The Doctrine Of Double Effect

Is causing harm ever justified?

The doctrine (or principle) of double effect is used to determine when an action which has two effects, one good and one evil, may still be chosen without guilt or sin.

This principle is attributed to St. Thomas Aquinas, who used it to show that killing in self-defence is justified (St. Thomas did not use the term “double effect” or refer to the principle).

It is is often invoked to explain 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.

According to this principle, sometimes it is permissible to cause a harm as a side effect (or “double effect”) of bringing about a good result even though it would not be permissible to cause such a harm as a means to bringing about the same good end.

To make such a determination, one must analyse an action on the basis of four conditions all of which must be met for the action to be morally justifiable.

The conditions of the principle of double effect are the following:

  1. The act-in-itself cannot be morally wrong or intrinsically evil.
  2. The bad effect cannot cause the good effect.
  3. The agent cannot intend the bad effect.
  4. The bad effect cannot outweigh the good effect; there is a proportionate reason to tolerate the bad effect.

One of the most cited thought experiment examples of the doctrine of double effect is the trolley problem.

This is an exciting day. We are going to tackle The Trolley Problem. You are driving a trolley when the brakes fail, and on the track ahead of you are five workmen that you will run over. Now, you can steer to another track, but on that track is one person you would kill instead of the five. What do you do?

The first trolley scenario was proposed by Philippa Foot and is about a trolley that is going down the tracks which is set on course to run down five people who are tied to the tracks. The driver of the trolley has the option to divert the trolley onto another track in which only one person is tied.

This famous ethical dilemma is hilariously illustrated in the US television series The Good Place where four people and their otherworldly ‘frienemy’ struggle in the afterlife to define what it means to be good.

Demon Michael (Ted Danson) snaps his fingers to make the trolley problem a brutal, bloody reality, forcing ethics professor Chidi (William Jackson Harper) repeatedly to cause the deaths of simulated innocents.

What would you do?

Other variations are: a bystander pulling a lever to divert the trolley, a fat man being thrown from a bridge to stop the trolley, a bystander pulling a lever to divert a trolley so that a fat man may be run over, and a bystander pulling a lever so that a fat man falls off from a bridge to stop the trolley.

This principle is used in serious argument about some important issues in ethics such as euthanasia and medical ethics, war and civilian deaths and abortions when the mother’s life is in danger.


The Doctrine of Double Effect: Philosophers Debate a Controversial Moral Principle edited by P.A. Woodward 

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