• the intended final end must be morally justified
• the intended means to the end must be morally justified
• the foreseen bad consequence must not itself be intended (willed)
• the good end must be proportionate to the bad consequence if it is to justify it

Elizabeth Anscombe discussed the principle of double effect at length in her 1961 book Intention and Jonathan Bennett made it the centerpiece of his 1980 Tanner Lectures at Oxford, "Morality and Consequences."

In his work on intentions, Michael Bratman looks back to Anscombe's descriptions of terror bombing in World War II and Bennett's similar cases, to give us the now popular case of the Strategic Bomber versus the Terror Bomber.

Both Terror Bomber and Strategic Bomber have the goal of promoting the war effort against Enemy. Each intends to pursue this goal by weakening Enemy, and each intends to do that by dropping bombs. Terror Bomber's plan is to bomb the school in Enemy's territory, thereby killing children of Enemy and terrorizing Enemy's population. Strategic Bomber's plan is different. He plans to bomb Enemy's munitions plant, thereby undermining Enemy's war effort. Strategic Bomber also knows, however, that next to the munitions plant is a school, and that when he bombs the plant he will also destroy the school, killing the children inside. Strategic Bomber has not ignored this fact. Indeed, he has worried a lot about it. Still, he has concluded that this cost, though significant, is outweighed by the contribution that would be made to the war effort by the destruction of the munitions plant.

Now, Terror Bomber intends all of the features of his action just noted: he intends to drop the bombs, kill the children, terrorize the population, and thereby weaken Enemy. In contrast, it seems that Strategic Bomber only intends to drop the bombs, destroy the munitions plant, and weaken Enemy. Although he knows that by bombing the plant he will be killing the children, he does not, it seems, intend to kill them. Whereas killing the children is, for Terror Bomber, an intended means to his end of victory, it is, for Strategic Bomber, only something he knows he will do by bombing the munitions plant. Though Strategic Bomber has taken the deaths of the children quite seriously into account in his deliberation, these deaths are for him only an expected side effect; they are not — in contrast with Terror Bomber's position — intended as a means. This, anyway, seems to be the commonsense view.

This supposed difference between the two bombers is thought by some to make an important moral difference. In particular, according to the principle of double effect this difference might make a crucial difference in the moral permissibility of the bombings. According to this principle it is sometimes permissible knowingly to bring about (or allow) some bad effect in the course of achieving some good end, even though it would not have been permissible to bring about (or allow) that bad effect as one's intended means to that good end. So it might be permissible for Strategic Bomber to bomb the plant and yet impermissible for Terror Bomber to bomb the school, even though in both cases it is known that the children will be killed and even though both bombing missions make the same contribution to weakening Enemy.

Of course, one can recognize the commonsense distinction between intending some means and merely expecting some side effect without supposing that this distinction can bear so much moral weight. My primary interest here is not in the moral principle but in the commonsense psychology that underlies it.

(Intentions, Plans, and Practical Reason, CSLI 1999, pp.139-140)

(Intention and Permissibility, Aristotle Society, 2000)

(Intentions, Plans, and Practical Reason, CSLI 1999, pp.139-140)

Chapter 3.7 - The Ergod	Chapter 4.2 - The History of Free Will
Part Three - Value	Part Five - Problems