What is the principle of double effect? The Stanford Encyclopedia of Philosophy speaks of it as explaining “the permissibility of an action that causes a serious harm, such as the death of a human being, as a side effect of promoting some good end.” The prototypical example is that someone is being attacked; perhaps he uses deadly force in order to protect himself, and his assailant dies as a result. The Encyclopedia cites Aquinas as introducing this principle in II-II Q64 A7 of the Summa Theologiae. Within this article he articulated how “moral acts take their species according to what is intended, and not according to what is beside the intention, since this is accidental”. The components of a moral action can be fundamentally broken down into intention, object, and circumstances. Here Aquinas is specifically referring to actions with good intentions, such as self-preservation: the object of the action properly speaking is to keep oneself safe. If there is a double effect in that the attacker is simultaneously harmed, this is accidental (as in, not essential) to the moral action. Of course, there is the consequence of getting some kind of help, funeral, etc for the party that was harmed, and perhaps the agent who was threatened will even go to court, but any culpability is legal in nature, not moral.
This is specifically regarding acts in which the intent is purely self-defense. Thomas nuances his discussion of the issue, clarifying how “it is not lawful for a man to intend killing a man in self-defense, except for such as have public authority.” Perhaps the difference is subtle, but previously the action was only permissible because the killing was not an element of the intention whatsoever. Any kind of loopholes to find how I can kill a man, but not incur sin because it is out of self-defense, would still be sinful. Indeed, the principle of double effect would dictate that you are not ultimately guilty of the harm that occurred, but the principle of double effect does not apply if murder was intended; at that point it is effectively just a single effect.
Thomas also nuances the point by proposing that “though proceeding from a good intention, an act may be rendered unlawful, if it be out of proportion to the end.” Thomas seems to have a particular appreciation for how sin is against reason and order; back in I-II Q71 A1 of the Summa (the first article of a kind of treatise on sin) he posited that “sin denotes an inordinate act; even as an act of virtue is an ordinate and due act”. If someone strikes back purely to defend himself but uses an unnecessary amount of force, Thomas thinks that is unreasonable enough to be sinful. Indeed, the Principle of Double Effect is quite important to philosophy, but it is useful to see what kinds of situations it does not concern at all. It is also useful to see how St. Augustine’s words on the subject compare to St. Thomas’s.
In Q64 A7, Thomas cites Augustine two paragraphs in a row. The first is a letter to a man named Publicola, in response to a letter Publicola had sent Augustine with some moral quandaries. The second is from On Free Choice of the Will– an almost Platonic dialogue between Augustine and the character Evodius. Thomas cites both of these in the Objections, which are claims he later refutes. Within the objections, both these works are used to support the notion that it is completely lawful to kill in self-defense. But Thomas has Replies to Objections, in which he clarifies that both the citations from Augustine would forbid killing for self-defense when the killing is intentional; Augustine is not speaking of the case when the killing is accidental. And it seems Augustine does agree with Thomas; yet a fascinating difference is, he does not seem to mention the normal situation of killing being accidental, either in the letter to Publicola or that section of On Free Choice of the Will.
A large amount of Publicola’s letter regarded various moral concerns about using things that had been dedicated to idols, and therefore much of Augustine’s letter focused on that issue as well. But in that letter Publicola also asks, “If a Christian is on the point of being killed by a barbarian or a Roman, ought he to kill the aggressor to save his own life? or ought he even, without killing the assailant, to drive him back and fight with him, seeing it has been said, ‘Resist not evil’?” Augustine starts the paragraph which responds to this with “As to killing others in order to defend one’s own life, I do not approve of this,” and the only exception he gives is for “a soldier or public functionary acting, not for himself”. Is the situation Publicola speaks of one in which killing the assailant is not permissible because the death is part of the original intention? Thomas sure seems to think so, and by his explanation Augustine forbids such defense because it is not permitted by the principle of double effect. Yet there is a subtle difference between deciding to kill out of self-preservation and deciding for self-preservation that results in killing, and discrepancies in translation can surely further muddle whether Augustine’s condemnation of killing is in agreement or disagreement with Thomas’s. Actually, Augustine goes on to clarify times when one does not incur sin for deaths they may technically have caused, but he is addressing cases of someone’s possessions incidentally killing others, since Publicola had also asked about culpability when others use a wall he built to kill an enemy. Perhaps this is also in the spirit of clarifying when harm done is accidental, but of course this matter is quite different from that of the principle of double effect.
