T 3.2.5.11

Nor is that knowledge, which is requisite to make mankind sensible of this interest in the institution and observance of promises, to be esteem'd superior to the capacity of human nature, however savage and uncultivated. There needs but a very little practice of the world, to make us perceive all these consequences and advantages. The shortest experience of society discovers them to every mortal; and when each individual perceives the same sense of interest in all his fellows, he immediately performs his part of any contract, as being assur'd, that they will not be wanting in theirs. All of them, by concert, enter into a scheme of actions, calculated for common benefit, and agree to be true to their word; nor is there any thing requisite to form this concert or convention, but that every one have a sense of interest in the faithful fulfilling of engagements, and express that sense to other |members of the society. This immediately causes that interest to operate upon them; and interest is the first obligation to the performance of promises.