§8. But, secondly, even if a promise has been made quite freely and fairly, circumstances may alter so much before the time comes to fulfil it that the effects of keeping it may be quite other than those which were foreseen when it was made. In such a case probably all would agree that the promisee ought to release the promiser. But if he declines to do this, it seems difficult to decide how far the latter is bound. Some would say that he is in all cases: while others would consider that a considerable alteration of circumstances removed the obligation---perhaps adding that all engagements must be understood to be taken subject to a general understanding that they are only binding if material circumstances remain substantially the same. But such a principle very much impairs the theoretical definiteness of the duty.
This difficulty assumes a new aspect when we consider the case already noticed, of promises made to those who are now dead or temporarily out of the reach of communications. For then there is no means of obtaining release from the promise, while at the same time its performance may be really opposed to the wishes---or what would have been the wishes---of both parties. The difficulty is sometimes concealed by saying that it is our duty to carry out the `intention' of the promise. For as so used the word Intention is, in common parlance, ambiguous: it may either mean the signification which the promisee attached to the terms employed, as distinct from any other signification which the common usage of words might admit: or it may include ulterior consequences of the performance of the promise, which he had in view in exacting it. Now we do not commonly think that the promiser is concerned with the latter. He certainly has not pledged himself to aim generally at the end which the promisee has in view, but only so far as some particular means are concerned: and if he considers these means not conducive to the end, be is not thereby absolved from his promise, under ordinary circumstances. But in the case supposed, when circumstances have materially changed, and the promise does not admit of revision, probably most persons would say that we ought to take into consideration the ulterior wishes of the promisee, and carry out what we sincerely think would have been his intention. But the obligation thus becomes very vague: since it is difficult to tell from a man's wishes under one set of circumstances what he would have desired under circumstances varying from these in a complex manner: and practically this view of the obligation of a promise generally leads to great divergence of opinion. Hence it is not surprising that some hold that even in such a case the obligation ought to be interpreted strictly: while others go to the other extreme, and maintain that it ceases altogether.
But again it was said that a promise cannot abrogate a prior obligation; and, as a particular application of this rule, it would be generally agreed that no promise can make it right to inflict harm on any one. On further consideration, however, it appears doubtful how far the persons between whom the promise passed are included in the scope of this restriction. For, first, it does not seem to be commonly held that a man is as strictly bound not to injure himself as he is to avoid harming others; and so it is scarcely thought that a promise is not binding because it was a foolish one, and will entail an amount of pain or burden on the promiser out of proportion to the good done to the promisee. Still, if we take an extreme case, where the sacrifice is very disproportionate to the gain, many conscientious persons would think that the promise ought rather to be broken than kept. And, secondly, a different question arises when we consider the possibility of injuring the promisee by fulfilling the promise. For when it is said to be wrong to do harm to any one, we do not commonly mean only what he thinks harm, but what really is so, though he may think it a benefit; for it seems clearly a crime for me to give any one what I know to be poison, even though he may be stubbornly convinced that it is wholesome food. But now suppose that I have promised A to do something which, before I fulfil the promise, I see reason to regard as likely to injure him. The circumstances may be precisely the same, and only my view of them have changed. If A takes a different view and calls on me to fulfil the promise, is it right to obey him? Surely no one would say this in an extreme case, such as that of the poison. But if the rule does not hold for an extreme case, where can we draw the line? at what point ought I to give up my judgment to A, unless my own conviction is weakened? Common Sense seems to give no clear answer.