The principle of double effect or doctrine of double effect is an idea around morality in philosophy.
It states that:
It can be morally good to do something that has a bad side effect so long as it also has a larger good effect.
Here, “double effect” means “second effect” or “side effect.”
Simply put, the principle of double effect means, “It is acceptable for actions to have bad side effects so long as the good effect is much larger.”
Thomas Aquinas (ca. 1225-1274), an Italian Dominican friar and priest, credited with introducing the principle of double effect, has been widely regarded as the foremost Scholastic thinker of the thirteenth century, as well one of the most influential Western philosophers and theologians.
In his discussion of the permissibility of self-defence in the Summa Theologica, killing one’s assailant is justified, he argues, provided one does not intend to kill him.
Aquinas observes that:
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-defence may have two effects: one, the saving of one’s life; the other, the slaying of the aggressor.
As Aquinas’s discussion continues, a justification is provided that rests on characterising the defensive action as a means to a goal that is justified:
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.
However, Aquinas observes, the permissibility of self-defence is not unconditional:
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-defence uses more than necessary violence, it will be unlawful, whereas, if he repels force with moderation, his defence will be lawful.
The passage can be interpreted as formulating a prohibition on apportioning one’s efforts with killing as the goal guiding one’s actions, which would lead one to act with greater viciousness than pursuing the goal of self-defence would require.
It’s important to note the Double Effect draws the distinction between causing a morally grave harm as a side effect of pursuing a good end and causing a morally grave harm as a means of pursuing a good end.
This can be summarised by saying that for certain categories of morally grave actions, for example, causing the death of a human being, the principle of double effect combines the claim that it can be morally permissible to cause a death incidentally as a side effect of pursuing a good end with a general prohibition on causing the death of an innocent human being for the sake of a good end.
The prohibition is absolute in traditional Catholic applications of the principle.
The Catholic Church provides four conditions for the application of the principle of double effect:
1. The act itself must be morally good or at least indifferent.
2. The agent may not positively will the bad effect but may permit it. If he could attain the good effect without the bad effect, he should do so. The bad effect is sometimes said to be indirectly voluntary.
3. The good effect must flow from the action at least as immediately (in the order of causality, though not necessarily in the order of time) as the bad effect. Put another way, the good effect must be produced directly by the action, not by the bad effect. Otherwise, the agent would be using a bad means to a good end, which is never allowed.
4. The good effect must be sufficiently desirable to compensate for the allowing of the bad effect.
The Catholic moral tradition provides useful clarification of the fourth “proportionality” condition by recognising five different dimensions of assessment:
1. the degree of badness of the effect
2. the degree of dependence of the bad effect on the act
3. the proximity of the effect to the bad act
4. the degree of certainty that the bad effect will occur and
5. the degree of obligation to prevent the bad act.
Examples of application
Pre-emptive Killing vs Active Self-defence
To kill a person whom you know to be plotting to kill you would be impermissible because it would be a case of intentional killing.
However, to strike in self-defence against an aggressor is permissible, even if one foresees that the blow by which one defends oneself will be fatal.
Terror bomber vs Tactical bomber
The terror bomber aims to bring about civilian deaths in order to weaken the resolve of the enemy. When his bombs kill civilians, this is a consequence that he intends.
The tactical bomber aims at military targets while foreseeing that bombing such targets will cause civilian deaths. When his bombs kill civilians, this is a foreseen but unintended consequence of his actions.
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.
Abortion vs Hysterectomy
A doctor who believed that abortion was wrong, even in order to save the mother’s life, might nevertheless consistently believe that it would be permissible to perform a hysterectomy on a pregnant woman with cancer.
In carrying out the hysterectomy, the doctor would aim to save the woman’s life while merely foreseeing the death of the foetus.
Performing an abortion, by contrast, would involve intending to kill the foetus as a means to saving the mother.
Euthanasia vs Pain relief that hastens death
A doctor who intends to hasten the death of a terminally ill patient by injecting a large dose of a drug would act impermissibly because he intends to bring about the patient’s death.
However, a doctor who intended to relieve the patient’s pain with that same dose and merely foresaw the hastening of the patient’s death would act permissibly.
The Trolley Problem (Simplified)
The trolley problem is a series of thought experiments in ethics, psychology, and philosophy involving ethical dilemmas of whether to sacrifice one person to save a larger number.
The series usually begins with a scenario in which a runaway trolley or train is on course to collide with and kill a person tied up on the track ahead, but the driver or bystander can intervene and divert the trolley or train to kill a number (say 5) people tied up on a different track by pulling a lever.
Next, the track the trolley or train is on is destined to kill the 5 people unless the driver or bystander pulls the lever, sealing the fate of the one.
While there are many variations to consider, I’ll add one of the more confronting examples, but not the most confronting, which involves adding one or more of your family members to the scenario.
Let’s take the second version outlined above, but this time you must manually push a large and heavy man off a cliff to hit the lever, causing the trolley or train to change tracks if you so choose, effectively minimising the number of deaths to two, instead of five.
Yes, the man that is pushed will die, along with the one tied up on the track as a result of your actions.
The Double Effect seems to fail the ‘side effect’ or ‘greater good’ (a bad means to a good end is never allowed) argument here because the only way to save the maximum number of people is to deliberately and with intent push the large and heavy man to his death.
But if you don’t, five people will die.
The question of course is what would you do?
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