Hume argues that there cannot be any act of the mind by means of which we create a moral obligation at will. I argue that nonetheless, on Hume’s view, to promise is, fundamentally, to obligate myself intentionally by my utterance (though not merely by an act of mind). The convention of promises, as he constructs it, gives an individual the power to change the sentiments of the community (regarding a certain action) by speaking. Given his account of obligation, that change makes the described action obligatory. This explains why Hume initially found promises mysterious and how the convention removes the mystery. It also reveals a sharp difference between Hume’s explanation of promissory obligation and that found in present-day assurance theories, according to which the obligation cannot be created merely by an intentional act of self-obligation but depends crucially on the mental states of the promisor and the promise’s recipient.