T 3.2.5.10

But tho' this self-interested commerce of men begins to take place, and to predominate in society, it does not entirely abolish the more generous and noble intercourse of friendship and good offices. I may still do services to such persons as I love, and am more particularly acquainted with, without any prospect of advantage; and they may make me a return in the same manner, without any view but that of recompensing my past services. In order, therefore, to distinguish those two different sorts of commerce, the interested and the dis|interested, there is a certain form of words invented for the former, by which we bind ourselves to the performance of any action. This form of words constitutes what we call a promise, which is the sanction of the interested commerce of mankind. When a man says he promises any thing, he in effect expresses a resolution of performing it; and along with that, by making use of this form of words, subjects himself to the penalty of never being trusted again in case of failure. A resolution is the natural act of the mind, which promises express: But were there no more than a resolution in the case, promises wou'd only declare our former motives, and wou'd not create any new motive or obligation. They are the conventions of men, which create a new motive, when experience has taught us, that human affairs wou'd be conducted much more for mutual advantage, were there certain symbols or signs instituted, by which we might give each other security of our conduct in any particular incident. After these signs are instituted, whoever uses them is immediately bound by his interest to execute his engagements, and must never expect to be trusted any more, if he refuse to perform what he promis'd.