Abstract
This essay examines the role of the Principle of Double Effect (PDE) in the moral philosophies of Thomas Aquinas and Elizabeth Anscombe. The PDE rests on the distinction between what an agent intends and what he foresees as a side effect of his intentional action. Aquinas employs this distinction in clarifying the morality of self-defense: while a private person may use necessary force to repel an attack, he may not intend the aggressor’s death, which, if it occurs, must be a side effect. I argue that this position allows Aquinas to reconcile Augustine’s reservations about defensive killing with the Biblical affirmation that defensive force may be licit. I then show how Anscombe deploys the PDE for two additional purposes. First, she uses the principle to defend absolute moral prohibitions against the consequentialist tendencies of modern moral philosophy. Second, she appeals to the PDE in her account of murder, distinguishing the “hard core” of intentional killing of the innocent from a “penumbra” of non-intentional killings. Anscombe also exposes a corruption of the PDE rooted in a Cartesian view of intention that enables rationalization of wicked actions. By comparing Aquinas and Anscombe, the essay demonstrates the continuing importance of the PDE for articulating the moral significance of intention and for resisting ethical theories that reject absolute prohibitions.