Principle of double effect

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The principle of double effect is 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 otherwise be obliged to avoid (sedation and a slightly shortened life). The first known example of double-effect reasoning is Thomas Aquinas' treatment of homicidal self-defense, in his work Summa Theologica.[1]



Intentional harm versus side effects[edit]

The principle of double effect is based on the idea that there is a morally relevant difference between an "intended" consequence of an act and one that is foreseen by the actor but not calculated to achieve his motive. So, for example, the principle is invoked to consider the 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 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.


  1. The nature-of-the-act condition. The action, apart from the foreseen evil, must be either morally good or indifferent.
  2. The means-end condition. The bad effect must not be the means by which one achieves the good effect. Good ends do not justify evil means.[3]
  3. 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. All reasonable measures to avoid or mitigate the bad effect must be taken.
  4. The proportionality condition. There must be a proportionately grave reason for permitting the evil effect.




In cases of terminally ill patients who would hasten their deaths because of unbearable pain, or whose caregivers would do so for them (euthanasia, medical aid in dying, 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 to relieve the pain suffered as it is considered harmful to the patient. The U.S. Supreme Court has voiced support for this principle in its deliberations over the constitutionality of medical aid in dying.[7]


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 will show that almost all good behavior proceeds from questionable intentions. Therefore, Mill argues, our moral analysis should ignore matters of motivation, and so we should reject DDE, which appeals to a distinction between intended and unintended consequences.[8] Mill further claims that scrutiny of motives will reveal a man's character, but utilitarianism does not judge character, only the rightness or wrongness of actions.

See also[edit]


  1. ^ Summa Theologiae, IIa-IIae Q. 64, art. 7
  2. ^ T. A. Cavanaugh, Double-Effect Reasoning: Doing Good and Avoiding Evil, p.36, Oxford: Clarendon Press
  3. ^ Mark Timmons, Moral Theory: An Introduction (Rowman & Littlefield 2003 ISBN 978-0-8476-9768-7), p. 80
  4. ^ McIntyre, Alison. "Doctrine of Double Effect". In Edward N. Zalta (ed.). Stanford Encyclopedia of Philosophy (Summer 2006 ed.). Retrieved 2007-08-18.
  5. ^ "Principle of Double Effect". Catholics United for the Faith. 2003. Archived from the original on 2007-09-27. Retrieved 2007-08-18. Cite uses deprecated parameter |deadurl= (help)
  6. ^ Delgado, George. "Pro-Life Open Forum, Apr 10, 2013 (49min40s)". Catholic answers. Archived from the original on 2 April 2015. Retrieved 2 September 2014. Cite uses deprecated parameter |deadurl= (help)
  7. ^ 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.
  8. ^ John Stuart Mill, Utilitarianism (London: Parker, Son and Bourn, 1863), page 26.

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