Double effect, principle of

‘Double effect’ refers to the good and bad effects which may foreseeably follow from one and the same act. The principle of double effect originates in Aquinas’ ethics, and is supposed to guide decision about acts with double effect where the bad effect is something that must not be intended, such as the death of an innocent person. The principle permits such acts only if the bad effect is unintended, not disproportionate to the intended good effect, and unavoidable if the good effect is to be achieved. The principle has wide relevance in the moral evaluation of acts which have foreseen double effects. Controversy arises over the identification of the agent’s intention in difficult cases, and over the use of the principle to resolve issues such as abortion, euthanasia, the use of pain-relieving drugs which hasten death, self-defence, and the killing of certain sorts of non-combatants in war.