Category: ethics
Associated thinker(s): Philippa Foot, Judith Jarvis Thomson
Original publication or approximate period: Foot, The Problem of Abortion and the Doctrine of the Double Effect (1967); Thomson’s later developments
Difficulty: Beginner
Themes: killing, intention, consequences
Related experiments: transplant, violinist
Kind: thought experiment
Research checked: 2026-10-30
A runaway trolley is heading toward five people who cannot escape. You can divert it onto a side track, where it will kill one person. For this simplified version, assume the numbers and outcomes are certain and there is no way to stop the trolley or warn anyone. The puzzle becomes sharper when compared with pushing a person from a footbridge to stop the trolley: five are still saved, but the person’s body is now part of the rescue mechanism.
When, if ever, may saving several lives justify foreseeably killing one?
The case tests the relationship between consequences, rights, intentions, and using someone as a means. A decision alone is not yet a theory: the task is to identify a principle that explains why similar numerical outcomes can receive different judgments.
To test a proposed rule, write it without mentioning trolleys, then apply it to both the switch and footbridge versions. If the rule changes, identify the new morally relevant feature. A rule such as “save the larger number” is clear but may generate uncomfortable implications elsewhere. A rule such as “never cause harm” must explain why diverting the trolley counts differently from declining to divert it. This exercise reveals whether your judgment rests on a stable principle or on a feature of the story whose relevance you have not yet defended.
Consequentialist approaches emphasize the better overall outcome, although their treatment of rules and uncertainty varies. Rights-based and double-effect approaches ask whether the death is intended as a means or merely foreseen. Thomson’s comparisons challenge overly simple versions of either distinction.
Foot’s original driver case and later bystander-switch versions are not historically identical. The footbridge case is a significant variation, not a second independent discovery. A classroom answer does not establish how anyone would behave in an emergency.
Thomson’s footbridge and loop-track variants test whether the victim is used as a means. Changing probabilities, consent, or special responsibilities creates further problems, but those changes must be stated rather than silently imported.
Historical association: Foot, The Problem of Abortion and the Doctrine of the Double Effect (1967); Thomson’s later developments. The sources above support the scenario and its intellectual context. Where an original work was not retrieved in full, its account is mediated by the named academic reference; no claim of full primary-source verification is made. The explanation and reflection questions here are original editorial prose.