Consider a fairly normal formulation of the Principle of Double Effect (PDE). An action that is foreseen to result in an evil is permissible if:
- the action is not intrinsically wrong
- at least one good is intended
- no evil is intended, either as a means or as an end
- the foreseen evils are not disproportionate to the intended good or goods.
Granted, the proportionality question is going to be affected by the command or vow, but proportionality does not do justice to the reasons that come from commands or vows. Proportionality weighs goods, while commands and vows give rise to exclusionary reasons, which make some goods no longer count. Of course, one could redefine proportionality to take into account all the reasons available, including the exclusionary ones, but if we do that, then (4) will presumably by itself be sufficient for permissibility. Moreover, the sort of "proportionality" that will let one adequately account for reasons arising from commands and vows will likely just be another word for permissibility, and then the account is trivial. Moreover, intuitively, even when one considers the evil of disobeying one's commander's prohibition, the bombing of the enemy headquarters could be proportionate. But when one's commander prohibits it, the bombing is no longer an act of war, but a private lethal act which one has no right to perform.
Now, one could add conditions like that the action is not forbidden by vow, promise or command. But the resulting PDE would look ad hoc, and I don't think we could be sure we listed everything needed.
So what is to be done about PDE? Here is a short and dogmatic suggestion. One of the basic deontic moral intuitions is that one should produce no evil. However, as soon as we start reflecting on the world around us, we realize that many of our actions have bad consequences for some people. A letter of recommendation that I write for my student is likely to either hurt my student or hurt my student's competitor. When I cross the road, I incur the harm of an increased risk of being run over. And so on, in various day-to-day things. Moreover, there are less day-to-day cases, such as the polio vaccine manufacturer who knows that the vaccine will kill some patients, but also knows it will save more lives. The consequentialist solution is to refine "Produce no evil" into "Do nothing that produces less utility than you could produce." I think it's easy to see that this doesn't do justice to the deontic "Produce no evil" insight.
The basic insight of double effect reasoning is that "Produce no evil" should be refined into as "Intend no evil", with a supplement of "Do nothing disproportionate." Discomfort over trolley cases then shows that we are sometimes unsure whether "Intend no evil" (together with the proportionality condition) really does capture all of the force of "Produce no evil", but I think "Intend no evil" does in fact come close to capturing the force. (I prefer: "Accomplish no evil.")
What is the PDE, then? It is simply an observation of the conditions under which the refinement of "Produce no evil" is satisfied. Seen in this way, it does not provide a sufficient condition for permissibility. It does provide a necessary condition for permissibility, and satisfaction of the conditions does show that the action is permissible insofar as the deontic question of the production of evil is concerned. But there may be other deontic questions. Seen in this way, the PDE can be simplified greatly. It simply says that "Produce no evil" is not violated when one intends no evil and does not act disproportionately.