A runaway train is racing toward five men who are tied to the track. Unless the train is stopped, it will inevitably kill all five men. You are standing on a footbridge looking down on the unfolding disaster. However, a fat man, a stranger, is standing next to you: if you push him off the bridge, he will topple onto the line and, although he will die, his chunky body will stop the train, saving five lives. Would you kill the fat man? The question may seem bizarre. But it's one variation of a puzzle that has baffled moral philosophers for almost half a century and that more recently has come to preoccupy neuroscientists, psychologists, and other thinkers as well.