The Principle of Double Effect

3. Four Key Objections

Book cover: Summa Theologica by Thomas Aquinas

Opponents of absolute moral rules claim four major problems for this kind of ethic. These difficulties are:

  1. By not distinguishing between voluntary acts or refusals and their foreseen consequences, absolutism either leads to self-contradiction or to a utilitarian type calculus.
  2. By not distinguishing between voluntary acts or refusals and their foreseen consequences, in some cases, absolutism would morally condemn whatever the agent does.
  3. By not recognising that some consequences of voluntary actions are foreseen but unintended, absolutism misses the core of morality.
  4. By not recognising that some consequences of voluntary actions are foreseen but unintended, absolutism is unnecessarily harsh in some of its judgements.

Advocates of PDE claim to have resolved these difficulties for an absolutist ethic. In the following sections, I will argue that proponents of the principle have not satisfactorily resolved problems 1, 2 and 3 and do not go far enough in resolving 4. I will deal with each problem and attempted solution in order.

3.1 Problem 1: Self-contradiction or Utilitarian Calculus

By not distinguishing between voluntary acts or refusals and their foreseen consequences, absolutism either leads to self-contradiction or to a utilitarian type calculus.

An absolutist ethic does not distinguish between a voluntary action or refusal and the foreseen consequences of this action or refusal, and deems the actor or refuser equally responsible for both. The problem with this ethic, it is claimed, is that it leads to conflicts between absolute rules and conflicts between applications of the same rule. For example, consider an absolutist ethic in which there are absolute prohibitions against intentionally killing an innocent person and stealing another person's possessions. Furthermore, suppose that I am under threat such that if I do not steal person X's car, person Y will be killed. If I am responsible for the consequences of what I do and do not do, then the prohibition against theft would require me to 'murder' Y and the prohibition against murder would require me to steal X's car.

Likewise, if I am under threat such that if I do not kill innocent person Z then person Y will be killed, applying the prohibition against murder to Z would require me to 'murder' Y. Applying the prohibition to Y would require me to murder Z. Here we have a conflict between applications of the same rule.

A method of resolving such conflicts is to allow for the weighing up of the merits and demerits of the alternatives; in these two cases, to judge whether stealing X's car and murdering Z is better or worse than 'murdering' Y. But this looks suspiciously like a form of utilitarianism under an absolutist veneer.

The proponent of the Principle of Double Effect is certainly correct in pointing out the linguistic fallacy in describing the person, in the above example, who does not give in to the blackmailer's demands as a 'murderer'. If this person cannot be Y's murderer, then he has not infringed upon the absolute principle, 'Do not intentionally kill an innocent person', if he refuses the blackmailer's demands and allows Y to die.[2]

The application of PDE has resolved the conflicts in these two cases, but can it do so in all cases? I do not think so. Consider a system of absolute principles that includes amongst its precepts, 'Do not intentionally break a promise' and 'Do not intentionally kill an innocent person'. Suppose that a visitor to a foreign country receives a letter from his closest friend telling him that the military has seized power in his home country and asking him to join a newly formed band of terrorists whose aim is to win victory by bombing the private homes of the military leaders, killing their wives and children. The visitor writes back to his friend, earnestly promising to join them in their mission to bomb civilians as soon as he possibly can. However, before arriving home and after much deliberation, he realizes that to intentionally kill innocent persons is absolutely wrong. A contradiction now arises in that to intentionally break his solemn promise to kill innocent family members is also absolutely wrong.

Book cover: Utilitarianism: For and Against by J. J. C. Smart and B. Williams

To generate such conflicts between rules is not difficult. One need only think of cases in which what is promised is the object of an absolute prohibition. This problem may be avoided by modifying the precept to read, 'Do not intentionally break a promise unless not to do so is to infringe upon another absolute rule'. It is doubtful whether this revised version can be properly regarded as an absolute rule because of its provision for exceptions. However, disregarding this problem, this move is of no help in cases in which two promises have been made, but through the unfortunate interplay of later circumstances, both cannot be kept, and neither of which has as its object what is absolutely prohibited. For example, I promise to attend my best friend's wedding. A week later, I receive a note in the post advising that the time of the wedding has been rescheduled for the same afternoon in which I was to have kept my promise to my partner to take her to the movies.

Let us consider a set of absolute rules that do not include an absolute prohibition against breaking promises. Take, for example, the Jewish Decalogue (Exod. 20; Deut. 5). It is not too difficult to imagine a conflict between the fifth commandment, 'Honour your father and mother', and the sixth commandment, 'Do not intentionally kill an innocent person'.[3] Suppose that a young medical scientist is working in Germany during the Second World War. His father, who heads the research team of which he is a member, instructs him to investigate the time it takes for a particular toxic chemical to kill thirty Jews in an enclosed room. Any refusal on the part of the son to comply with his father's orders would be seen by his father, who is a Nazi sympathizer, as a mark of disrespect. The son must make a choice between dishonouring his father and killing a number of innocent Jews. A similar conflict will arise between the fourth commandment, 'Keep the Sabbath', and the fifth commandment, for a son whose father orders him to work on the Sabbath.

These conflicts can neither be avoided by ranking the rules in order of strictness nor by weighing the consequences of each act. Doing so would leave the advocate of the Principle of Double Effect open to the charge of pseudo-absolutism, a criticism that they had previously sought to avoid. It seems that PDE does not provide a logical guarantee against conflicts between rules. It may be possible to so restrict the number and type of absolute rules such that it is empirically impossible for a conflict to occur. However, I find it doubtful that such a system, in which the great bulk of morality is signified by a weighing of consequences and non-absolute rules, can be called absolutist.


  1. [2] See Anscombe [1970: 50f] and Mackie [1977: 161f].
  2. [3] For the sixth commandment, there are difficulties in translating the Hebrew text. The translation that I have given here is the most difficult one from which to generate conflicts.

Copyright © 2015

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