laws.14 Finally, there is the third criterion of good intention. It’s here that Aquinas’ most philosophically fruitful idea comes in. It’s called the “doctrine of double effect.” The classic text on this is found in his treatment of a question that we’ve seen to be closely related to the topic of just war: is it all right to kill an attacker in self-defense (ST 2.2 Q40 a7)?15 Aquinas says yes, and reasons as follows. It may be that you use force against an attacker solely with the intention of protecting yourself and kill the attacker in the process. The fact that you didn’t intend to kill him
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