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The principle of double effect — also known as the rule of double effect; the doctrine of double effect, often abbreviated as DDE or PDE, double-effect reasoning; or simply double effect — is a set of ethical criteria which Christian philosophers, and some others, have advocated for evaluating the permissibility of acting when one's otherwise legitimate act (for example, relieving a terminally ill patient's pain) may also cause an effect one would normally be obliged to avoid (sedation and a slightly shortened life). Double-effect originates in Thomas Aquinas's treatment of homicidal self-defense, in his work Summa Theologiae.
This set of criteria states that an action having foreseen harmful effects practically inseparable from the good effect is justifiable if the following are true:
Although different writers state and employ double effect differently, they share the position that consequentially similar acts having different intentional structures make for ethically different acts. So, for example, advocates of double effect typically consider the intentional terror bombing of non-combatants having as its goal victory in a legitimate war morally out of bounds, while holding as ethically in bounds an act of strategic bombing that similarly harms non-combatants with foresight but without intent as a side effect of destroying a legitimate military target. Because advocates of double effect propose that consequentially similar acts can be morally different, double effect is most often criticized by consequentialists who consider the consequences of actions entirely determinative of the action's morality.
In their use of the distinction between intent and foresight without intent, advocates of double effect make three arguments. First, that intent differs from foresight, even in cases in which one foresees an effect as inevitable. Second, that one can apply the distinction to specific sets of cases found in military ethics (terror bombing/strategic bombing), medical ethics (craniotomy/hysterectomy), and social ethics (euthanasia). Third, that the distinction has moral relevance, importance, or significance.
The doctrine consists of four conditions that must be satisfied before an act is morally permissible:
The principle of double effect is frequently cited in cases of pregnancy and abortion. A doctor who believes abortion is always morally wrong may still remove the uterus or fallopian tubes of a pregnant woman, knowing the procedure will cause the death of the embryo or fetus, in cases in which the woman is certain to die without the procedure (examples cited include aggressive uterine cancer and ectopic pregnancy). In these cases, the intended effect is to save the woman's life, not to terminate the pregnancy, and the effect of not performing the procedure would result in the greater evil of the death of both the mother and the fetus. 
In cases of terminally ill patients who would hasten their deaths because of unbearable pain, or whose caregivers would do so for them (euthanasia, physician-assisted suicide, etc.), a principle of "double effect death" could be applied to justify the deliberate administration of a pain-killer in potentially unsafe doses — not in an attempt to end life, but with a conscious disregard of the possibility. The U.S. Supreme Court has voiced support for this principle in its deliberations over the constitutionality of physician-assisted suicide.
Consequentialists, in particular, reject the notion that two acts can differ in their moral permissibility, if both have exactly the same consequences, or expected consequences. John Stuart Mill, a nineteenth-century advocate of the utilitarian version of consequentialism, argues that it is a mistake to confuse the standards for right action with a consideration of our motives to perform a right action: "He who saves a fellow creature from drowning does what is morally right, whether his motive be duty, or the hope of being paid for his trouble; he who betrays the friend that trusts him, is guilty of a crime, even if his object be to serve another friend to whom he is under greater obligations." According to Mill, scrutiny of motives or intentions will show that almost all good behavior proceeds from questionable intentions. Therefore, Mill argues, our moral analysis should ignore matters of intention, and so we should reject DDE, which appeals to a distinction between intended and unintended consequences.