  Promises are invented in order to build upon the advantages afforded by property.  The invention of mere ownership suffices to make possession stable. The introduction of transfer by consent permits some trade, but so far only simultaneous swapping of visible commodities. Great advantages could be gained by all if people could be counted on to provide goods or services later for benefits given now, or exchange goods that are distant or described generically. But for people without the capacity to obligate themselves to future action, such exchanges would depend upon the party who performs second doing so out of gratitude alone; and that motive cannot generally be relied on in self-interested transactions. However, we can devise better ways to satisfy our appetites “in an oblique and artificial manner...” (T 3.2.5.9). First, people can easily recognize that additional kinds of mutual exchanges would serve their interests. They need only express this interest to one another in order to encourage everyone to invent and to keep such agreements. They devise a form of words to mark these new sorts of exchanges (and distinguish them from the generous reciprocal acts of friendship and gratitude). When someone utters this form of words, he is understood to express a resolution to do the action in question, and he “subjects himself to the penalty of never being trusted again in case of failure” (T 3.2.5.10), a penalty made possible by the practice of the group, who enforce the requirement to keep promises by the simple expedient of refusing to contract with those whose word cannot be trusted . This “concert or convention” (ibid.) alters human motives to act. One is moved by self-interest to give the promising sign (in order to obtain the other party’s cooperation), and once one has given it, self-interest demands that one do what one promised to do so as to insure that people will exchange promises with one in the future. Some interpreters say that this enlightened self-interest remains the only motive for keeping one’s promise, once the practice of promising has been created. But Hume says the sentiment of morals comes to play the same role in promise-keeping that it does in the development of honesty with respect to property (T 3.2.5.12); so there is evidence he thinks the moral sentiment not only becomes “annex’d” to promise-keeping but further motivates it. In larger, more anonymous communities, a further incentive is needed besides the fear of exclusion; and a sentiment of moral approval of promise-keeping arises as the result of sympathy with all who benefit from the practice, aided by a “second artifice,” the well-meaning psychological manipulation of the people by parents and politicians, which yields a near-universal admiration of fidelity and shame at breaking one’s word (T 3.2.5.12). This may provide a moral motive for promise-keeping even in anonymous transactions.