A thought experiment:
Scene A: Imagine you're at the controls of a trolley and the brakes have failed. You're approaching a fork in the track at top speed. On the left side, 5 rail workers are fixing the track. One the right side, there is 1 worker. If you do nothing, the trolley will bear left and kill the 5. The only way to save the 5 is for you to steer to the right...which will certainly kill the 1.
Scene B: Now, imagine you're watching the runaway trolley from a footbridge. This time there is no fork in the track, but the 5 workers are there, facing certain death. You're standing next to a really big guy, and you know that if you sneak up and push him off the footbridge and into the path of the trolley, his body will stop the trolley and save the 5 men.
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