St. Thomas Aquinas, the 13th-century philosopher who established the Doctrine of Double Effect.

Hulton Archives/Getty Images

The Double Effect

The trolley problem presents a case of two similar, but vastly different moral dilemmas. Those who subscribe to the philosophical theory of utilitarianism would say that both are justified. Utilitarianism is a no-frills view of consequences. If the outcome for five people is good and the outcome is bad for one, the action is justified, permissible and even obligatory.

That gnawing little feeling you get in the back of your head when you consider the consequences of pushing a man in front of a trolley is expressed in the Doctrine of Double Effect. This notion, first introduced by St. Thomas Aquinas in the late 13th century, gives a name to the reason we have trouble accepting that it's OK to push the man onto the tracks.

This doctrine says that for an act to be morally permissible, it has to fit certain criteria. For starters, the outcome has to be a good one. Both examples in the trolley problem have that -- five people survive a terrible accident. Secondly, the outcome has to be at least as important as the action taken. Both examples cover that, too -- five lives outweigh one. Thirdly, the action can't be taken for the purposes of evil, even if it does result in beneficial good. In other words, you can't pull the lever just because you want to kill the man standing in front of the sand pit.

Lastly, the good effect has to be produced by the action taken, not by the bad effect. And here we reach the reason why pulling the switch is preferable to pushing the man onto the tracks. By pulling the lever, we are taking an action that indirectly results in the death of the man on the track. In the second example, we are intentionally pushing the man to his death. Although five people's lives will still be saved, according to Aquinas (and to many philosophers), an evil act never justifies a greater good.

Aquinas used the example of self-defense to prove his argument. As long as the victim's intent is to save his or her own life (a good intent) and not to kill his or her attacker (an evil intent), then self-defense, he reasoned, is justified and allowable [source: Stanford Encyclopedia of Philosophy].

Although Aquinas lived 700 years ago, his views on human morality are a cornerstone of Western legal systems. Even today, defendants who can prove they killed a person in self-defense are acquitted.

The Doctrine of Double Effect is based on Aquinas' observations of human morality. But where does it come from? Read the next page to find out how science is looking into our brains to uncover the source of our knowledge of right from wrong.