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.
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.