Principle of double effect

The principle of double effect or doctrine of double effect is an idea in philosophy. It says that it can be morally good to do something that has a bad effect so long as it also has a larger good effect. Here, "double effect" means "second effect" or "side effect." In plain English, 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."^[1][2][3]

Historians think Thomas Aquinas was the first person to write about the principle of double effect but they are not sure. Thomas Aquinas lived in the 1200s. He wrote about killing in self-defense. Philosophers from before Thomas Aquinas, for example Augustine of Hippo, said it was never acceptable to kill another person, not even in self-defense. Thomas Aquinas said it was so long as the person did not kill their attacker on purpose and so long as the person does not fight harder than they need to. Here, the first effect is saving one's own life. The double effect (side effect) is killing the attacker. He wrote this in Summa Theologica.^[1]

In the principle of double effect, it is okay to foresee a bad effect but not to intend it. This means a person who knows their action will cause something bad might still be acting within the principle of double effect, but a person who wants the bad effect definitely is not.^[1]

The New Catholic Encyclopedia says that for an action to follow the principle of double effect:^[1]

• The action itself must be neutral; it must not be good or bad.
• The person must not want the bad thing to happen; if there is another action that would cause the good effect without the bad one, then the person must do that instead.
• The good effect must happen before or at the same time as the bad effect. It must not happen after the bad effect. Then, it might have been caused by the bad effect.
• The good effect must be bigger or more important than the bad effect, enough to be worth the bad effect.

The principle of double effect is part of Roman Catholic philosophy.^[4] Medical doctors can also use it when deciding how to treat patients, especially patients who are in pain and want to die.^[3]

The principle of double effect is not the same as "the ends justify the means." In the principle of double effect, the goal is to make something good happen, and something bad happens too. The good and bad thing both come from the same action. In the ends justify the means, the person does something bad to make a good thing happen. The good thing comes from the bad action.^[1]

Some philosophers say that foreseeing a bad effect (knowing it will happen) and intending a bad effect (wanting and meaning it to happen) are not different enough for the principle of double effect to be real. Philosophers have used the trolley problem to study the principle of double effect.^[1]

• Trolley problem
• Absolutism
• Consequentialism