Trolley Problem

technique · philosophy · philosophical-tradition

Foot and Thomson's thought experiment probing intuitions about killing vs letting die.

The Trolley Problem is the family of thought experiments originated by Philippa Foot in her 1967 paper 'The Problem of Abortion and the Doctrine of Double Effect' and substantially elaborated by Judith Jarvis Thomson in 'Killing, Letting Die, and the Trolley Problem' (1976) and 'The Trolley Problem' (1985). The basic case: a runaway trolley will kill five people on the main track unless diverted to a side track where it will kill one. Most people judge it permissible to divert. The Footbridge variation (Thomson): you could stop the trolley by pushing a large person off a footbridge into its path, killing him but saving the five. Most people judge this impermissible — even though the consequentialist arithmetic is identical (one death to save five). The puzzle is what moral feature distinguishes the cases. Candidate explanations include the doctrine of double effect (intending vs foreseeing harm), the distinction between using a person as means vs side-effect harm, deontological constraints on doing vs allowing, and various philosophical specifications. The Trolley Problem became one of the most analyzed thought experiments in 20th-century moral philosophy and has had substantial influence in moral psychology research (Joshua Greene's neuroethics work using fMRI), AI ethics (autonomous-vehicle decision-making), and applied ethics. Some contemporary critics including Foot herself in later work, and Thomson in 2008's 'Turning the Trolley', have substantially revised or retracted earlier conclusions, and the problem's relevance to actual moral life is contested.

Originators

Philippa Foot (1967, original); Judith Jarvis Thomson (substantial elaboration, 1976 and 1985) high

Year / Decade

1967 (Foot); 1976 and 1985 (Thomson elaborations); 2008 (Thomson's 'Turning the Trolley' partial retraction) high

Primary sources

Foot, P. (1967). 'The Problem of Abortion and the Doctrine of Double Effect', Oxford Review, Thomson, J.J. (1976). 'Killing, Letting Die, and the Trolley Problem', The Monist, Thomson, J.J. (1985). 'The Trolley Problem', Yale Law Journal, Thomson, J.J. (2008). 'Turning the Trolley', Philosophy & Public Affairs high

Core components

Original Foot case (divert trolley)
Thomson's Footbridge variation (push large person)
Loop case and other variants
Doctrine of double effect (intended vs foreseen harm)
Distinction between using as means vs side-effect harm
Killing vs letting die distinction
Connection to deontological constraints
Joshua Greene's moral-psychology research
Recent applications to AI ethics and autonomous vehicles

Primary use case

Foundational thought experiment in normative ethics; teaching tool for ethics courses; basis for substantial moral psychology research (Greene and others); contemporary applications in AI ethics and autonomous-vehicle decision-making; reference framework in applied ethics across many domains.

Common criticisms

Substantial backlash from contemporary moral philosophers (Bernard Williams, Barbara Herman, others) arguing artificial dilemmas distort moral analysis by abstracting from real-world context, agent character, and the social fabric in which ethical decisions actually occur
Thomson herself in 'Turning the Trolley' (2008) significantly revised her earlier conclusions, arguing the original judgment that diverting is permissible may be mistaken
cross-cultural variation in trolley intuitions complicates appeals to 'universal' moral judgment
Greene's neural-correlate research has been argued to misuse trolley intuitions (showing brain regions activate without showing what should be inferred normatively)
applications to autonomous-vehicle ethics often misuse the trolley structure — real autonomous-vehicle decisions are not trolley problems and importing the framework can distort engineering choices
the iconic status of trolley reasoning in popular philosophy often outpaces its actual analytic value.

Lineage

Siblings: Veil of Ignorance Method, Ship of Theseus