The Principle of Double Effect




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?

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. 

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 …

Originally appeared on Daily Philosophy Read More



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