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Philippa Foot's trolley problem and Judith Jarvis Thomson's variations are canonical thought experiments probing the moral significance of doing versus allowing harm, and of using someone as a means versus a side effect. In the standard case, diverting a runaway trolley from five people to one seems permissible; in the footbridge case, pushing a large man off a bridge to stop the trolley seems impermissible—though both save five by killing one. The cases are used to examine the doctrine of double effect (DDE), which permits acting with a bad foreseen but unintended side effect when other conditions are met. They reveal how moral intuitions can conflict with consequentialist arithmetic and motivate deontological side-constraints.
Work through several variants systematically: switch case, footbridge case, loop track, transplant surgeon. In each, identify what the consequentialist and deontologist each say, and check whether the DDE applies.
From your prerequisites, you know that consequentialism evaluates actions by their outcomes and deontology evaluates them by the nature of the action itself. The trolley problem is a testing machine for exactly this conflict: it constructs situations where consequentialist arithmetic points clearly in one direction while deontological intuitions resist. The philosophical point is not to tell you whether to pull the lever—it is to use your reaction to the cases to reveal the implicit moral distinctions you already hold.
In the switch case (Foot's original): a runaway trolley will kill five people unless you pull a lever diverting it to a side track where it will kill one. Most people judge diverting permissible. In the footbridge case (Thomson's variation): the only way to stop the trolley from killing five is to push a large man off a bridge into its path, killing him. Most people judge pushing impermissible. The consequentialist arithmetic is identical—five lives saved at the cost of one. Yet the intuitions diverge sharply. Something morally significant is different between the cases, and the task is to identify what.
The doctrine of double effect (DDE) is the traditional framework for distinguishing the cases. It holds that an action with a harmful effect is permissible when: (1) the action itself is not intrinsically wrong; (2) the agent intends only the good effect, not the bad one; (3) the bad effect is a foreseen but unintended side effect; and (4) there is proportionate reason for allowing the bad effect. In the switch case, the death of the one person is a foreseen side effect—the trolley would have endangered that track regardless. In the footbridge case, the man's death is the mechanism by which harm is prevented—his body is used as a means to stop the trolley. The DDE permits the side effect but forbids using someone as a means.
The doing/allowing distinction reinforces this asymmetry. In the switch case, you redirect an existing threat—the causal chain originated elsewhere and you merely alter its course. In the footbridge case, you initiate a new causal chain using another person's body. Deontologists argue that using someone as a means—treating them as an instrument for others' benefit—violates their status as a person with rights. This captures why pushing feels like murder in a way that switching does not. Consequentialism, which evaluates only outcomes, cannot explain the moral asymmetry between the cases; its inability to do so is precisely what motivates deontological side-constraints against treating persons merely as means.
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