George Tarr

Flaws in the Doctrine of Double Effect


The purpose of this essay is to critically assess the doctrine of double effect as it appears in traditional just war theory. Its approach rests on an important premise: a rejection of the thesis that the ethics of war must begin at practical conduct and work backwards, using analytical rigour only as a disgraced informant. Rather, it holds that analytically sound ethical justifications are a pre-requisite for developing true and practically applicable principles of conduct. While the ‘fog of war’ has the power to obscure the distinction between right and wrong, it bears no capacity to undermine that distinction. Acknowledgement of the fog should manifest in the sympathetic treatment of soldiers who disobey ethical principles less wittingly or under duress, rather than in a revision of the principles themselves.

This essay begins with a brief description of the just war tradition and the doctrine of double effect. Having clarified the content of the doctrine, two central controversies are noted. Subsequently, several objections are outlined, the most decisive of which is that the doctrine is helplessly imprecise: In almost all cases most pertinent to war, an intended harm could be re-described as an unintended side-effect. Moreover, the doctrine is undermined by counteracting intuitions and a reliance on the cognitive states of agents, which is both excessive and misplaced.

Just War Tradition

The just war tradition is a set of guiding principles for the instigation and conduct of war. In accordance with jus ad bellum, the agent must satisfy the conditions of legitimate authority, just cause, right intention, public declaration, reasonable success, last resort, and proportionality of ends (Coleman, 2013: 67; Burke, 2015: 108). In accordance with jus in bello, the agent must satisfy the conditions of discrimination, proportionality, and military necessity.

Objections to the just war tradition pull in opposite directions. On one hand, the realist argues that the instigation and conduct of war is beyond moral reason. On the other hand, the pacifist argues that war stands under the jurisdiction of moral reason, but may never satisfy its demands, being always impermissible. The just war theorist holds that war is both subject to moral consideration and capable of being morally right. If this holds true, then war does not merely answer to the practical considerations of politics, but also demands ethical rigour in order to attain justification. Rather than being isolated from the ethical realm, war must cohere logically with a set of true ethical principles.

The Doctrine of Double Effect

The ethical coherence of war is at once threatened by three premises appearing together. As noted by Lee, the just war theorist seems committed to each of the following:

  1. Just war theory: at least some wars may be morally justified
  2. The Principle of Discrimination: a just war may not involve attacks on unarmed civilians
  3. Empirics: civilians are almost always, if not always, attacked during war (Lee, 2004: 234).

The doctrine of double effect is a component of just war tradition which seeks to reconcile these premises. It does so by pointing to the equivocal use of the verb “attack”, which has different referents in the second and third premises (Lee, 2004: 234). While war normally entails that citizens suffer harm, it does not entail that they are intentionally caused to suffer harm. The doctrine of double effect is designed, therefore, to account for the permissibility of seriously harmful actions in cases where harms are merely side-effects of an independent pursuit of good ends (McIntyre, 2004: 1). Originating in Thomas Aquinas’s ‘Summa Theologica’, it distinguishes between these harmful side-effects and harms which are caused as a means of promoting further ends.

According to the doctrine, “a person may licitly perform an action that he foresees will produce a good effect and a bad effect provided that four conditions are verified at one and the same time” (Mangan, 1949:43). The first condition is Aim: the action must pursue a good or neutral end. The second condition is Intent: It must be this good end, but not the harmful effect, which is intended. According to Bennett, “what you intend is determined by which of your beliefs explain or give reasons for your behaviour” (Bennett, 1980: 99). Nevertheless, the harmful effect may be foreseen: an effect which “the actor does not seek to bring about, but which she recognises to be inevitable or likely by-products of the action” (Lee, 2004b: 1). The third condition is Means: The harm must not be used as a means to the good end. One way to understand this condition is that the occurrence of the harm must not be necessary in order for the good effect to come about. Rather, it may only be a by-product, which, if it did not occur, would not itself prevent the achievement of the good end. The fourth condition is Proportionality: There must be “a proportionally grave reason for permitting the evil effect” (Mangan, 1949: 43). Mangan’s phrasing is useful in that it is made clear that consequentialist reasoning is not logically necessary to meet the fourth condition. Authors often ally this condition to the requisite that “agents strive to minimise foreseen harm” (McIntyre, 2004: 1).

