NOTE:  The principle of double effect is also treated in LGL pp. 182-185.  The following is taken and/or adapted from  Copyright © 1993-2000 by Father David C. Trosch - All Rights Reserved.  Permissions granted for non-profit purposes.

Principle of Double Effect:  Definition.  The principle of double effect is a rule of conduct frequently used in moral theology to determine when a person may lawfully perform an action from which two effects will follow, one bad, and the other good.

Conditions. Theologians commonly teach that four conditions must be verified in order that a person may legitimately perform such an act.

  1. The act itself must be morally good or at least indifferent.
  2. The good effect must be intended, the bad effect only tolerated.  That is, the agent may not positively will the bad effect but only permits 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 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. There must be a sufficient or proportionate reason to allow the evil effect.  That is, the good effect must be sufficiently desirable to compensate for the allowing of the bad effect. In forming this decision many factors must be weighed and compared with a care and prudence proportionate to the importance of the case.


    a)  Thus, an effect that benefits or harms society generally has more weight than one that affects only an individual;

    b)  An effect sure to occur deserves greater consideration than one that is only probable;

    c)  An effect of a moral nature has greater importance than one that deals only with material things.

Of these four conditions the first two are general rules of morality. With respect to the first, a person is never allowed to perform a morally bad action. With respect to the second, one may never positively will an evil effect of an action, even though the act would otherwise be lawful.  Thus a doctor who is required to use painful means to cure the ailment of a child may not directly will to cause the pain or take pleasure in causing it.  The third and fourth conditions enumerated above pertain specifically to the principle of the double effect.

Typical Situations. Situations calling for the application of this principle occur frequently in connection with pregnancy. Thus, a pregnant woman bearing a child is found to have a cancerous womb that will cause her death if it is not excised as soon as possible. The operation of hysterectomy is morally lawful, for this operation is permissible in itself as a normal means of saving the woman's life. She does not positively will the death of her child, but permits it as an unavoidable evil. Both the benefit to her health and the death of the child follow from the surgery with equal directness or immediacy in the order of causality, though the death of the child is prior in the order of time. The woman's chance of restoration to health (the good effect) is sufficiently desirable to compensate for the death of her child (the bad effect), which would probably not survive even if the operation were not performed.   

In this case, if there were cause to think that the child could survive if the woman refused the hysterectomy – or rather decided to postpone it until after the birth of her child, it is morally permissible for her to do so even though it is medically certain that she would die from the cancer as a result.  That is, the woman may have the hysterectomy but is not morally obliged to do so.  

However, if the woman is suffering from kidney disease, heart trouble, or tuberculosis, which would be easier to care for if she were relieved of the pregnancy, it would be immoral to perform an abortion. For in such a case the third condition for the proper use of the principle of the double effect would be lacking as the good effect would flow directly from the bad effect – indeed a direct act of killing.  That is, the relief to the woman would come as an effect of the abortion, not directly as an effect of a curative the surgery as in the preceding example. Hence, a bad means would be employed to produce a good end.

Even if the woman's life would be gravely endangered unless an abortion were performed (a situation rarely verified in view of modern medical progress), it would be a grave violation of God's law to kill directly an innocent child to save her life.

The Old Testament (I Maccabees 6) contains a striking example of a lawful application of the principle of the double effect. Eleazar, a Jew, was fighting in the army of the Maccabees against an enemy force. When he saw an elephant on which he believed the king of the opposing side was riding to battle, Eleazar ran under the beast and slew it, knowing that he himself would he crushed, but hoping that he would thus kill or disable the leader of the enemy. This narrative is presented in the Scriptures as something commendable. 

Applications. Actually, the principle of the double effect is often used in the ordinary affairs of human life by persons who are unaware of the speculative requirements of this principle but are acting on common sense. Thus, the aviator who tests planes in order to improve aeronautic equipment, the doctor who treats patients affected with contagious diseases, the policeman who attempts to capture an armed criminal are all lawfully using the principle of the double effect, the bad effect being the hazards they are incurring to their own life or health, the good effect being the benefits they are conferring on society.

