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Clarifying the Principle of Double Effect Through Anscombian Action Theory

It is often difficult to determine the goodness of an action, even more so when the action might have good as well as bad effects. While it has been disputed for numerous reasons, the Principle of Double Effect (hereafter abbreviated PDE) is widely accepted as an effective tool that can assist an agent’s moral reasoning. Rather than being substantive in itself, the PDE is a human tool for evaluating and making clear of certain complex moral situations, specifically by aiding the agent in determining when he may perform an action from which two effects follow, one being an unintended bad effect, the other an intended good effect. The PDE states that an agent may never directly and voluntarily intend evil, whether as an end or as a means to an end; evil may however be allowed as a side-effect of a morally good action under certain specific conditions.

     This origin of the PDE is found in Aquinas’ Summa Theologiae[1] where he discusses how an agent may properly act in cases of self-defense.[2][3] The modern interpretation of the PDE, which will be used in this essay, consists of the following four conditions; an action with both a good and a bad effect must meet all four conditions for it to fall under the scope of the PDE:[4]

   1. The act itself must be morally good, or at least indifferent.

   2. The agent may not positively will the bad effect but may permit it. If he
    could attain the good effect without the bad effect he should do so. The bad
    effect is sometimes said to be indirectly voluntary.

   3. The good effect must flow from the action at least as immediately (in the
    order of causality, though not necessarily in the order of time) as the bad
    effect. In other words the good effect must be produced directly by the
    action, not by the bad effect. Otherwise the agent would be using a bad
    means to a good end, which is never allowed.

   4. The good effect must be sufficiently desirable to compensate for the
    allowing of the bad effect.

The PDE applies to a large range of cases, from common, seemingly morally insignificant, acts such as taking medication with negative side-effects, to more serious issues such as self-defense, abortion, and the formulation of Just War Theory. Though the PDE has helped evaluate many complex moral situations, it also lends itself to significant disputes over when it may be applied, how one is to judge circumstances, and how to accurately interpret Aquinas’ complex presentation of the principle.

     Because of these concerns there is need for clarification of the PDE. Philosopher Gareth B. Matthews says that, “Not everyone finds the Principle of Double Effect plausible. Although it does strike some medical decision-makers with the force of revelation, it seems to others little more than sophistry. Precisely this disparity makes it an especially important principle to subject to careful examination and reflection.”[5] Many philosophers and theologians have attempted to clarify the PDE, either by giving numerous examples but never isolating a rule or rules, or by reducing the principle in an attempt to simplify it. Another approach is to offer effective and simple tools to aid the agent in discerning whether his action falls under the scope of the PDE. While Anscombe’s action theory offers several of these methods and tools such as an in-depth account of what constitutes moral action, how to determine the intention with which the action was done, how the practical syllogism aids one in determining the morality of their action, and how intention is related to the order of causality, one of the most effective tools is the moral action description(s) of one’s action.

     An action description is a tool for explaining specifically what an action consists of. It should mention circumstances and intention, and should imply some goodness or badness. All human actions have multiple action descriptions. For example, if someone is driving his car the action could be described “he turned the wheel and hit the gas,” “he drove in order to get to the store,” or any number of other ways. Note that “indifferent” action descriptions are incomplete because human action, by definition, cannot be indifferent. An example of this is the indifferent action description “throwing pebbles into a lake”; while this describes a human action, it does not imply any moral goodness or badness or intention, and so it is incomplete in that way. Thus while this kind of description may be useful, it may also be misleading. Moral action descriptions, on the other hand, suggest some specific goodness or badness that falls under the action, and they also include (foreseen) circumstances that may change the object of the act as well as the agent’s intention. For example, an observer witnessing a man shooting another man might say that that action is an evil, murderous action. If one looks at the circumstances and intention of the agent, however, he might see that the moral action description is, “I shot the man to protect my life because he was attacking me with a knife.” Under this moral action description the action would be considered self-defense and would not be an evil act, whereas if the action description were, “I shot the man to get revenge because he killed my brother,” it would be an evil, murderous action. Thus, though an action description can “change” the character of the act, it does not change the action itself. It is only a tool for clarifying the morality of an action.

