Thomas Aquinas’ Theory of Double-Effect
The theory of double-effect is a development from Aquinas’ explanation for how killing an aggressor in self-defense is morally justified:
Nothing hinders one act from having two effects, only one of which is intended, while the other is beside the intention. … Accordingly the act of self-defense may have two effects, one is the saving of one’s life, the other is the slaying of the aggressor. Therefore this act, since one’s intention is to save one’s own life, is not unlawful, seeing that it is natural to everything to keep itself in “being,” as far as possible. And yet, though proceeding from a good intention, an act may be rendered unlawful, if it be out of proportion to the end. Wherefore if a man, in self-defense, uses more than necessary violence, it will be unlawful: whereas if he repel force with moderation his defense will be lawful.
ST II-II, 64, 7
While there have always been strict Christian pacifists who in order to obey the Fifth Commandment, “Thou shalt not kill,” will not take a human life under any circumstances, the long tradition of Catholic moral theology has seen the preservation of life, especially the safety and wellbeing of the community (state, country, city or even family), as a moral duty. Thus, it has always seen that sometimes it is necessary for people to kill in order to defend innocent life or the state (as police or soldiers).
Saint Thomas Aquinas developed the Principle of Double-Effect to explain how one may morally kill an aggressor in order to save one’s own life or that of another, even though the Fifth Commandment seems to forbid any killing. Moral theologians have applied this principle in other areas where, while one can never directly intend to cause an evil effect, an unintended evil side-effect can be tolerated to bring about a proportionately greater good.
Traditionally, Thomistic ethicist and moral theologians have recognized in Aquinas’s text four conditions in applying the Principle of Double-Effect:
- One may only intend a good or morally neutral action. One may never intend an intrinsically evil action, an action that is inherently evil. Intrinsically or inherently evil acts have an object which is disordered, i.e., which reason recognized as opposed or thwarting some human good. Some examples intrinsically evil actions are lying, fornication, adultery, murder (intentionally, directly killing another human being), suicide.
- The good action, or at least a morally neutral action, that one does intend has two effects: a good effect which one intends to achieve, and an evil side-effect, which one does not intend (even though it may be foreseen), but which is tolerated.
- The evil effect cannot be the means of achieving the good effect, for this would be equivalent of intending evil to bring about good, and so go against Romans 3:8.
- There must be proportionality between the good intended and the evil tolerated. The good to be achieved must outweigh the evil tolerated; one cannot allow significant evil effects for a trivial reason.
Aquinas’s reasoning was specifically meant to address a private person who is trying to stop an aggressor threatening innocent lives, and so the direct intended effect is just to stop the aggressor, not to kill him or her. The aggressor’s death is a secondary, unintended (though possibly foreseen) effect. Thus, one has to have a good intention, and not try to kill, or try not to kill, even though one knows the death of the aggressor will result. Certainly, one cannot intend their death, or fight in anger or out of vengeance, or even delight in their death, as all of these emotional states result from intentional killing.
Critique: I think Aquinas is, in general, right — one may morally kill another in self-defense — and his conditions are true in theory. But in the heat and panic of an actual attack, I’m not sure one would have the presence of mind to intend only the stopping of an attacker, but not their death, when one is actually employing a deadly weapon, like a gun, for instance. In practice, it would seem very difficult to kill an aggressor as a side-effect of intending to stop him without intending to kill him.
The Principle of Double-Effect applies in cases beyond self-defense, such as when efforts to save the life of a mother results in the death of a fetus. It would be morally wrong to directly intend to kill a fetus, regardless of whether the place he/she is developing is not viable in the long run (as in an ectopic pregnancy where the fetus is developing in a woman’s fallopian tubes, or if the unborn baby is growing in uterus afflicted with cancer). The direct killing of a fetus in abortion is an intrinsically evil act. But, the Principle of Double-Effect applies when a doctor intends to save the woman’s life by removing the diseased uterus or fallopian tube (even though it holds the fetus, and the unborn baby’s death can be foreseen). The doctor intends a good act, the removal of the woman’s diseased tissue (not the death of the fetus in the intrinsically evil act of abortion), but this good act has an evil side-effect, the death of the baby. Saving the life of the mother is, furthermore, proportionate to the life of the fetus which is lost.
The same basic principles apply to the other cases of killing when it is less obvious it would be moral: war and executions. These are basically morally permissible (maybe morally required) in order to defend the community (and the innocent lives that make it up).
But as it is unlawful to take a man’s life, except for the public authority acting for the common good, as stated above (Article 3), it is not lawful for a man to intend killing a man in self-defense, except for such as have public authority, who while intending to kill a man in self-defense, refer this to the public good, as in the case of a soldier fighting against the foe, and in the minister of the judge struggling with robbers, although even these sin if they be moved by private animosity.
Ibid.
Even here, though, Aquinas allows that killing someone is morally permissible only when the death of an aggressor (nation or criminal) is absolutely necessary to preserve life (negotiation has failed or incarceration is not possible), under strict conditions of the right intention, and minimizing (or being prohibited by) accompanying evils.
Double-Effect does not apply in cases of euthanasia when one intends to end the suffering of a patient dying of terminal cancer by means of directly and actively administering a lethal dose of morphine with the intention of ending the patients life. In euthanasia, one intends to kill a suffering patient as a means to ending their suffering, which is admittedly a good, but indirect, intended end. The means used to end the suffering, however, is an intrinsically evil act, i.e., the direct, intentional killing of the person with morphine.
For other discussion on morally permissible or forbidden killing please see:
Thomistic Philosophy Page 