Clarifications and Controversy

Some clarifications are in order. First, the doctrine neither presupposes nor proves that no harm can be caused intentionally as a means to an end. Rather, it proposes that there are cases where the same harm could not justifiably be caused intentionally as a means, but could be caused unintentionally but foreseeably as a side-effect. Second, as Bennett notes, this distinction between foreseeing and intending must neither be equated to, nor said to include, the moral distinction between allowing and causing (Bennett, 1980: 105). When the Enola Gay dropped a nuclear bomb on Hiroshima, even if it were true that civilian deaths were not intended by the bomber, it would be wildly unclear that such deaths were not caused by the bomber. The question would be left: what did cause the civilian deaths? Third, the locus of the controversy must be clarified. It is obvious that war involves many instances of harm which demand justification if war is to remain ethical. The controversy rather centres on:

  1. Whether these instances of harm, intended or not, require a justification beyond their promotion of better consequences;
  2. If they do, whether the doctrine of double effect is, in fact, a unified and true (McIntyre, 2004: 1) justification of all these instances of unintended harm.


A Narrative Approach

The mode of argumentation used by proponents of the doctrine make resolving these controversies particularly difficult. Rather than supporting the argument with a set of analytical premises, proponents take a narrative approach. Readers are entreated to reflect upon their intuitions in a series of comparative examples, these intuitions supposedly falling in favour of the doctrine. While this approach is accessible, and has the appearance of ready-made practicality, this paper’s first assessment is thus: There is often far too grave an uncertainty regarding whether or not a particular example is actually an instance of the doctrine in operation. This speaks to a lack of clarity in the doctrine, but more critically, it brings into question the ‘stark’ intuitions which the doctrine relies upon.

Two narratives are useful in prying open the doctrine. The first is the infamous trolley problem. A train is travelling rapidly towards ten innocent people tied to the track. In Case A, an agent may switch the track, in which case the train will hit just one innocent person. In Case B, an agent may push a fat man in front of the train, stopping it from hitting the ten people (Foot, 1967: 3). The doctrine here claims to explain why pushing the fat man is impermissible and switching the tracks is permissible. In the second story, more pertinent to just war, we have a ‘terror bomber’ who seeks to kill civilians and thereby procure some political impact through terror. In parallel, a strategic bomber seeks to destroy military targets, foreseeing the likelihood of civilian deaths, but not intending them (McIntyre, 2001: 220). The number of civilians killed is equal – or rather, from the perspective of the agents, the probable number of deaths foreseen is equal. The doctrine claims to provide an explanation for why the actions of the strategic bomber, but not the terror bomber, are permissible.

The challenges of this purely narrative approach are further revealed in the complexity of the underlying differences here. As Bennett observes, we can supposedly distinguish between the terror and strategic bomber by asking them the question: “If you had believed that there would be no civilian deaths, would you have been less likely to go through with the raid?” (Bennett, 1980: 100). Yet, answers would only differ if the question were interpreted very specifically: namely that eliminating the civilian deaths would affect “what causally follows downstream” but not upstream (Bennett, 1980: 102). In other words, the strategic bomber would still destroy the building, but the terror bomber would not produce any terror. If both would still otherwise achieve their ends, both would respond “no” to the question. The focus is very specific, tenuous, and in large-part unexplained.


