  Suppose the practice of giving and receiving promises did not depend on a socially-defined convention. In that case, what could we mean by the utterances we use to make them, and what would be the origin of our obligation to fulfill them? Where the words are used (uncharacteristically) in a way that does not purport to reveal the agent’s will (as when the person is joking or play-acting), we do not think a promise is really being made; we only take a speaker to have promised, and so to be bound to perform, if he understands the words he uses, in particular as purporting to obligate him. Thus for effective use there must be some act of the speaker’s mind expressed by the special phrase “I promise” and its synonyms, and our moral obligation results from this act of the mind. (This seems to be Hobbes’s assumption in Leviathan, where the implicit signs of covenant — as distinct from the explicit ones — are clear signs of the person’s will.) The requisite mental act or mental state, though, could not be one of mere desire or resolution to act, since it does not follow from our desiring or resolving to act that we are morally obligated to do so; nor could it be the volition to act, since that does not come into being ahead of time when we promise, but only when the time comes to act. And of course, one can promise successfully (incur obligation by promising) even though one has no intention to perform; so the mental act requisite to obligation is not the intention to perform. The only likely act of mind that might be expressed in a promise is a mental act of willing to be obligated to perform the promised action, as this conforms to our common view that we bind ourselves by choosing to be bound.