 Thomas Aquinas is credited with introducing the principle of double effect in his discussion of the permissibility of self-defense in the Summa Theologica (II-II, Qu. 64, Art.7). Killing one’s assailant is justified, he argues, provided one does not intend to kill him. In contrast, Augustine had earlier maintained that killing in self-defense was not permissible, arguing that “private self-defense can only proceed from some degree of inordinate self-love.” Aquinas observes that “Nothing hinders one act from having two effects, only one of which is intended, while the other is beside the intention. … Accordingly, the act of self-defense may have two effects: one, the saving of one’s life; the other, the slaying of the aggressor.” As Aquinas’s discussion continues, a justification is provided that rests on characterizing the defensive action as a means to a goal that is justified: “Therefore, this act, since one’s intention is to save one’s own life, is not unlawful, seeing that it is natural to everything to keep itself in being as far as possible.” However, Aquinas observes, the permissibility of self-defense is not unconditional: “And yet, though proceeding from a good intention, an act may be rendered unlawful if it be out of proportion to the end. Wherefore, if a man in self-defense uses more than necessary violence, it will be unlawful, whereas, if he repel force with moderation, his defense will be lawful.”