Principle of double effect
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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 Christians, and some others, use for evaluating the permissibility of acting when one's otherwise legitimate act (for example, relieving a terminally ill patient's pain) will also cause an effect one would normally be obliged to avoid (for example, the patient's death). 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:
- the nature of the act is itself good, or at least morally neutral;
- the agent intends the good effect and not the bad either as a means to the good or as an end itself;
- the good effect outweighs the bad effect in circumstances sufficiently grave to justify causing the bad effect and the agent exercises due diligence to minimize the harm.
Intentional harm versus side effects 
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 nature-of-the-act condition. The action must be either morally good or indifferent.
- The means-end condition. The bad effect must not be the means by which one achieves the good effect.
- The right-intention condition. The intention must be the achieving of only the good effect, with the bad effect being only an unintended side effect.
- The proportionality condition. The good effect must be at least equivalent in importance to the bad effect.
A vaccine manufacturer typically knows that while a vaccine will save many lives, a few people may get sick or die from side effects of vaccination. The manufacture of a drug is in itself morally neutral. Lives are saved as a result of the vaccine, not as a result of the deaths due to side effects. The bad effect, the deaths due to side effects, does not further any goals of the manufacturer, and hence is not intended as a means to any end. Finally, the number of lives saved is much greater than the number lost, and so the proportionality condition is satisfied. This is more a case of side effects/benefit analysis than of a real principle application and is common in medicine.
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.
The principle appears useful in war situations. In a war, it may be morally acceptable to bomb the enemy headquarters to end the war quickly, even if civilians on the streets around the headquarters might die as, in such a case, the bad effect of civilian deaths is not disproportionate to the good effect of ending the war quickly, and the deaths of the civilians are side effect and not intended by the bombers, either as ends or as means.
On the other hand, to bomb an enemy orphanage in order to terrorize the enemy into surrender would be unacceptable, because the deaths of the orphans would be intended, in this case, as a means to ending the war early, contrary to the second condition.
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.
- Summa Theologiae, IIa-IIae Q. 64, art. 7
- T. A. Cavanaugh, Double-Effect Reasoning: Doing Good and Avoiding Evil, p.36, Oxford: Clarendon Press
- Mark Timmons, Moral Theory: An Introduction (Rowman & Littlefield 2003 ISBN 978-0-8476-9768-7), p. 80
- McIntyre, Alison. "Doctrine of Double Effect". In Edward N. Zalta. Stanford Encyclopedia of Philosophy (Summer 2006 edition ed.). Retrieved 2007-08-18.
- "Principle of Double Effect". Catholics United for the Faith. 2003. Retrieved 2007-08-18.
- Vacco v. Quill and Washington v. Glucksberg, both in 1997. See: Tucker, Kathryn E., “Legal Advocacy to Improved Care and Expand Options at the End of Life,” in Physician-Assisted Dying: The Case for Palliative Care & Patient Choice, edited by T.E. Quill and M.P. Battin (Johns Hopkins University Press, 2004). The Court made it clear that a “medical death” hastened by palliative measures was permissible.
- John Stuart Mill, Utilitarianism (London: Parker, Son and Bourn, 1863), page 26.