You cannot divert the trolley. You consider jumping off the bridge, in front of the trolley, thus sacrificing yourself to save the five people in danger, but you realize that you are far too light to stop the trolley.

The only way you can prevent the trolley from killing five people is by pushing this large stranger off the footbridge, in front of the trolley. If you push the stranger off, he will be killed, but you will save the other five.

The only thing you can do to prevent these five deaths is to throw a switch that will divert the trolley onto a side track, where it will kill only one person. When asked what you should do in these circumstances, most people say that you should divert the trolley onto the side track, thus saving a net four lives.

A colleague shared the story of a patient who told the porter pushing her on a trolley to the operating theater that she had been looking forward to the operation. Once she was cured, she said, she could start a family.

The first thing we did was we took a little bicycle mirror and we Band Aided it here, onto a gurney, a hospital trolley, so that when you were wheeled around by a nurse or by a doctor, you could actually have a conversation with them.

Joshua Greene, a philosophy graduate now working in psychology who has recently moved from Princeton University to Harvard, studied how people respond to a set of imaginary dilemmas. In one dilemma, you are standing by a railroad track when you notice that a trolley, with no one aboard, is heading for a group of five people.