The nature of moral _approbation_ being thus solved, there remains the nature of _obligation_; by which the author means to enquire, if a man having a view to his own welfare, will not find his best account in the practice of every moral virtue. He dwells upon the many advantages of social virtue, of benevolence and friendship, humanity and kindness, of truth and honesty; but confesses that the rule that 'honesty is the best policy' is liable to many exceptions. He makes us acquainted with his own theory of Happiness. How little is requisite to supply the _necessities_ of nature? and what comparison is there between, on the one hand, the cheap pleasures of conversation, society, study, even health, and, on the other, the common beauties of nature, with self-approbation; and the feverish, empty amusements of luxury and expense? Thus ends the main treatise; but the author adds, in an Appendix, four additional dissertations. The first takes up the question started at the outset, but postponed, how far our moral approbation is a matter of _reason_, and how far of _sentiment_. His handling of this topic is luminous and decisive. If the utility of actions be a foundation of our approval of them, _reason_ must have a share, for no other faculty can trace the results of actions in their bearings upon human happiness. In Justice especially, there are often numerous and complicated considerations; such as to occupy the deliberations of politicians and the debates of lawyers. On the other hand, reason is insufficient of itself to constitute the feeling of moral approbation or disapprobation. Reason shows the means to an end; but if we are otherwise indifferent to the end, the reasonings fall inoperative on the mind. Here then a _sentiment_ must display itself, a delight in the happiness of men, and a repugnance to what causes them misery. Reason teaches the consequences of actions; Humanity or Benevolence is roused to make a distinction in favour of such as are beneficial. He adduces a number of illustrations to show that reason alone is insufficient to make a moral sentiment. He bids us examine Ingratitude, for instance; good offices bestowed on one side, ill-will on the other. Reason might say, whether a certain action, say the gift of money, or an act of patronage, was for the good of the party receiving it, and whether the circumstances of the gift indicated a good intention on the part of the giver; it might also say, whether the actions of the person obliged were intentionally or consciously hurtful or wanting in esteem to the person obliging. But when all this is made out by reason, there remains the sentiment of abhorrence, whose foundations must be in the emotional part of our nature, in our delight in manifested goodness, and our abhorrence of the opposite.