The trolley problem is a series of thought experiments in ethics and psychology, involving stylized ethical dilemmas of whether to sacrifice one person to save a larger number. The series usually begins with a scenario in which a runaway tram or trolley is on course to collide with and kill a number of people (traditionally five) down the track, but a driver or bystander can intervene and divert the vehicle to kill just one person on a different track. Then other variations of the runaway vehicle, and analogous life-and-death dilemmas (medical, judicial etc.) are posed, each containing the option to either do nothing, in which case several people will be killed, or intervene and sacrifice one initially "safe" person to save the others.
Opinions on the ethics of each scenario turn out to be sensitive to details of the story that may seem immaterial to the abstract dilemma. The question of formulating a general principle that can account for the differing judgements arising in different variants of the story was raised in a 1967 philosophy paper by Philippa Foot, and dubbed "the trolley problem" by Judith Jarvis Thomson in a 1976 article that catalyzed a large literature. Thus, in this subject the trolley problem refers to the meta-problem of why different judgements are arrived at in particular instances, which are called trolley cases, examples, dilemmas, or scenarios.
The most basic version of the dilemma, known as "Bystander at the Switch" or "Switch", goes:
There is a runaway trolley barreling down the railway tracks. Ahead, on the tracks, there are five people tied up and unable to move. The trolley is headed straight for them. You are standing some distance off in the train yard, next to a lever. If you pull this lever, the trolley will switch to a different set of tracks. However, you notice that there is one person on the side track. You have two (and only two) options:
Do nothing, in which case the trolley will kill the five people on the main track.
Pull the lever, diverting the trolley onto the side track where it will kill one person.
Which is the more ethical option? Or, more simply: What is the right thing to do?
