A moral principle; a way of judging whether an act is morally acceptable or unacceptable. It says that it's sometimes ok to do a good act that we know will have bad consequences, but it's never ok to do a bad act for the sake of its good consequences (rejecting "the end justifies the means").

It makes two main claims:
A. There are some intentions that are morally impermissible (eg, intending to kill innocent people). An act motivated by one of these intentions is morally unacceptable. And...
B. Just having good intentions isn't enough -- an agent is also responsible for the forseeable but unintended consequences (by-products, side-effects) of his or her actions. If those consequences are bad enough, then the act is impermissible.

Claim A requires input -- it needs a further theory of what the impermissible intentions are. The doctrine of double effect is mute on this; it can be applied using whatever theory of impermissible intentions you like.

Claim B relies on a distinction between (a) consequences the agent (the person doing the act) intends, and (b) consequences s/he may forsee but not intend. For example: a military strategist intends to take out an important munitions dump; it is forseeable that the munitions dump will explode and harm nearby civilian neighborhoods, but the strategist doesn't intend this. We can tell s/he doesn't intend it because, for example, if the dump could be taken out in a way that wouldn't harm the civilians, s/he would choose to do it that way. The civilian deaths are by-products, side-effects of the real intent.

The doctrine of double effect says that in determining whether an act is morally acceptable, we should follow this procedure:
Step 1. Check the agent's intentions. If they're morally unacceptable, then the act is unacceptable, and you don't need to proceed to step 2. *
Step 2. Check the forseeable but unintended consequences. They must be "proportional" or "not excessive" given the agent's goal. If they're excessively bad, the act is unacceptable.

* The intentions include both the ends and the means, that is, both the agent's larger goals and the methods the agent decides to use to reach those goals. For example, one might intend to bomb civilians with the larger goal of destroying morale and winning the war. The doctrine of double effect says both intentions (the intended means and the intended ends) are subject to scrutiny.

Try this with our earlier case:
Step 1. The strategist intends only to take out the munitions dump. This is ok.
Step 2. Whether this is ok depends on how many civilians will forseeably get killed. If it's 8, say, the act is acceptable. But if it's 8,000, then the act is unacceptable. (This judgment depends on the importance of the target, of course, but hopefully you get the picture.)

The doctrine has its origins in medieval Christian thinking, specifically the works of Thomas Aquinas. There are strong criticisms of it, mainly that it leaves two big loopholes for naughty people to justify their bad actions:
Loophole 1. The distinction between intended consequences and mere by-products. (Pretty easy to fudge this)
Loophole 2. The judgment of whether the bad consequences are "excessive", given the goal. (Ditto.)

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