Aquinas developed what has become known as the principle of double effect. Simply put, double effect means that if a person performs an act with an intent of doing good, yet unintentionally causes harm, the action is morally justified. To illustrate the argument, Aquinas uses the example of self defense. He believes killing is wrong in all cases, but he ponders what one should do if she is violently attacked. Aquinas argues that the person is justified in killing her attacker to save her own life despite the evils of killing another person. In his own words," Nothing hinders one act from having two effects, only one of which is intended, while the other is beside the intention. Now moral acts take their species according to what is intended, and not according to what is beside the intention, since this is accidental."[127]
Aquinas foresaw the dangers inherent in this mode of thinking. It is easy for a person to hide his or her intentions, using the principle of double effect to justify a manifestly unjust action. To try to account for this, Aquinas argued that any act justified by double effect must be proportional to the justification. For example, a person who murders someone who stepped on her toe would not be justified. Thus, though proceeding from a good intention, an act may be rendered unlawful, if it be out of proportion to the end.[128] This principle of double effect would be developed further and would come to play a crucial role in the churchs theology of death.