The Principle of Double Effect
Philosophy and current affairs
Double effect means that our actions sometimes have two effects (or outcomes): one that was intended and one that was predictable but not intended. The principle of double effect explains when we are allowed to accept a morally bad effect as a consequence of trying to bring about a morally good outcome.
This is the third part of a series on the ethics of war. Find the first part here: The Ukraine Conflict and the Ethics of War.
What is a “double effect”?
I’ve just come from a class teaching philosophy students. It was a good class. They learned something useful … or did they?
Employment statistics for philosophers are a little hard to come by, but let’s assume for the sake of this argument that the cliche is right: a philosophy degree will cause you to end up unemployed or serving fries in a fast-food joint. Then, what I just did in the past two hours was to knowingly diminish my students’ chances of getting a good job and having a happy and successful life in the future. Assuming that I knew that they’d earn a lot more as accountants, why isn’t it immoral to keep teaching them philosophy instead? I am effectively robbing them of a better future, even of a specific amount of money for every month in the future where they won’t be earning an accountant’s salary. Am I a thief?
There are many cases like that. Say, your house is burning and the fire department comes along with their water hose and they flood the place. After they are gone, you discover that they ruined your computer, which contained the only copy of that spicy memoir you had just finished, not to speak of the Picasso on the wall that they also destroyed. Destroying someone’s work and prized possessions is surely an evil action. So were the firefighters evil men?
Or look at Covid vaccinations. Every vaccine and every medicine, however well tested, has some small probability of causing adverse effects. Some people, very few in the grand scheme of things, do die from vaccines that are given to them. So when the government is forcing a population to get vaccinated, they can be statistically almost certain that a small number of recipients of that vaccine will be harmed or even killed. If they insist on the mandatory vaccination, are they not committing murder?
And what about me driving my car to work? The pollution from my engine’s exhaust, together with the particles that my brakes and tyres leave on the streets, are a major source of roadside pollution that kills millions every year. I know that, even as I step into my car. And I still drive that car to my office. Am I doing something morally bad?
Thomas Aquinas and Double Effect
Cases like these have interested philosophers since ancient times. It so happened that it was a medieval Catholic philosopher, Thomas Aquinas (1225–1274), who first gave a relatively clear description of a case like that in one of his works. He is asking the question whether one may kill another person in self-defence and writes:
I answer that there is nothing to prevent one act from having two effects, of which only one is intended by the agent and the other is outside of his intention. Now, moral actions receive their character according to what is intended, and not from what is outside of the intention … Therefore, from the act of a person defending himself a twofold effect can follow: one, the saving of one’s own life; the other, the killing of the aggressor.
Such an act, therefore, insofar as the conservation of one’s own life is intended, is not illicit, since it is natural to every being to preserve its life as far as possible. Nevertheless, an act which proceeds from a good intention may be rendered illicit, if it is not proportioned to the end intended.
Hence, if one uses greater violence than is necessary in defending his own life, his act will be illicit. But, if with due moderation he repels the violence offered him, his defense of himself will be licit … Yet, since it is unlawful to kill a man except by public authority for the common good, … it is, therefore, wrong for a man to intend to kill another as a means to defend himself. (Summa Theologica, II-II, q. 64, a. 7 c; emphasis by me)
So here things get a bit more complex. A man may kill another while defending himself, but he may not intend to kill the other. If the aggressor’s death is only an unintended consequence, even if it can be foreseen, then it is fine to defend oneself, even if this means to kill the aggressor. If, on the other hand, one intended to kill the aggressor (rather than just to defend one’s own life) then the action would be immoral.
This has later been picked up by various other authors, both within the Catholic tradition and outside of it, and has become a well-known and much-disputed principle of ethics: the Doctrine of Double Effect.
The idea is that an action can have two effects, just as Thomas Aquinas explains it in the passage above: one is the intended effect, the effect we are aiming at with our action. The second is the unintended effect. It may be foreseeable, but as long as it is not intended (and a few other conditions hold), we cannot be held responsible for that unintended, second effect.
What other conditions?
Means and ends in the doctrine of Double Effect
One condition is that the bad effect must be strictly a side effect, not a means to the good end.
For example, in many wars (and one might clearly disagree with the practice) it is said that a small number of accidental civilian victims have to be accepted as “collateral” damage as long as the warring parties aim to target only other combatants. Here, the accidental civilian victims are not a necessary part of the attempt to target the enemy.
On the other hand, in the Second World War, the Allied forces bombed German cities in order to discourage the population from fighting on. Bombing civilians in this case was the whole point: a means to the end of ending the war. As good as that end may be, it is not permissible to use the deaths of innocent people as means.
The same applies to the other cases we mentioned above. If a few vaccine recipients die as a side-effect of a vaccination drive, then this is an unintended side-effect. As long as it is outweighed by thousands of saved lives due to the vaccine, one might argue that these cases don’t make the vaccination evil. But let’s say I am an anti-vaccination campaigner and I am looking forward to using these deaths for propaganda purposes. I want people to die so that I can make their deaths public and in this way achieve my political goals. Now I am using them as means to my ends. In this case, I would be doing something wrong.
Weighing good and bad effects
In order for my action to be permissible, the (intended) good effect must clearly outweigh the (unintended) bad effects. A few deaths in a vaccination campaign that will save thousands of lives might be seen as an acceptable price to pay. On the other hand, causing the same few deaths because one wanted to save money in the development and testing of the vaccine is clearly not right. One must achieve an overwhelmingly good result in order to justify the bad effects of one’s action.
If multiple effects are possible, we would also think that one must try, as much as possible, to minimise the harm done. If I have the choice of two vaccines, one of which is known to be safer than the other, then I cannot use the doctrine of Double Effect to justify using the less safe one.
