Thomson, Judith Jarvis. "Killing, Letting Die, and the Trolley Problem." The Monist, vol. 59, no. 2, 1976, pp. 204-217.
"Killing, Letting Die and the Trolley Problem" tackles what has become one of the best-known moral philosophy problems around, thanks to several rounds on Twitter and a full episode of "The Good Place." This version, also from Thomson, finds you on a runaway trolley approaching a switch. If you do nothing, your trolley will run over five innocent people on the tracks. If you activate the switch, you will divert the trolley onto a side track, where you will run over one innocent person.