Scenario 1:
It is the afternoon and you are the only passenger on a trolley going downhill. The brakes suddenly fail and there is no backup system. The situation causes the conductor to have a panic attack and he passes out. You do have the ability to steer the trolley but not stop it. You see a fork in the tracks coming up. If you do nothing then it is certain the trolley will kill five unaware people. If you intervene to choose the other set of tracks then the lives of the five people will be spared, but one person on the other set of tracks will be killed instead. What would you do?

Scenario 2
You are an amazing surgeon. Any surgery you perform WILL NOT FAIL. You have five people in need of transplants. One needs a heart, two need a lung and two patients need kidneys. Without these organs very soon all five will certainly die. By some stroke of luck one healthy person walked into the hospital. They are a perfect match to all five of your patients. Do you kill the one healthy person to save the five patients, or do you let the five die?

Most people would help in one but let events take their course in the other, though it is difficult to pinpoint what makes intervention in one situation acceptable and unacceptable in the other.

EDIT: Here is a related article sent to me by a friend:

“The Vexing Mental Tug-of-War Called Morality” – Discover Magazine July-August 2011