The trolley problem is a thought experiment in moral philosophy and moral psychology with many variations, involving hypothetical ethical dilemmas about whether to sacrifice one person to save a larger number of people. The series usually begins with a scenario in which a runaway trolley (tram) or train is on course to collide with and kill a number of people tied to the tracks, 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 are posed, each containing the option either to do nothing—in which case several people will be killed—or to intervene and sacrifice one initially "safe" person to save the others.