There was a post at "3 Quarks Daily" recently on Hume's account of promises. One thing that is worth noting, however, is that all of Part II of Book III of Hume's Treatises (he discusses promises in 3.2.5) is a treatise on the natural foundations of jurisprudence. This is quite clear, not only from the topics discussed, but also from the fact that Hume regularly uses terms and addresses topics that were standard in jurisprudence texts.
Thus when Hume talks about justice, what he means by 'justice' is respect for private property, or, from the other side, property rights (his major examples are restoring a loan and abstaining from taking someone else's property). From this perspective, it is not at all surprising that he insists that justice is an artificial virtue that depends on human convention and public interest (3.2.1); he then goes on to discuss how rules governing property develop and why we end up taking them to be a moral matter at all (3.2.2); after which follows an account of what the rules of property are (3.2.3); then we get the need for mutual exchange and commerce or the transference of property by consent (3.2.4); and the discussion of property follows immediately on that (3.2.5) before finishing up the discussion of justice with some confirming arguments (3.2.6) and going on to discuss allegiance to government (3.2.7-3.2.10), international law (3.2.11), and the foundation of marriage law (3.2.12). So what is Hume doing in the discussion of promises? Well, it occurs literally in the midst of a discussion of property, right after the section establishing the need to transfer property by consent. And how do we transfer property by consent? The answer to that question makes everything clear. By 'promise' Hume means 'contract'.
And everything Hume says in the section makes perfect sense in light of that fact. Contracts are not established by mere resolution or desire to perform a service, but by expressing that one wills to take on an obligation, which one uses the contract to create. Contracts do not express any one act of mind or single action, and one's sense that one should follow a contract is generally exhausted by one's sense that it is an obligation. There are two kinds of commerce affecting transfer of property by consent: disinterested, which is that among friends and family, and requiring no contract; and interested, in which, while not necessarily caring about you at all I exchange some good or service in order to get some good or service. To establish the latter and distinguish it from the former, we invent special forms of words to show that both parties seriously expect some benefit from the contract, and that each seriously intends to uphold the responsibility given to each in the contract, on pain of no longer being trusted in this kind of exchange if they fail. It takes very little experience with contracts to recognize just how useful they are, and thus they receive the sanction of society in general and the moral force that involves. And that is Hume's account of 'promises'. He is talking about contracts.
The first six sections of Part II discuss what Hume calls 'the laws of nature' -- by which he means the most basic structures constituting human society -- which are three: "the stability of possession, of its transference by consent, and of the performance of promises" or, as we would put it, Property, Exchange (or Trade), and Contract. All three of these are in some sense more fundamental for Hume than the next big topic, Government, and do not presuppose it; all human societies have some basic principles of property, exchange, and contract, but Hume denies that all human societies have government. (He thinks, however, that government is so advantageous, and sometimes inevitable, that it will necessarily be common.) And he goes on to use his account of promises in order to attack one of the most prominent early modern accounts of the origin of government: the social contract theory.