It is morally permissible to perform an action with bad effects (e.g. killing one person to save another) iff:
1. The act itself, considered in independently of its effect, is not wrong.
2. Only the good effect is directly intended (the bad is merely foreseen).
3. The bad effect is not a means for achieving the good.
4. The good effect outweighs the bad
In a nutshell: An act is not permissible if the intention is to do a bad thing to achieve a good consequence of that bad thing. But an act is permissible if the intention is to do a good thing that simply has bad consequences (outweighed by the good).
Double effect in action
1. Dropping a bomb on a military base, knowing it will result in hundreds of civilian deaths. But the destruction of the base will end the war, resulting in many more lives saved.
2. Dropping a bomb on civilians, resulting in hundreds of deaths, the resulting terror leading to surrender and many more lives saved.
Notice the consequences are identical. According to Double Effect 1 is permissible but 2 is not. Many find this intuitively right. Now consider...
Two unconscious men are lying at either end of a plank suspended from a runaway balloon by a fraying rope that will soon break, plunging both men to their death. The plank passes a window. You can save one man by grabbing him. But that will tip the plank and kill the other. What should you do? Grab one man and you save him. But as a result of your action the other will immediately die. Or do nothing, and they will both soon die.
Intuitively, it seems right to me to save one even if as a result the other immediately dies. It seems right to kill an innocent person so that a life might be saved.
Doctrine of Double Effect concurs. But now consider...
Two unconscious men are lying at either end of a plank suspended from a runaway balloon by a fraying rope that will soon break, plunging both men to their death.
The plank passes your window. You can save one man by firmly shoving the other, nearer man off the plank to his death (he is far to heavy for you to grab). Shove him off and the plank will tip and the other man will drop safely onto an adjacent roof. Do nothing and both men die. What should you do?
Intuitively - push the man to his death. In Case 2, it also seems right to save one man by directly and intentionally killing the other (his death is not an unintended but foreseen bad consequence), just as in Case 1. But Double Effect says you should not shove the man off the plank. You should do nothing, with the result both die.
But does it really make any moral difference that one is grabbing a man rather than shoving him off? Isn't this difference morally irrelevant?