T 3.2.5.12

Afterwards a sentiment of morals concurs with interest, and becomes a new obligation upon mankind. This sentiment of morality, in the performance of promises, arises from the same principles as that in the abstinence from the property of others. Public interest, education, and the artifices of politicians, have the same effect in both cases. The difficulties, that occur to us, in supposing a moral obligation to attend promises, we either surmount or elude. For instance; the expression of a resolution is not commonly suppos'd to be obligatory; and we cannot readily conceive how the making use of a certain form of words shou'd be able to cause any material difference. Here, therefore, we feign a new act of the mind, which we call the willing an obligation; and on this we suppose the morality to depend. But we have prov'd already, that there is no such act of the mind, and consequently that promises impose no natural obligation.