Can you do a writeup of the Doctrine of Double Effect? In particular, I'd like to know what you think the best arguments in favor of it are since I haven't been able to find one that I don't think is complete horse shit (the distinction between a bad effect that's used as a means to a desired end and a foreseen bad effect that's "merely" an unavoidable side effect of producing that end seems completely bogus and I don't see how it could ever be morally relevant). Thanks!
Holy shit, the ol' Doctrine of the Double Effect, huh? It starts all the way back with Thomas Aquinas, who was talking about what things we can and can't do - what is and is not permissible. His discussion was actually pretty short; he's like, "What if a guy is going to kill you and you have to really injure him to stop you? Uh, well, you don't mean to hurt him because you're not a dick; you're just trying to save yourself, so as long as you're not excessive, you're all good brah!" And then he's off to other topics and you're all, "WAIT COME BACK THAT SOUNDS SUPER IMPORTANT I HAVE SO MANY QUESTIONS" but it's too late. Fortunately, plenty of others have taken up the Doctrine since then, and we've got a pretty robust discussion going.
Sometimes an act has two different effects - one good and one bad. You might even say it has a... Double Effect. For example, hitting that guy in self-defense has a good effect (saving you) and a bad effect (harming another guy). There are lots of situations like that: sometimes I don't go out partying, and I have a shitty night but I'm really productive the next day. (That actually never happens, but roll with me here.) Sometimes you bomb an enemy base, which brings a war to an end but also some civilians also get killed. (I admit, that got dark quick. Welcome to moral philosophy!)
According to the Doctrine of Double Effect (DDE), you can only do an act that has both a good and a bad consequence if the following four conditions are all met at the same time:
1. The actual thing you're doing has to be good, or at least not evil. So you can't, say, drown a baby, even if that means you win a billion dollars, because baby-drowning is bad. BAD.
2. The thing you're trying to achieve can't be the evil thing. Like, if there was a way to get the good effect without the bad effect, you totally would. But you can't. So you have to suck it up and accept the bad thing, but it's absolutely not what you're going for.
3. The evil thing can't be a means to the good thing. You can't do something that has an evil effect that in turn has a good effect - the good effect has to come from your action, not from the evil effect.
4. The good effect has to outweigh the bad effect. You can't, like, accidentally allow a city to burn down in order to save a kitten. Not that I don't love kittens; I am from the Internet after all. There just has to be proportionality there, is all I'm saying.
Let's say there's a general who is a pretty awful dude. Real dickface of a general, in fact. We'll call him General Dickface. General Dickface, general dickface that he is, is planning to assassinate the President, President Reallygreatguy, who is (as you'd imagine) just a super nice guy. You also know that General Dickface has an estranged daughter named Delightfulgirl (she goes by "Dee" for short) who lives with his ex-wife, the former Mrs. Dickface. Dee is a delightful girl - hard worker, volunteers at a children's hospital, can quote Monty Python extensively, just everything you'd hope for in a decent human being. If anything happened to his daughter, General Dickface would be so stricken with grief he would kill himself instead of going through with his plan.
Now, you have to save President Reallygreatguy. Let's say that Dee has to walk over a particular bridge to get to the children's hospital, and General Dickface has to drive a tank over the same bridge to kill the President. He's almost there - your last chance to stop him is on this bridge. Unfortunately, Dee is crossing over at the exact same time. But you have to save President Reallygreatguy. WHAT DO YOU DO?
Maybe you could shoot Dee right in front of General Dickface, and he'd be so shocked and upset he'd stop what he's doing long enough to be pulled out of the tank and subdued. You've saved President Reallygreatguy! But according to the DDE, can't just shoot Dee. Why? Because shooting an innocent person is itself an evil thing, so you fail condition 1. That seems straightforward, right? Here's a morally important difference for you. "How'd you save the President?" "I shot an innocent girl on her way to a children's hospital, which shocked the assassin just long enough to arrest him!" "...Oh. Well... oh. Uh... great?"
And you can't get out of it by saying, "Man, I was just pulling a trigger! And it had this weird effect of killing Dee, which caused General Dickface to kill himself." Even if we agree that "pulling a trigger" is not intrinsically bad, that still uses the death of Dee, which is a bad, bad effect (remember we like Dee, she's seriously wonderful) that causes the good effect of saving President Reallygreatguy. The bad effect is causally linked to the good one. So that's condition 3.
Also, you can't just nuke the bridge and accidentally wipe out the entire fucking city. Sure, you save President Reallygreatguy, but the cost is way too high. So that's condition 4.
Conditions 1, 3, and 4 seem more or less straightforward to anyone who isn't a utilitarian. If you're a utlitarian and you try to think about the double effect, you're gonna have a bad time. No, it's condition 2 that is really tricky - why does it matter what you intend?
So let's say you plan to blow up the bridge, halting the tank's progress and (accidentally) killing Dee. Blowing up bridges isn't necessarily evil - sometimes you need to demolish them to build a better bridge, after all, so you meet condition 1. Killing Dee isn't the means to stopping the General here, it's just an unfortunate accident, so you meet condition 3. And this seems proportional - saving the nicest President ever probably outweighs blowing up this bridge and hurting this awesome girl. It sucks that Dee will die, but that's the only way to save the President. There's condition 4, clear. So, we can blow up the bridge, right?
Hold your fucking horses. What do you intend? If you're trying to save the President, then by all means, blow that fucker right up. But what if the guy holding the button went on a couple dates with Dee, and then she dumped him, and now he just hates her? The Double Effect says he cannot blow up the bridge, because he intends to kill Dee, which is not okay. Which is fucking weird, right? You mean he can't, save the President, just because he doesn't like this girl? If there was another guy there with him, a guy who doesn't know Dee, new guy would be allowed to push the button, but this guy can't? That's really strange, that something like intention can prevent something as important as saving the President.
As usual, some people bite the bullet and say something like, "well, that sucks, but he shouldn't really blow up that bridge." But that isn't the approach most people take. Several philosophers have suggested that the Double Effect isn't particularly helpful in deciding hard cases - it just tells you what the hard cases are. T.M. Scanlon spent a chapter of his recent Moral Dimensions discussing (and critiquing) the double effect; ultimately, he accounts for our intuitions by suggesting that an evil intention doesn't affect the permissibility of the act, it only affects the blameworthiness of the act. So maybe this guy is allowed to blow up the bridge (because of other considerations) but he doesn't get any credit for being a hero because he's actually kind of a jerk.
I don't really have room for a thorough treatment of the entire history of the Double Effect, but in general, that's what it looks like. At bottom, it doesn't seem like the Double Effect helps us determine rights and wrongs in a broad sense - it doesn't tell us what sorts of acts are right or wrong. Instead, it helps us weigh rights and wrongs already established elsewhere, in a larger moral framework.
On the other hand, it seems like there's a difference between accidentally killing civilians in an attempt to, say, end a war by bombing a factory, and killing civilians who live around a factory in order to terrorize them into ending the war. Even if either of those ends up destroying the factory and killing the same number of civilians, one of them makes you a much bigger douchebag in a morally relevant way. So one way to go about not being a douchebag is, when you do things, don't aim for evil shit to happen. I think we can all agree on that.
You can read Aquinas' original formulation of the Double Effect here.
T.M. Scanlon's Moral Dimensions has a good treatment of some of the problems which have been raised against the Double Effect in the first chapter.
Both the SEP and Wikipedia have pages on the Double Effect.