Here’s one for the ethicists — and you can blame the renowned moral philosopher Philippa Foot for this one. This thought experiment, of which there are now many variations, first appeared in Foot’s 1967 paper, “Abortion and the Doctrine of Double Effect.”

Imagine that you’re at the controls of a railway switch and there’s an out-of-control trolley coming. The tracks branch into two, one track that leads to a group of five people, and the other to one person. If you do nothing, the trolley will smash into the five people. But if you flip the switch, it’ll change tracks and strike the lone person. What do you do?

(Credit: We Love Philosophy)

Utilitarians, who seek to maximize happiness, say that the single person should be killed. Kantians, because they see people as ends and not means, would argue that you can’t treat the single person as a means for the benefit of the five. So you should do nothing.

A second variation of the problem involves a “fat man” and no second track — a man so large that, if you were to push him onto the tracks, his body would prevent the trolley from smashing into the group of five. So what do you do? Nothing? Or push him onto the tracks?

This thought experiment reveals the complexity of morality by distinguishing between killing a person and letting them die — a problem with implications to our laws, behavior, science, policing, and war. “Right” and “wrong” is not as simple as it’s often made out to be.