The trolley problem is a set of thought experiments, introduced by Philippa Foot and developed by Judith Jarvis Thomson, that ask whether it is permissible to bring about one death in order to prevent several, and why intuitions differ across structurally similar cases.
A runaway trolley is heading toward five people who will be killed unless it is diverted. The trolley problem asks what an onlooker may do, and it has become one of the most discussed thought experiments in moral philosophy. Its interest is not the headline question of whether saving five at the cost of one is good, but the puzzle that small changes in how the one death is brought about seem to change what most people judge permissible. Those shifting judgments are evidence that ethical theories must explain.[1]
Philippa Foot introduced the original case in a discussion of abortion and the doctrine of double effect. Judith Jarvis Thomson later refined the examples and gave the problem its modern shape, contrasting cases that a purely outcome-based view would treat alike. NoteThe name "trolley problem" was coined by Thomson, building on Foot’s earlier example of a runaway tram.
The two main cases
The discussion turns on a pair of scenarios that share an arithmetic of one life against five but differ in their mechanics.
The switch
A bystander can pull a lever that diverts the trolley onto a side track, where it will kill one person instead of the five on the main track. Most people judge that pulling the lever is permissible, and many think it is required. The death of the one is foreseen but appears to be a side effect of saving the five rather than the means of saving them.[2]
The footbridge
In the second case, the only way to stop the trolley is to push a large bystander off a footbridge into its path, where his body will halt it and save the five. The arithmetic is identical, yet most people judge that pushing is impermissible. The difference between the cases is what the problem asks theory to account for. NoteHere the one person’s death is the means by which the five are saved, not a mere side effect.
Proposed explanations
Several principles have been offered to mark the difference. The doctrine of double effect distinguishes harm intended as a means from harm merely foreseen, permitting the latter more readily. A related idea holds that using a person’s body as an instrument is a distinct wrong. Others appeal to the difference between redirecting an existing threat and introducing a new one. Each principle fits some cases and is strained by others, which is why the problem has produced a long series of further variants designed to isolate the relevant factor.[3]
Killing and letting die, even where the numbers are the same, may not be on a par. a theme drawn from Foot and Thomson
Criticism
Not everyone takes the cases to be illuminating. Critics object that the scenarios are too artificial to ground claims about real moral life, that the elicited intuitions may track features of the description rather than genuine moral structure, and that heavy reliance on isolated cases risks mistaking quirks of psychology for ethical principle. Defenders reply that idealized cases are a legitimate way to hold confounding factors fixed, much as controlled conditions function elsewhere in inquiry.[4]
Use and influence
Beyond ethics, the trolley problem has been taken up in moral psychology, where responses to the two cases are studied experimentally, and in debates over automated systems that must be programmed to act in unavoidable-harm situations. For the broader contrast between rule-based and outcome-based theories that the cases probe, readers may consult related entries on normative ethics; for the method of weighing such case judgments against principles, see reflective equilibrium, and for a parallel limit on free agency, see free will and determinism.