§6. Is, then, this obligation intuitively seen to be independent and certain?
It is often said to be so: and perhaps we may say that it seems so to unreflective common sense. But reflection seems at least to disclose a considerable number of qualifications of the principle; some clear and precise, while others are more or less indefinite.
In the first place, thoughtful persons would commonly admit that the obligation of a promise is relative to the promisee, and may be annulled by him. And therefore if the promisee be dead, or otherwise inaccessible and incapable of granting release, there is constituted an exceptional case, of which the solution presents some difficulty. 
Secondly, a promise to do an immoral act is held not to be binding, because the prior obligation not to do the act is paramount; just as in law a contract to do what a man is not legally free to do, is invalid: otherwise one could evade any moral obligation by promising not to fulfil it, which is clearly absurd. And the same principle is of course applicable to immoral omissions or forbearances to act: here however, a certain difficulty arises from the necessity of distinguishing between different kinds or degrees of obligatoriness in duties; since it is clear that a promise may sometimes make it obligatory to abstain from doing what it would otherwise have been a duty to do. Thus it becomes my duty not to give money to a meritorious hospital if I have promised all I can spare to an undeserving friend; though apart from the promise it might have been my duty to prefer the hospital to the friend. We have, however, already seen the difficulty of defining the limits of strict duty in many cases: thus (e.g.) it might be doubted how far the promise of aid to a friend ought to override the duty of giving one's children a good education. The extent, therefore, to which the obligation of a promise overrides prior obligations becomes practically somewhat obscure.