Should we forcibly harvest a man’s organs to save five people? Should we plug the whole of the humanity into a machine that could ensure the maximal amount of aggregate pleasure if we could? Are we obligated to save a child drowning in a pond? These are all famous thought experiments that philosophers have posited over the past century to sway us over to their way of thinking and appeal to our most basic intuitions. But this practice, argues Edouard Machery, is dangerously flawed.

You’ve probably heard of the famous Trolley case. Here is one version: suppose a runaway trolley is about to hit five workers who, by accident, happen to be working on the track. The only way to prevent their death is by pushing a switch that will redirect the trolley onto another track. Unfortunately, there is another worker on this sidetrack who will be killed if the switch is pushed. The question: Is it permissible to push the switch, saving five people but killing one?

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You probably know that this scenario, and many other similar ones, have been used to study moral judgment, or as many say, moral intuition both in neuroscience and psychology. You might even know that they have been used to study the design of automated vehicles. But what you perhaps do not know is that Trolley cases have been a mainstay of moral philosophy for decades. They are not only used to study how people think about the right and the wrong or about the permissible and the forbidden, but also about the right and the wrong or about the permissible and the forbidden themselves. The goal is to determine what is morally right and wrong, required, permissible, and forbidden based on these very cases.

Moral philosophy is not an outlier in philosophy in its use of thought experiments such as the Trolley cases. Metaphysicians use them and epistemologists are addicted to them in their effort to understand knowledge, justification, and other important epistemic notions. Here is a famous one, called the “Gettier case”: Suppose you see a clock that says it is 3:00pm and that you come to believe it is 3:00pm as a result. It is in fact 3:00pm, and your belief is thus true, but the clock is broken and always says 3:00pm. The question is: Do you know that it is 3:00pm?

As you would expect from philosophers, there is an intricate and at times heated debate about the exact role of thought experiments and of the judgments or intuitions they elicit in philosophy. Some deny that intuitions play any role in philosophy whilst others claim that philosophers merely argue for their claims about thought experiments (e.g., “Yes, it is permissible to push the switch!”).

These lively debates notwithstanding, most philosophers would agree, I believe, that thought experiments are often used in philosophy and that the judgments philosophers make in response to them are used in some fashion to support or, more commonly, undermine philosophical theories about knowledge, wisdom, fairness, causation, or permissibility.

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How people respond to the different versions of Trolley cases depends on the order in which they read them

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