Having assessed its narrative approach, it is now necessary to assess the doctrine by the standards of its primary defendant: intuition[1]. The doctrine has the support of many intuitions. Even if we do not think that using bad means to good ends is always wrong, there is an intuition that in otherwise equivalent cases, using something bad as a means is worse than an equivalent bad occurring as a side-effect: It is not that the events differ in badness, but that the acts behind them seemingly differ in moral wrongness. Moreover, there is a strong intuition supporting the moral significance of intent. If a teenager plays a game which ends in his little brother being crushed under a heavy wardrobe, we might begin by saying ‘he didn’t mean to’, by way of defence. Indeed, mens rea, or establishing intent, is essential for legal prosecution (Kamm, 2000: 45).

However, there is also an intuition that foresight undermines some, or perhaps all, of this sympathy. If it appears that the teenager thought his brother’s being crushed was quite probable, we accuse him swiftly. Anthony Kenny provides an example. Someone who drinks heavily foreseeing a hangover is unlikely to have intended it. Nevertheless, if he drinks heavily the night before he flies a jet, and the jet is crashed, he is considered culpable (Keown, 1997: 37). In other words, there is a competing intuition that foreseeability might do all the work.  Moreover, there is an intuition that a lack of intent does not really matter, if the actor would still go back in time and do the same thing. As Bennett writes: “Although perhaps [the strategic bomber] does not intend the deaths in themselves, nevertheless he rather wishes that the civilians die than that the raid be called off” (Bennett, 1980). Intuitions suggest that whatever about our internal state is morally significant, the willingness or unwillingness to cause harm may account for much of it.  These examples illustrate that while intuition can be in favour of the doctrine, it can also go against it. This web of ‘initial’ intuitions provides markers, but is not the ultimate determinant of the theory’s success.


The most decisive objection to the doctrine is an analytical one: It seems seriously difficult to distinguish harm as a means to an end and harm as a side-effect of acting toward an end. Let us compare the agent who elects to push the fat man with the agent who diverts the train. The end in both cases is identical: to stop the train from progressing along a track towards 10 people. In neither case, however, is the death of an innocent person strictly necessary for this good end. In both cases, the harm itself could, under some description, be understood as a side-effect. The fat man need not die in order to obstruct the trolley; he simply needs to obstruct the trolley with his body, an act of which his death is a probable or inevitable consequence. Foot uses the example of a team of cave explorers who blow up a tunnel blocked by a diver in order to escape a cave (Foot, 1967: 2). Is the diver’s death an unintended side-effect? It is simply not clear. Fried would argue in response that intending that someone is hit by a train simply is intending to harm him. However, as Bennett rightly answers, this “is” does not denote conceptual or essential involvement (Bennett, 1980: 108). It is contingent. Moreover, even if “the fat man being hit by a train” and “the fat man dying” were semantically equivalent – which they are not – this would not be sufficient to conclude that if one was intended, so too was the other. If an actor simply did not perceive the two as semantically equivalent, he could easily intend only one of them.

Quinn’s formulation of ‘direct’ and ‘indirect’ agency attempts to resolve this problem, referring to the ‘involvement’ rather than the harm of other agents. More specifically, he refers to “deliberately involving [the victims] in something in order to further his purpose precisely by way of their being so involved” (Quinn, 1989: 4). However, we do not ordinarily think it impermissible to involve any person in order to achieve a given good end. What we cannot do is harm them as a means to an end: And the component of the person’s involvement which constitutes the ‘harm’ is often not the component which is necessary in order for the end to be achieved.

Foot calls the problem one of ‘closeness’, although this seems somewhat untidy; there is no clear understanding about the content of ‘closeness’. The word grasps at the fact that the doctrine of double effect suggests the existence of a line, delineating the justified from the unjustified, but does not in fact indicate where this line is. This seems an indisputable blow: the doctrine does not do as it promises. Given that cases similar to that of the diver are common in war, the doctrine seems to lose its legs.

Interestingly, this objection analytically deconstructs the practical applicability of the doctrine: It logically unties it to the point of sophistry, but with the consequence of highlighting how imprecise and therefore impractical the doctrine is. Unless one already had the principle of discrimination in mind, distinguishing the features of an act which are intended as a means and those which are mere side-effects seems an impossible task.


