Author, Simon.
Published, April 23, 2009.

It is important, here, to expand on Aquinas’ doctrine as he wrote it in Summa Theologica. Saint Aquinas wrote that:

proceeding from a good intention, an act may be rendered unlawful, if it be out of proportion with the end. Wherefore if a man, in self-defence, uses more than the necessary violence, it will be unlawful.  

Within these sentences is the principle of proportionality, which combined with the principle of discrimination, means that one can kill innocents in war and appeal to the doctrine of double effect only if the killing of innocents was not intended.  As Aquinas himself says, actions are rendered moral or immoral by their intention, not by what is an accidental by-product of the action.  Acts are lawful if they proceed “from a good intention.” They can be rendered otherwise if their results are disproportionate. An example would be a man who is being attacked by an unarmed teenager. He chooses to defend himself with a flame-thrower. The intention is good, and wise. The outcome is possibly the same as if he had used his bare hands; his attacker is warded off. The method he uses to defend himself against his assailant uses violence far and above what is necessary.

Part 5, which explains a war-like example, is here.