Not only physical evils, such as danger to life and bodily welfare, but also spiritual evils, such as the occasions of sin, can enter into the application of the principle of the double effect. Thus, a young couple soon to be married may be in the proximate occasion of sins of impurity when they are together. Nevertheless, through a reasonable application of the principle of the double effect, such association may be permitted, the good effect, which consists in the normal benefits of lawful courtship with a view to marriage in the near future, compensating for the spiritual danger. Of course, they must take suitable measures to avoid sin.

Passive scandal can also be justified at times by the use of the principle of the double effect. This means that for a sufficiently good reason one may perform a lawful action even though another person makes this action the occasion of sin. Thus, a priest is justified in visiting the sick in a hospital even though a man residing nearby, on seeing the priest, becomes enraged and blasphemes.

In modern warfare the principle of the double effect is frequently applicable. Thus, in waging a just war a nation may launch an air attack on an important military objective of the enemy even though a comparatively small number of noncombatants are killed. This evil effect can be compensated for by the great benefit gained through the destruction of the target. This would not be true if the number of noncombatants slain in the attack were out of proportion to the benefits gained, as is clear from the fourth condition explained above. Furthermore, if the direct purpose of the attack were to kill a large number of noncombatants so that the morale of the enemy would be broken down and they would sue for peace, the attack would be sinful because the third condition for the lawful use of the principle would not be fulfilled. It would be a case of the use of a bad means to obtain a good end.  Nor can non-combatants ever be directly targeted. 

Taken and/or adapted from  Copyright © 1993-2000 by Father David C. Trosch - All Rights Reserved Permissions granted for non-profit purposes.

NOTE:  The principle of double effect is also treated in LGL pp. 182-185. 



I.  HEALTH CARE, Death and Dying – General Norms.  Two principles are balanced in the following stipulations: the right to life and the right to die with dignity. 

A.  It never morally permissible to take the life of a sick person (innocent life may never be attacked).

B.  It is permissible to allow the imminent death of person (death is inevitable).

C.  Extraordinary means are never required (extraordinary means as defined by the Church teaching and moral norms, not the medical profession for which some “extraordinary means” are used every day. 

D.  In all cases the ordinary means of sustaining life are always required.  Ordinary means are

1.  Water

2.  Food

3.  Basic medicines.

E.  Painkillers used to make a patient comfortable are permissible even if they may have the effect of shortening the person’s life.1

II.  HEALTH CARE, death and dying – principles which pertain to the terminally ill:

A.  With the consent of the patient, it is permissible to make use of the most advance means available even if they are still in experimental stages and not without some element of risk. 

B.  It is also permissible to interrupt or discontinue such means if the results are disappointing either because the suffering the means inflict outweighs their benefits or because the costs (instruments and personnel) is disproportionate to the foreseeable  results. 

C.  No one is obliged to have recourse to a type of remedy which, although commonly used, is too onerous or has dangers attached. 

1.  This refusal is not the equivalent of suicide. 

2.  Rather is might indicate an acceptance of death, or to avoid things named in B above. 

III.  HEALTH CARE - discontinuing medical treatment in the case of terminal illness. 

A.  The right to life in the case of the terminally ill is defined as the right to die in serenity with human and Christian dignity.  This is a right to experience dying in a human and Christian way – not a right to kill oneself or to be killed by another. 

B.  When death is imminent it is legitimate to refuse treatments that would only secure a precarious and painful prolongation of life while continuing the normal treatment due a person in the circumstances.  The administration of food and liquids, even if artificially, is part of normal treatment always due a person when these are not burdensome

C.  The decision to discontinue medical treatment that is burdensome, dangerous, extraordinary, or disproportionate to the expected outcome can be legitimate – it is the discontinuation of “over-zealous” treatment.  The decision should be made by the patient if he is competent or, if he is not, by those legally entitled to act for the patient.2

1.  Heath-care proxy

2.  Living will. 









1 Adapted from:  Aurelio Fernández and James Socias, Our Moral Life in Christ: a Basic Course on Moral Theology (Princeton: Scepter Publisher, Inc., 1997)  254.

2Adapted from:  Aurelio Fernández and James Socias, Our Moral Life in Christ: a Basic Course on Moral Theology (Princeton: Scepter Publisher, Inc., 1997) 270-71.