A fan-favorite episode of NBC’s The Good Place is called “The Trolley Problem.” If you’ve watched it, or if you’re one of the quarter of a million people who follow the “Trolley problem memes” page on Facebook, you know at least a little bit about Judith Jarvis Thomson’s work — even if you’ve never heard her name.

The prehistory of this philosophical puzzle goes back to Philippa Foot. In an essay crammed with examples intended to illustrate the complexities of an obscure idea in moral philosophy called the “doctrine of double effect,” she introduces the “driver of a runaway tram which he can only steer from one track to another.” If he does nothing, he’ll kill five workers doing repairs on the track. If he steers onto an alternate track, he’ll only kill one. Foot thought it was obvious that “we should say, without hesitation, that the driver should steer for the unoccupied track.”

I’ve introduced dozens of introductory classes of students to this example over the course of the decade and a half that I’ve taught philosophy classes, usually illustrating it with crude stick figure drawings on the chalkboard. When I’ve asked for a show of hands, I’ve never had a class where more than two or three students didn’t share Foot’s intuition that this would be the right thing to do. If that was all there was to it, no one would think there was a “problem” about the runaway trolley.