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, abbreviated to DDE; double-effect reasoning; or simply double effect, is a set of ethical criteria 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 the thought of Thomas Aquinas (in his treatment of homicidal self-defense found in his Summa Theologiae, IIa-IIae Q.  64, art. 7).
This set of criteria states that an action having foreseen harmful effects practically inseparable from the good effect is justifiable if upon satisfaction of the following:
- 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 tactical 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/tactical bombing), medical ethics (craniotomy/hysterectomy), and social ethics (euthanasia). Third, that the distinction has moral relevance, importance, or significance.

# Examples

## Medicine
A vaccine manufacturer typically knows that while a vaccine will save many lives, a few people may 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 nevertheless 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 unborn child.

## Palliative care
It is often thought that the administration of a high dosage of opioids is sometimes allowed for the relief of pain in cases of terminal illness, even when this can  cause death as a side effect. This argument played a great part in the 1957 acquittal of suspected serial killer Dr. John Bodkin Adams, a case which established the principle in British law.
Some, including most Catholic ethicists, hold that this concept is morally different from deliberate euthanasia for the relief of pain. In addition, support for the view that palliative care and euthanasia are close companions is based on the assumption of a fine line between pain relief, or relief of severe distress, and causing death.   In practice, opioids have a very wide safety margin when used appropriately and in the context of pain relief that is titrated (adjusted) to the individual patient. Similarly, sedatives are not lethal when used only to relieve distress and at the lowest dose to avoid dangerous advserse effects. Today, palliative care experience and research has shown that it is possible to manage pain or distress without hastening death (see opioids) and double effect is not viewed as being part of palliative care practice.

## War
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.  For, 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 condition 2.

## Passive suicide by refusal of medical treatment
Suppose a patient has been diagnosed with a terminal illness where the chances of survival are even or better if treated, but certainly lethal if not.   Suppose that the patient has no great desire to live.   By the principle of double effect, the patient could choose to end his or her life simply by refusing the prescribed medical treatment.    The law usually provides that any patient may choose to refuse treatment for any reason, or for no reason at all.  The effect would be the same as that of a suicide, but it would not actually involve the direct action of ending one's own life.

# Controversy
Despite some apparent plausibility, the doctrine of double effect is controversial. Consequentialism|Consequentialists, in particular, reject the notion that two acts can differ in their permissibility, if both have exactly the same consequences.
Was Kevorkian found guilty simply because he said he euthanized patients and would he have been acquitted if he said he didn't but merely caused death due to double effect?  ManyTemplate:Who argue that the notion of double effect is just a semantic loop-hole around anti-euthanasia laws.
A major argument against the DDE is the hypothetical case where some evil must actively be done to bring about an enormous good. For example, suppose a nuclear bomb has been planted in a major city, and a person is held in custody who knows where it is, but who refuses to disclose the bomb's location. May the interrogators torture this person's family in front of his or her eyes, exploiting the family attachment to extract information and save millions of lives?
Even in such an extreme case, the DDE would not permit evil to be done prior to good consequences, whereas the consequentialist position states that the order of events is irrelevant. The argument against DDE thus becomes a question of how high must the stakes be before any evil is permissible for good ends, with the DDE position maintaining that evil is never permissible as an instigator to good ends.
In the past few years in the UK, at least two doctors undergoing murder trials for giving large doses of opioids to ill patients, have used the defence of double effect.