If you have not done so, please read parts 1, 2, 3, and 4 before proceeding.
We have now arrived at the fourth and final criteria of the Principle of Double Effect:
4. The amount of good that is gained from the good effect must be able to compensate for the harm from the bad effect.
This fourth criteria is also known as the criteria of Proportionality. It essentially means that one must not have a better way of accomplishing the desired end result then by doing the stated action with the known good and bad effect; but if you can avoid the bad effect entirely or minimize it through a different act while still achieving the good effect then that different act is the proper act to do. Doing an act with a good and a bad effect when the bad effect could be avoided in another way would violate the fourth criteria and thus make it immoral for you to do the act.
Suppose, for instance, that you have soreness in your arm and you want to relieve that soreness by having it amputated. The fourth criteria would say that if the only way that soreness can be relieved is by having it amputated then it may be morally acceptable to do. But if there is a way to avoid having it amputated and yet relieve the soreness such as, say, by taking an aspirin then it would be wrong for you to have your arm amputated. Just take the aspirin and call it a day.
Let’s go back to the train analogy. Say you meet all three of the first criteria: you push the button to switch the train onto different tracks in order to save your mother, even though you know it will kill the other three strangers. The act itself, switching the train onto different tracts, is morally neutral (which satisfies criteria 1), your intention is not to kill the three strangers but to save your mother’s life (which satisfies criteria 2), and you are not saving your mother through the killing of the three strangers (which satisfies criteria 3). The fourth criteria says that if there is no way for you to save your mother’s life without the other three strangers being killed then it can be morally acceptable to switch the train onto the other train tracks. However, if there is a way for you to avoid that bad effect then that is the path that must be made. If you are in some other way capable of stopping the train to avoid killing everyone then hitting the button to switch the train would be gravely immoral even though you satisfied the first three criteria since there was a known way for you to avoid the death of the three strangers and yet that way was not chosen.
The same thing applies to the analogy of self-defense. If there is a way for you to defend yourself or others without resulting in the death of the attacker then you are obligated to achieve it in that way. If the only way to defend yourself is by killing the attacker, however, then the fourth criteria is satisfied, making it morally acceptable to defend yourself despite the bad effect.
The fourth criteria seeks to help you choose the best course of action of possibly several options. For there will be times when the first three criteria are met and yet there is a better way for you to accomplish your established objective. Whenever you can avert or reduce the bad effects you should do so.