Cory Doctorow:

The Trolley Problem is an ethical brainteaser that’s been entertaining philosophers since it was posed by Philippa Foot in 1967:

A runaway train will slaughter five innocents tied to its track unless you pull a lever to switch it to a siding on which one man, also innocent and unawares, is standing. Pull the lever, you save the five, but kill the one: what is the ethical course of action?

The problem has run many variants over time, including ones in which you have to choose between a trolley killing five innocents or personally shoving a man who is fat enough to stop the train (but not to survive the impact) into its path; a variant in which the fat man is the villain who tied the innocents to the track in the first place, and so on.