In Book I, section 5 of On Free Choice of the Will, Augustine demonstrates disapprobation for a similar use of self-defense. Here it seems a bit more obvious to me that the action is impermissible, since it speaks of allowing “a travel to kill a highway robber in order to keep from being killed himself”. Indeed, Thomas did not permit the kind of moral acrobatics of trying to kill in a way that is permissible; what if this is a case of killing or punishing someone for a crime he has not done yet? That is an abysmal attempt at moral calculus, in which none of the curves are differentiable. It seems that if people were to propose that Augustine would not hold to the principle of double effect, they would use the first excerpt more than the second. Yet a fascinating point from the second is how the civic law allows such killing of assailants even though the killing is unjust and the law is just. Is this not a contradiction in terms? Well, the civic law is meant to keep society civil, not forbid all things humans should not do. That is God’s prerogative: “It seems to me, therefore, that the law written to govern the people rightly permits these killings and that divine providence avenges them.”
All this discussion of medieval philosophy may come across as really pie-in-the-sky, but it actually has implications for praxis regarding modern technology and phenomena (props to the Stanford Encyclopedia of Philosophy for giving me a couple helpful modern examples). For example, there is the distinction of terror bombing versus tactical bombing. These would both be particular to wartime, but by terror bombing I mean bombing in order to kill civilians of the enemy nation in order to weaken their resolve, whereas tactical bombing is meant primarily to destroy military targets. Terror bombing both foresees and intends the death of civilians–those deaths are kind of the whole point. Yet civilian deaths are not the purpose of tactical bombing– it is permissible because, although such deaths are foreseen, they are not intended. The Stanford Encyclopedia of Philosophy explains how, “Even if it is equally certain that the two bombers will cause the same number of civilian deaths, terror bombing is impermissible while tactical bombing is permissible.” Perhaps a bomber knows that his action will cause civilian deaths, but that is not the end his action is ordered towards. If he could, he would wipe out the military target without any civilians dying. He suspects that is impossible, but it is morally permissible for him to wipe out the target anyway, with the civilian deaths being an unintended double effect.
Another distinction is between euthanasia and pain relief that hastens death. Actually, the Stanford Encyclopedia of Philosophy explains how a doctor giving the exact same dose of morphine to a terminally ill patient could follow under one category or the other based on whether the doctor’s intention is bringing death or relieving pain. It is permissible if the doctor gives the morphine for the purpose of relieving pain, although he foresees that it will also hasten death. The principle of double effect applies and makes the act permissible if, as in the previous example, the doctor wants to ease the pain without causing death, but sees that as not feasible.
There are two conditions that must apply for a case of double effect to be truly permissible; first, the action can not cause a bad effect, which in its turn causes a good effect. For this would mean that the agent allowed something evil so that good would result in it, which allowance is evil because a good action requires a good object and good intention, not merely an evil object for a good intention. Thus the Atomic bombing of Hiroshima and Nagasaki in 1945 was immoral; a direct effect of it was the deaths of thousands of civilians. Now, those deaths significantly contributed to the end of WWII, thereby preventing many more deaths that have surely occurred; yet (assuming the primary intention of the bombing was to kill civilians), the bombers used an evil effect of the bombing to cause a secondary good effect, which is impermissible. Perhaps a more succinct way of phrasing this condition is that “The good effect must flow from the action at least as immediately…as the bad effect.” The other condition is simply that the primary effect is sufficiently desirable to balance out the foreseen allowance of the bad, secondary effect–even when the bad effect is not intended, if it is still allowed there ought to be a good reason why!
Bibliography
Augustinus, Aurelius, et al. On Free Choice of the Will. Hackett, 1993. K10plus ISBN.
CHURCH FATHERS: Letter 46 (St. Augustine). https://www.newadvent.org/fathers/1102046.htm. Accessed 30 Dec. 2025.
CHURCH FATHERS: Letter 47 (St. Augustine). https://www.newadvent.org/fathers/1102047.htm. Accessed 30 Dec. 2025.
Logos Virtual Library: Saint Augustine: Letter 46. https://www.logoslibrary.org/augustine/letters/046.html. Accessed 30 Dec. 2025.
McIntyre, Alison. “Doctrine of Double Effect.” The Stanford Encyclopedia of Philosophy, edited by Edward N. Zalta and Uri Nodelman, Winter 2023, Metaphysics Research Lab, Stanford University, 2023. Stanford Encyclopedia of Philosophy, https://plato.stanford.edu/archives/win2023/entries/double-effect/".
“The Bombing of Nagasaki, August 9, 1945.” The National WWII Museum | New Orleans, 9 Aug. 2020, https://www.nationalww2museum.org/war/articles/bombing-nagasaki-august-9-1945
Thomas. St. Thomas Aquinas Summa Theologica: Complete English Edition in Five Volumes. Vol. 3: IIa IIaeQQ. 1-148: With Synoptical Charts. Reprint, Christian Classics. K10plus ISBN.
Thomas. St. Thomas Aquinas Summa Theologica: Complete English Edition in Five Volumes. Vol. 2: 1a IIaeQQ. 1-114: With Synoptical Charts. Reprint, Christian Classics. K10plus ISBN.