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.
 The act itself must be MORALLY GOOD or at least indifferent.
 The agent MAY NOT positively WILL THE BAD EFFECT but may merely 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.
 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.
 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 care and prudence proportionate to the importance of the case. Thus, an effect that benefits or harms society generally has more weight than one that affects only an individual; an effect sure to occur deserves greater consideration than one that is only probable; 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. A person is never allowed to perform a morally bad action. Nor may one ever positively will an evil effect of an action, even though the act would otherwise be lawful. Thus, a censor of books, who is allowed to read obscene literature, may not take deliberate pleasure in the evil thoughts arising in consequence, though he necessarily permits them to enter his mind. 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 nonviable fetus 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 the fetus (the bad effect), which would probably not survive even if the operation were not performed.
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. The relief to the woman would come as an effect of the abortion, not directly as an effect of the surgery. 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 ch. 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. Seeing 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 told in the inspired book as something commendable.
Historical Development. The discussion of the principle in its technical details is comparatively recent in Catholic theology. St. Thomas may have visualized it when he argued in the Summa theologiae (2a2ae, 64.7) that a person may kill an unjust aggressor when this is necessary to save his own life. One difficulty in the application of the principle to this case, however, is that it seems to lack the third condition, since the preservation of one's own life seems to follow from the killing of the aggressor. Hence, others would decide that in this case God gives permission to the victim to protect himself, if necessary, by a direct slaying of the unjust assailant.
The principle of the double effect was developed by the theologians of the 16th and 17th centuries, especially by the Salmanticenses. However, the greatest credit in modern times for the thorough exposition of this principle as a norm applicable to the whole field of moral theology is owed to the Jesuit theologian Jean Pierre Gury.
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 all these are 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. However, company keeping involving the proximate danger of sin is wrong for a couple who have no intention of marrying, at least within a reasonable period, since there is no adequate reason to compensate for the moral danger.
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.
New Catholic Encyclopedia Volume 4, 1967
Bibliography: THOMAS AQUINAS, ST 2a2ae, 64.7. Salamanca Collegium Fratrum Discalceatorum . . ., Cursus theologicus . . ., v.7 (Paris 1877) 21 1 213. J. P. GURY, Compendium theologiae moralis (5th ed. New York 1874), tract. 1, ch. 2, nn. 6 10. Merkelbach SumThMor 1:173 177. 0. LOTTIN, Morale fondamentale (Tournai 1954) 265 268. F. J. CONNELL, Outlines of Moral Theology (2d ed. Milwaukee 1964) 22 24. J. MANGAN, "An Historical Analysis of the Principle of the Double Effect," ThSt 10 (1949) 40-61. [F. J. CONNELL)
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