Assume deontology. Can one make a promise so strong that one shouldn't break it even if breaking it saves a number of lives? I don't know for sure, but there are cases where such promises would be useful, assuming deontology.
Fred needs emergency eye-surgery. If he doesn't have the surgery this week, he will live, but he will lose sight in one eye. The surgery will be done under general anesthesia, and if Fred is not brought out of the general anesthesia he will die. But there is a complication. A terrorist has announced that if Fred lives out this week, ten random innocent people will die.
Prima facie here are the main options:
- Kill Fred. Nine lives on balance are saved.
- Do nothing. Fred loses binocular vision, and ten people are killed.
- Perform surgery as usual. Fred's full vision is saved, he lives, but ten people are killed.
(A legalistic deontologist might try to suggest another option: Perform surgery, but don't bring Fred out of general anesthesia. The thought is that the surgery is morally permissible, and bringing Fred out of general anesthesia will be prohibited, but Fred's death is never intended. It is simply not prevented, and the reason for not preventing it is not in order to save lives, but simply because Double Effect prohibits us from bringing Fred out of general anesthesia. The obvious reason why this sophistical solution fails is that there is no rational justification for the surgery if Fred is't going to be brought back to consciousness. I am reminded of a perhaps mythical Jesuit in the 1960s who would suggest to a married woman with irregular cycles that worked poorly with rhythm--or maybe NFP--that she could go on the Pill to regularize her cycles in order to make rhythm/NFP work better, and that once she did that, she wouldn't need rhythm/NFP any more. That's sophistical in a similar way.)
What we need for cases like this is promises that bind even at the expense of lives. Thus, the anesthesiologist could make such a strong promise to bring Fred back. As a result, Double Effect is not violated in solution (3), because proportionality holds: granted, on balance nine lives are lost, but also a promise is kept.
In practice, I think this is the solution we actually adopt. Maybe we do this through the Hippocratic Oath, or perhaps through an implicit taking on of professional obligations by the anesthesiologist. But it is crucial in cases like the above that such promises bind even in hard cases where lives hang on it.
All that said, the fact that it would be useful to have promises like this does not entail that there are promises like that. Still, it is evidence for it. And here we get an important metametaethical constraint: metaethical theories should be such that the usefulness of moral facts be evidence for them.