T 3.2.5.6

The same truth may be prov'd still more evidently by that reasoning, which prov'd justice in general to be an artificial virtue. No action can be requir'd of us as our duty, unless there be implanted in human nature some actuating passion or motive, capable of producing the action. This motive cannot be the sense of duty. A sense of duty supposes an antecedent obligation: And where an action is not requir'd by any natural passion, it cannot be requir'd by any natural obligation; since it may be omitted without proving any defect or imperfection in the mind and temper, and consequently without any vice. Now 'tis evident we have no motive leading us to the performance of promises, distinct from a sense of duty. If we thought, that promises had no moral obligation, we never shou'd feel any inclination to observe them. This is not the case with the natural virtues. Tho' there was no obligation to relieve the miserable, our humanity wou'd lead us to it; and when we omit that duty, the immorality of the omission arises from its being a proof, that we want the natural sentiments of humanity. A father knows it to be his duty to take care of his children: But he |has also a natural inclination to it. And if no human creature had that inclination, no one cou'd lie under any such obligation. But as there is naturally no inclination to observe promises, distinct from a sense of their obligation; it follows, that fidelity is no natural virtue, and that promises have no force, antecedent to human conventions.