Judith Jarvis Thomson

Philippa Foot’s treatment of the trolley problem

The trolley problem, as initially described by British philosopher Philippa Foot, concerns a “tram” whose driver seems to face a difficult decision:

The driver of a runaway tram…can only steer from one narrow track on to another; five men are working on one track and one man on the other; anyone on the track he enters is bound to be killed.

Should the driver do nothing and thereby allow the tram to run over five men, or should he redirect the tram to run over one man? According to Foot, most people would agree that the driver should redirect the tram, despite knowing, and in some sense intending, its negative consequences. Foot then compared the tram problem with another case involving an undesirable result:

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Suppose that a judge or magistrate is faced with rioters demanding that a culprit be found for a certain crime and threatening otherwise to take their own bloody revenge on a particular section of the community. The real culprit being unknown, the judge sees himself as able to prevent the bloodshed only by framing some innocent person and having him executed.

Although the execution in this case would be another known and intended negative consequence for a beneficial purpose, most people would agree that the judge should not frame and execute an innocent person, no matter what the benefits of the plan. How then are the two actions morally different?

Foot explained that they differ on the basis of whether the negative means or ends are directly or only indirectly intended—as noted above, whether the negative means or ends are the purpose or only a foreseeable consequence of the action itself. In the tram case the death of the single track worker is not directly intended but only foreseen as a negative consequence of the positive act of saving five workers on the other track. In the execution case, however, the death of the innocent man is not only foreseen but directly intended in order to prevent the threatened assault. A revealing difference between the cases is that the unexpected survival of the single track worker would not be opposed or countered by the tram driver, while the unexpected survival of the targeted man would have to be followed with another execution attempt.

In her groundbreaking article, Foot began by noting that the doctrine of double effect has been invoked by Roman Catholic moral theorists as a means of determining the circumstances in which abortion may or may not be morally permissible. According to one assessment, if a pregnant woman finds herself suffering from lethal uterine cancer, a hysterectomy, which would save the woman but also kill the child, is justified, because the child’s death would not be directly intended but only foreseen as a negative consequence of saving the woman’s life. However, if the birth of the fetus would kill the mother, then killing the fetus to save her is not justified, because the child’s death would be directly intended and not merely foreseen as a negative consequence of saving the woman’s life.

Foot then noted that the doctrine of double effect as commonly formulated is subject to counterexamples and thus weaker than it might appear. For example, merchants who sell a concoction that they know to be poisonous as cooking oil are not free of blame merely because they do not directly intend their customers’ deaths but instead foresee them as a negative consequence of their efforts to make money.

Although the doctrine of double effect correctly predicts moral intuitions in many cases, a more reliable account, according to Foot, is given simply by distinguishing between “positive” and “negative” duties—duties to benefit others in a given way and duties not to harm others in a given way—and by treating negative duties as significantly outweighing positive duties. The solution also assumes that, in cases of conflicting duties of the same kind (positive or negative), the duty that should be carried out is the one that either maximizes aid or minimizes harm.

Thus, the tram driver faces a conflict between the negative duty not to kill five track workers and the negative duty not to kill one. Because the circumstances make it impossible to act on both duties, the driver must choose the duty that entails the least number of deaths. And in the case of the scapegoat, the intuition of most people that the judge should not carry out the execution is explained by the assumption that the negative duty not to kill one man is more important than the positive duty to save five.

Judith Jarvis Thomson’s rethinking of double effect

In her articles “Killing, Letting Die, and the Trolley Problem” (1976) and “The Trolley Problem” (1985), Thomson introduced provocative variants of Foot’s tram problem that seem to undermine the latter’s duty-based analysis. In one variant, the driver of the trolley faints after realizing that the trolley’s brakes have failed, and a bystander on the ground, understanding the emergency, notices a switch that could be thrown to divert the trolley onto the one-worker track. Most people would agree that it would be at least morally permissible for the bystander to throw the switch. On Foot’s analysis, the bystander would thereby violate a negative duty not to kill one person. If the bystander does nothing, however, the negative duty not to kill five people would not be violated, since, in doing nothing, the bystander would not be engaged (in any reasonable sense) in active killing—as would the driver of the trolley, who is understood to involuntarily drive the vehicle into the worker(s) in both Foot’s account and Thomson’s. At most, the bystander would be violating a positive duty to save five people. Foot’s analysis, therefore, incorrectly predicts that most people would consider it morally wrong for the bystander to throw the switch. Thomson also offered a similar example in which the “bystander” is a passenger on the trolley, who likewise would not be driving the trolley into the five workers if he did nothing.

Another much-discussed variant by Thomson involves two bystanders who witness the emergency from a footbridge over the track leading to the five workers. One of them, understanding that the trolley can be stopped only if a heavy object is thrown in its path, pushes the other, a “fat man,” off the bridge and onto the track, thereby halting the trolley and saving the five workers—but of course killing the fat man. If the pushing takes place, the pusher will have violated a negative duty not to kill one person. If, on the other hand, the bystander does nothing, no violation of a negative duty not to kill five people would occur (because the bystander would not have engaged in any active killing); at most, the bystander will have violated a positive duty to save five people. As Thomson noted in a later essay, “Turning the Trolley” (2008), the case of the fat man is similar to Foot’s case in which a judge frames and executes a scapegoat to save members of a threatened community, and it also resembles a surgeon killing healthy people against their will and transplanting their vital organs into patients who need them to survive. (Foot herself had discussed a similar negative case involving “killing people in the interests of cancer research or to obtain…spare parts for grafting on to those who need them.”)

Although Foot’s duty-based analysis correctly predicts that most people would consider it morally wrong to push the fat man off the bridge, its apparent failure to account for most people’s moral intuitions in the cases involving the bystander on the ground and the passenger on the trolley indicates that there must be other, heretofore unnoticed, differences between the cases in which the action taken seems permissible and the cases in which it seems wrong. Likewise, there must be other similarities between the cases in which the action seems wrong and other similarities between the cases in which it seems permissible.

In “Killing, Letting Die, and the Trolley Problem,” Thomson tentatively suggested that the relevant similarities between the “wrong” cases could be explained in either of two ways:

1. The person killed has more of a claim on a benefit or good of which he is deprived, or more of a claim against the harm that he suffers, than do the other person(s) involved.
2. The action immediately taken involves doing something to the person deprived or harmed rather than doing something to some other thing, which then results in that person being deprived or harmed.

Thus, Foot’s examples of the executed scapegoat and the person killed for body parts, as well as Thomson’s examples of the fat man and the involuntary donor of vital organs, exhibit feature 2. By comparison, the organ and body parts cases exhibit not only feature 2 but also feature 1—the latter because the victims obviously have a decisive claim on what is taken from their bodies. In contrast, the original trolley problem as well as the cases of the bystander on the ground and the passenger in the trolley exhibit neither feature.

Thomson’s aforementioned essays, written over the course of more than three decades, contain several other variants and analyses of the trolley problem. Interestingly, in “Turning the Trolley” Thomson argued that the common intuition that it would be permissible for the bystander on the ground to divert the trolley is mistaken.

The academic literature that her work has inspired encompasses descriptive as well as normative accounts and contributions from psychologists, physiologists, and legal scholars as well as philosophers. Somewhat simplified versions of the problem have also been presented in nonacademic publications.