The metaphysical department of the Treatise is a system with a great leading principle throughout, of which its author intended that all the details should be but the individual applications. If his reasoning in that department of his work be accurate, he sweeps away all other systems of the foundation of knowledge, and substitutes another in their stead. But the third book, "on Morals," like the second, on "the Passions," has no such pretension. The leading principles of the metaphysical department are certainly kept in view, but the details are not necessarily parts of it. They have a separate existence of their own: they are an analysis of phenomena which we witness in our daily life; and the reader assents or dissents as the several opinions expressed correspond with or diverge from his own observation of what he sees passing in the world around him, without, in that mental operation, either receiving or rejecting any general theory. In short, it is to a considerable extent a series of observations of human conduct and character; and as such they are admitted or denied, are sympathized with or contemned, according to the previous feelings and opinions of the reader. Among the prominent features of the theoretical part of this book, is the admission of a moral sense,[121:1] but the negation of an abstract code of morality, separately existing, and independent of the position of the persons who are applying this sense. The work in some measure foreshadows the systems which have been respectively called the utilitarian and the selfish; the former applying as the scale of moral excellence the extent to which an action is beneficial or hurtful to the human race; the latter referring the actions of mankind, whether good or bad, interested or disinterested, to self, and to impulses which are always connected with the individual in whom they act, and his passions or desires.
In this respect it had its influence, when joined to other hints thrown out by philosophers, in supplying the texts on which Helvetius, Beccaria, and Bentham discoursed at greater length and with a clearer application to definite systems. The utilitarian principle Hume afterwards extended and rendered systematic, in pursuance of the views announced in his correspondence with Hutcheson. In connexion with what is called the "selfish system" of morals, he went no farther than to point out that the source of every impulse must have its relation to the individual person on whom that impulse acts. If it be the sordid impulse of the miser, it must be because the man who feels it loves gold; if it be the profuse impulse of the spendthrift, it must be because the individual who spends has a corresponding desire within himself; if it be the charitable impulse of the person who feeds the poor, it must be because that person is under the influence of inducements which incline him rather to do so than not do so. If the principle be applied to a martyr suffering for conscience sake, or to a soldier who prefers death to submission, it is still because the person who acts fulfils impulses acting on himself. But this is a subject from which Hume appears to have shrunk in his subsequent works. He seems to have disliked the character of being connected with "the selfish school;" and he thus failed to revert to a subject on which his rigid and clear examination would have been a matter of greater interest, than his merely arguing against self-interest being the proper rule of action--an argument that with him amounts to nothing more than a protest against that vulgarization of the system, which charges it with such a doctrine for the purpose of rendering it odious. We shall afterwards find that he had a correspondence on this subject with Helvetius, who wished to bring him over to the admission of his own opinions.