see an object, I give expression to the fact, that a certain impression is made on the retina of my eye. If I convey to the person I am speaking to an accurate notion of what I mean, I awaken in his mind ideas left there by previous impressions, brought thither by his sense of sight.[70:1] Thus, in the particular case of the external senses, when they are considered as in direct communication between the mind and any object, there are impressions: when the senses are not said [71]to be in communication with the object, the operations of the mind in connexion with it, are from vestiges which the impressions have left on the mind; and these vestiges are called ideas, and are always more faint than the original impressions themselves. And a material circumstance to be kept in view at the very threshold of the system is, that there is no specific and distinct line drawn between impressions and ideas. Their difference is in degree merely—the former are stronger, the latter weaker. There is no difference in kind; and there is sometimes doubt whether that which is supposed to be an impression may not be a vivid idea, and that which is supposed to be an idea a faint impression.