T 3.2.5.15

We may draw the same conclusion, concerning the origin of promises, from the force, which is suppos'd to invalidate all contracts, and to free us from their obligation. Such a principle is a proof, that promises have no natural obligation, and are mere artificial contrivances for the convenience and advantage of society. If we consider aright of the matter, force is not essentially different from any other motive of hope or fear, which may induce us to engage our word, and lay ourselves under any obligation. A man, dangerously wounded, who promises a competent sum to a surgeon to cure him, wou'd certainly be bound to performance; tho' the case be not so much different from that of one, who promises a sum to a robber, as to produce so great a difference in our sentiments of morality, if these sentiments were not built entirely on public interest and convenience.