     Problems may arise, however, in determining which action description the action should be evaluated under to see whether it falls under the PDE. The answer is that the most accurate description will be a moral action description which mentions the intention with which, which is the agent’s relative highest reason for doing some specific action. To determine the correct moral action description, it is helpful to draw out the practical syllogism behind one’s action; for the moral action description consists of a combination of the first premise and the conclusion of the practical syllogism. Take the following practical syllogism, for example:

   1. I want to protect my own life from danger.

   2. This aggressor is putting my life in grave danger.

   3. Slaughtering this aggressor will stop him.

   4. Stopping him will keep him from putting me in danger.

   5. Therefore, I should slaughter him in order to stop him.

Here the most prime moral action description would be, “I slaughtered the man in order to stop him and so protect my life from danger.” While there are still other correct moral action descriptions, the one that mentions the intention with which will be the most complete and accurate for determining the morality of an action.

     As we have seen with the case of the man shooting another man in self-defense, a moral action description can change the perceived morality of an action. This is important in cases where an action may be both good and bad, meaning “good only in a certain respect, and bad in others.”[6] In rare cases, bad actions can be justified as good under some moral action descriptions. Anscombe gives the example of an amputation: cutting off a human limb is generally an evil act, but when it is chosen as a “lesser evil for that human being’s physical condition or integrity”, which is chosen for the sake of the greater good of the person’s health or integrity, this supposed evil action is inflicted on the person for the sake of a more valuable good and so becomes a good action under that action description.[7] This is precisely what occurs with actions that fall under the PDE: an evil is allowed when it falls under a moral action description that justifies the evil due to some greater good; the evil is still restricted, however, in that it must be an unintended side-effect.

[1] Respondeo, II-II, q.64, a.7.

[2] While it is generally assumed that the PDE is taken from Summa Theologiae, II-II, q. 64, a. 7, pieces of it can also be found in Aquinas’s De Malo 1, 3, ad 15.

[3] The origin of the modern presentation of the PDE as given in this paper is widely disputed. Anscombe herself attributed the origin to Aquinas I-II, q.20, a.5 (Human Life, Action and Ethics, 225-226). Others say that the origins of the PDE can be found in Aristotle (see: Moral Psychology and Human Action in Aristotle by Michael Pakaluk and Giles Pearson). Some scholars also claim that this ‘modern interpretation’ of the PDE is not equivalent to that found in Aquinas q. 64. For more on this see Saint Thomas and the Principle of Double Effect by Gareth B. Matthews.

[4] F.S., Connell. New Catholic Encyclopedia. Edited by Beard L. Marthaler. 2nd ed. Farmington Hills, MI: Catholic University of America Press, 2003. 880.

[5] Mcdonald, Scott, and Eleonore Stump. Aquinas’s Moral Theory. Ithaca, New York: Cornell University Press, 1999. 63.

[6] Anscombe, G. E. M. Human Life, Action and Ethics. Edited by Mary Geach and Luke Gormally. Charlottesville, VA: Imprint Academic Philosophy Documentation Center, 2005. 212.

[7] Anscombe, G. E. M. Human Life, Action and Ethics. Edited by Mary Geach and Luke Gormally. Charlottesville, VA: Imprint Academic Philosophy Documentation Center, 2005. 218.



  1. Aquinas, Thomas. Summa Theologiae., accessed April 30th, 2015.
  2. Aquinas, Thomas. Summa Theologiae., accessed Feb. 12th, 2015.
  3. Anscombe, G.E.M. Intention. Cambridge: First Harvard University Press, 2000.
  4. Anscombe, G.E.M. Human Life, Action and Ethics, edited by Mary Greach and Luke Gormally. Charlottesville, VA: Imprint Academic Philosophy Documentation Center, 2005.
  5. Boyle, Joseph M. Jr. The Thomist. Volume 42, Number 4, 649-65. “Praeter Intentionem” in Aquinas. Accessed April 21st, 2015.
  6. Connell, F.S. “Double Effect, Principle of” The New Catholic Encyclopedia, edited by Beard L. Marthaler. Farmington Hills, MI: The Catholic University of America Press, 2003. 2nd edition.
  7. Mcdonald, Scott and Eleonore Stump. Aquinas’s Moral Theory. Ithaca, New York: Cornell University Press, 1999.
  8. McIntyre, Alison. “Doctrine of Double Effect”, The Stanford Encyclopedia of Philosophy (Winter 2014 Edition), edited by Edward N. Zalta,, accessed Feb. 12th, 2015.