I consider this problem of specificity a serious blow to the doctrine of double effect, particularly with respect to its role in the just war tradition. Nevertheless, I also add one concern with the doctrine’s agent centrality. In particular, the doctrine seems to rely too exclusively on the agent’s cognition, and more seriously, on the wrong aspects of the agent’s cognition. In the trolley problem, it is often assumed that in switching tracks, one does not intend to kill the man tied to the track. However, it seems entirely possible that the agent might fail to conceive of his action in this way. He may think “I am instead going to turn and kill this tied up man” and the way in which he conceives of it may be no different to how he would process pushing the fat man. Quite simply, not every soldier or train conductor is a philosopher, and internally, the relationship of the driver’s faculty of intent to each of these two acts may well be cognitively identical. The observer can distinguish the acts only because the observer can see that the fat man is used as a means in some way that the tied man is not. The agent may not be aware of this distinction in a way which causes a cognitively different attitude in each case. This is particularly likely given the previous objection, where this distinction is shown to be imprecise and ambiguous. If a driver did possess identical cognitive states in Case A and in Case B, however, this would not seem to undo or explain the perceived moral distinction between the cases. This calls the doctrine into further question.


This essay assesses the doctrine of double effect as it appears in the just war tradition, using the analytical ethical approach favoured by revisionists. In assessing the doctrine of double effect, the stakes seem very high. If we are in the business of punishing crimes, for example, we are likely to unintentionally but foreseeably punish at least some innocent people. If what the doctrine sets out to do is not achieved at all, many central features of state and society would become unjustifiable, to paralysing effect. This essay concludes that the doctrine is philosophically unsound, failing to do as it proposes, and being ultimately imprecise and non-prescriptive. Moreover, it is counterbalanced by multiple intuitions, and pays unjustified and excessive attention to the wrong aspects of agents’ cognitive states. These objections appear to defeat the doctrine. However, there remains the question of an alternative. The options seem clear: Either, as the consequentialist believes, there is no difference at all between “throwing a trolley at a person and throwing a person at a trolley” (Harris, 2002) if both result in the same consequences; or, the doctrine of double effect is right to note a difference, but faultily explains the difference by reference to a unified principle centred on intent, instead of an amalgamation of different, cross-cutting moral factors.


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Coleman, S. (2012). Military ethics: An introduction with case studies. Oxford University Press.

Foot, P. (1967). The problem of abortion and the doctrine of double effect. Oxford Review, 5, 5-15.

Harris, J. (2000). The moral difference between throwing a person at a trolley and throwing a trolley at a person: A reply to Frances Kamm. Proceedings of the Aristotelian Society, 2000.

Huemer, M. (2007). Ethical Intuitionism. Springer.

Kamm, F. M. (2000, July). The Doctrine of Triple Effect and Why a Rational Agent Need not Intend the Means to His End. In Aristotelian Society Supplementary Volume (Vol. 74, No. 1, pp. 21-39). The Aristotelian Society.

Keown, J. (1997). Euthanasia examined: ethical, clinical and legal perspectives. Cambridge University Press.

Lee, S. (2004). Double effect, double intention, and asymmetric warfare. Journal of Military Ethics3(3), 233-251.

Mangan, J. T. (1949). An historical analysis of the principle of double effect. Theological Studies10(1), 41-61.

McIntyre, A. (2001). Doing away with double effect. Ethics111(2), 219-255.

McIntyre, A. (2004). Doctrine of Double Effect. Stanford Encyclopaedia of Philosophy. Accessed 30 Dec, 2018:

Quinn, W. S. (1989). Actions, intentions, and consequences: The doctrine of doing and allowing. The Philosophical Review98(3), 287-312.

[1] Given that moral properties are likely non-natural, it seems our moral intuitions are either frequently correct or our capacity to obtain any moral truth is seriously undermined (Huemer, 2005). As such, satisfying intuition is an important component of ethical theorising, not to be confused with ‘instinct’ or lack of reason.

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