  The just war tradition, however, allows that innocent noncombatants may be killed according to the principle of double effect. This idea is derived in the Christian tradition from Aquinas, who holds that a single act may have two effects. Aquinas uses an example of killing in self-defense: the act has one effect of saving a life but another effect of killing the aggressor.  Killing an aggressor in self-defense may be permitted, if the death of the aggressor is not the primary intention of the defensive act.  It is significant that Aquinas does not expand this discussion to make it permissible to kill an innocent third party. But the just war tradition has employed this idea in a way that makes it permissible to kill innocent third parties (i.e., noncombatants) in war, so long as the primary intention is to accomplish a legitimate war aim and so long as the killing of innocents is merely a foreseen but unintended secondary effect. Absolute pacifists will argue against this employment of the doctrine of double effect and will claim that the killing of the innocent in war is always wrong, even if it is an unintended effect.