The relevance of the action itself
And, finally, I have to make sure that the action itself is a good, or at least a neutral action. I cannot justify robbing or killing someone with the good consequences that this will bring about; at least not using the doctrine of Double Effect.
In July 1944, a number of conspirators attempted to assassinate Adolf Hitler and thus bring the Second World war to an end. Although the ultimate intention was a good one, such an action would not quality as Double Effect, because the action itself, assassinating another person, is not a good or neutral action. It is generally seen as a morally impermissible action, and therefore does not fulfil the criteria for a Double Effect justification.
So the actions that can be justified using Double Effect arguments must be themselves good or neutral: driving my car to work, teaching my students philosophy.
The doctrine of Double Effect
The principle of Double Effect says that an action that has two effects, one morally good and the other morally bad, might be justified if:
- The action is in itself good or at least indifferent;
- Only the good effect is intended; the evil effect might be foreseen, but it must not be intended;
- The good effect may not be produced by means of the evil effect; and
- The good effect must clearly outweigh the bad consequences.
Sometimes a fifth condition is added to these: that we must do what we can to minimise the harm caused by the action, even if some harm is inevitable.
Let’s now look back once again at the examples mentioned at the beginning of this article.
When I am teaching my students philosophy, the action of teaching philosophy is good or indifferent, but it is certainly not in itself morally bad. My intention while teaching them is to make them into better, more educated people, which is a good effect. Even if I know that they might end up unemployed, it is not the unemployment that I intend. I merely foresee it. The good effect (the philosophical education of the students) is not produced by means of their unemployment. And, finally, becoming an educated and philosophically-minded person outweighs the bad effects of unemployment (one hopes), especially since young people can always find another, more lucrative occupation, even if they have a philosophy degree.
In the same way, when the fire department extinguishes your house fire and ruins the Picasso on the wall, putting out the fire is a good action in itself; the intention is not to destroy the artwork, but to save the house; the destruction of the Picasso is not the means by which the house is saved from the fire; and putting out the fire (and saving the other artworks in the house) outweighs (one would hope) the damage done – at least this is what the firemen can be expected to assume when they are called to a fire without knowing what exactly is inside the house.
Is the doctrine of Double Effect right?
The Double Effect doctrine is one of those classic arguments in philosophy that immediately seem to make sense and to solve almost every problem. From teaching useless subjects to bombing civilians, Double Effect considerations seem to provide just the right advice in every case, the one that agrees most with our moral sense.
But is the Double Effect doctrine really the best way of judging what’s right?
One of the earliest examples of how it can go wrong was provided, unwittingly, by another Catholic philosopher, Cardinal Cajetan, Thomas de Vio (1469–1534). Joseph T. Mangan writes :
He [Cajetan] maintains that to intend to kill an innocent person as an end in itself or as a means to an end is contrary to all rights. But to kill an innocent person per accidens, by doing something that is lawful and necessary, as one does who is administering a public office, is not contrary to natural law, divine or written law. Cajetan makes this assertion when he is explaining how a judge can condemn to death a man who from private knowledge he knows to be innocent, although the evidence in the court indicates the man as guilty.
Read that last sentence again: It is morally right, according to the Cardinal, to condemn to death a man whom the judge privately knows to be innocent, just because public office and court evidence say so.
Another problem is that intentions are private: like every other mental state, they are only available to the agent themselves and not to anyone else. In addition, we are often enough mistaken about our own goals and intentions, which is, after all, the starting point of psychoanalysis. So when a general in war orders the bombing of a military installation in a city centre, do they intend to destroy only that installation? Is the demoralising effect of the expected, collateral civilian deaths really only foreseen but not at all intended? And can even the general himself reliably judge their own intentions in a situation of war, after having seen their comrades killed, their friends wounded, their country occupied?
One classic illustration of the principle of Double Effect is, once again, the trolley case. I’m sure most readers have heard of this: a trolley is out of control, racing towards a fork in the tracks. Unfortunately, where the trolley is heading, five workers are standing on the track, working away, oblivious to the fact that they will be dead in a few seconds, run over by the racing trolley. But you are standing there, able to reach a switch that will direct the trolley towards the other side of the fork: to a place where the trolley will only kill one person. Should you flick the switch?
This case fulfils all the criteria for a Double Effect defence: You’d be doing something indifferent, flicking a switch, changing the track of a trolley. The intention is to save the five, not to kill the one. The saving of the five is not produced “by the means” of killing the one. The death of the one worker is just a side-effect of redirecting the trolley. And, finally, killing one rather than five clearly minimises the harm done.
The problem is that in this case we don’t seem to actually need the doctrine of Double Effect in order to reach the conclusion at all. If we are utilitarians, we will naturally prefer to save five instead of one, no further justification needed. And if we are, say, Kantians, we will insist that all lives are equally valuable and that we cannot trade one for five – and that, therefore, any attempt to justify an action that leads to a death will be immoral.
This is because at the core of the doctrine of Double Effect is that idea of “minimising harm,” and judging that is left to the agent and their own value systems. Is bombing a city minimising harm? Is bombing a military target minimising harm? Was bombing Japan with the atomic bombs minimising harm?
And is it even providing real value to anyone to learn about philosophy?
For Cardinal Cajetan, the court proceedings had more value than actual justice. One could argue that all the clattering mechanics of the Double Effect calculation only serve to obscure that, in the end, we are making a judgement based only upon our own intuitions about what is really valuable and what constitutes benefit or harm.
So perhaps I should ride to my office by bike, after all.
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 Mangan, J. T. (1949). An historical analysis of the principle of double effect. Theological Studies, 10(1), 41-61. Available in Google Scholar.