To promise is to do something stranger than it first appears: to create, by the mere utterance of words, an obligation that did not exist a moment before. A stranger's mild inconvenience carries no special claim on me until I say I will help him move house on Saturday, at which point a new fact enters the world -- not a fact about my desires or his need, but about what I now owe. This is the peculiar magic of the promise, and it is also its peculiar burden, for the obligation, once created, does not simply dissolve when it becomes inconvenient to keep.
It is tempting to think that what makes promise-breaking wrong is simply the harm it causes -- the friend left stranded, the plan thrown into disarray. But this account cannot be the whole story. Consider a promise whose breaking causes no harm at all: I promise to meet a friend for coffee, but before the appointed hour she forgets the plan entirely and suffers no disappointment when I fail to appear. Something has still gone wrong, even though no one is worse off for it. What has gone wrong is not an injury to her but a betrayal of the structure I myself erected: I asked to be trusted on specific terms, she extended that trust, and I have now made the trust groundless without her ever learning of it. The wrong, in other words, attaches to the broken structure of obligation, not merely to its visible wreckage.
This raises an uncomfortable question about promises extracted under pressure. If a traveller, held at knifepoint, promises his assailant that he will not report the robbery, has he incurred a genuine obligation? Most of us think not -- and the reason cannot be that no harm would follow from breaking such a promise, since harm is beside the point already established. The better explanation is that a promise requires not merely the words of commitment but a will that is free to withhold them; where consent is coerced, the ceremony of promising is performed without the substance that makes it binding. The same reasoning extends, more controversially, to a promise to do something wrong: a promise to help conceal a crime does not generate a genuine obligation to keep it, because the practice of promising exists to stabilise cooperation among people pursuing legitimate ends, not to manufacture new duties to do harm. A promise, on this view, is not a magic word that overrides every other consideration; it is a mechanism that operates within, and depends upon, a broader framework of what may permissibly be asked and granted.
None of this diminishes the ordinary seriousness of promises honestly made and freely broken. It only clarifies what that seriousness consists in. We do not merely dislike having our plans disrupted; we rely on a shared practice in which words of commitment can be taken at face value, and every unpunished, unexplained breach corrodes that practice a little for everyone, not only for the person immediately let down. To keep a promise even when it has become costly is not sentimentality but maintenance -- the ordinary, unglamorous upkeep of a structure that makes coordinated human life possible at all.