Again, let us set the same condition for Peter, Paul, and Thomas; but Peter is not able to cut himself loose. John, a fourth person, can cut Peter loose and save him, but can do no more; he must let Paul go down with the corpse of Thomas. May John cut Peter loose? Certainly he may, on the principle quod liceat per se licet per alium. This is another case of double effect, with the extenuating circumstances as above.
Suppose, however, Peter represents a living infant in the womb of Ann, and that she is in labor; further, this infant cannot be delivered owing to the contraction of Ann's pelvis. May John, a physician, cut away Peter by craniotomy and so save Ann's life? Certainly he may not. John here directly brains Peter to save Ann, although Peter is not an unjust aggressor; he does a murder to get a good effect, and the end does not justify the means. There are two effects, but the good effect follows from the bad one, and not immediately from the causal act.
Take another example: Peter is a swimmer disabled by cramps and about to drown; Paul, going to save Peter, is seized by Peter, and both are now in danger of drowning; John goes to help Peter and Paul. He cannot get Peter's grip loose by ordinary means, and he sees he can save only one man, either Peter or Paul. May John knock Peter senseless to loosen his grip from Paul, bring in Paul, and thus leave Peter to drown? Certainly he may. You have the double effect here also. Moreover, Peter is a materially unjust aggressor; he is like a maniac trying to kill Paul. In the craniotomy case the child is not a materially or formally unjust aggressor, it is not doing anything at all. It is where the mother put it, and it has a full right to its position and its life.
John most probably might also knock Paul senseless and save Peter, if through affection or similar motive he preferred this course. He would then be justified by the double-effect principle alone, although Paul is in no sense an aggressor. The intention of the blow would have to be solely to loosen Paul's hold.
In a just war a commander may shell an enemy's works and indirectly thereby kill non-combatants. The gunners that cause the death of the non-combatants do not intend this death; they permit it as the evil effect which comes immediately with the good effect (the capture of the works) from the causal act of firing the guns.
If we keep within the bounds of a just defence we may protect ourselves against an unjust aggressor to the effusion of his blood, or even, if need be, to killing him. An aggressor is any one who does injury to us contrary to our rights and the ordination of right. A formally unjust aggressor is a sane intelligent person who intentionally attacks us; a materially unjust aggressor is one who is not intelligent, not responsible, as an insane person, a child, or a sane person who is injuring us unintentionally. This question is important in medicine because the fetus in utero is often erroneously called an unjust aggressor.
It is a primary law of nature that every human being should and will strive to resist injury and destruction. Justice requires a moral equation, and if one right prevails over another it must be superior to the right it supersedes. At the outset both the aggressor and the intended victim have equal rights to life, but the fact that the aggressor uses his own life for the destruction of a fellow man sets the aggressor in a condition of juridic inferiority to the victim. The moral power of the aggressor here is equal to his inborn right to life, less the unrighteous use he makes of it; while the moral power of the intended victim remains in its integrity, and has therefore a higher juridic value.
The right of self-defence is not annulled by the fact that the aggressor is irresponsible. The absence of knowledge saves him from moral guilt, but it does not alter the character of the act considered objectively; it is yet an unjust aggression, and in the conflict the life assailed has still a superior juridic value. In any case the right of wounding or of killing in self-defence is not based on the ill will of the aggressor, but on the illegitimate character of the aggression.
The condition's of a blameless defence (moderamen inculpatae tutelae) are: (1) that the aggressor really threatens the defender's life, and there is no means of offsetting that violence except like violence; (2) that no more violence is used than is adequately required: if the aggression can be stopped by wounding the aggressor the defender is not to kill him; (3) that the violence in the defence is used with the intention of defence, not in revenge, hatred, anger, or the like motives.
We may do an act good in itself from which a double effect immediately follows, one good, to which the agent has a right, and the other bad, which the agent is not obliged to omit if permitted by him and not intended; but in the case of a necessary defence of life against an unjust aggressor, made even with the death of the aggressor, the defence is such an act, provided the moderation of a blameless defence is observed.
The evil effect here is not a means to the good effect, nor does it more immediately follow from the act done. The evil effect is an effect per accidens, and thus not directly voluntary, either in itself, because it is not intended, or in its cause. It lacks the condition necessary to make it voluntary in cause as regards the accidental effect since the act is not prohibited precisely because this accidental effect follows.
The act in the case is good in itself; it is an application of physical force in defence of a proper right, and any right supposes a compulsive power. The two effects of this double-effect act are: (a) the preservation of the defender's life, and (b) the death of the aggressor. The first effect is good because the defender has a right to his own life; the other effect is evil, not only physically for the one who dies, but morally inasmuch as the death conflicts with the dominion of God. This death, however, is an accidental effect of the act, because in general the defensive act is not directed by its nature to that death but to the preservation of the defender's life; nor does the death follow more immediately than the preservation. Thus it is not a means of the defence. Finally, the defensive act is not prohibited precisely lest that death follow: not in justice, for there is no justice in any right of the aggressor which requires from the defender an omission of defence unto the loss of life; there is no obligation in charity, since charity does not oblige us to love another more than ourselves, or to exalt the good of another above our own.
In an aggression which is merely material—say, in an attack by an insane man—the defender has a right to the infliction of such damage as is necessary and proportionate to an efficacious defence. The right of the aggressor yields to the superior right of the defender, not through the fault of the aggressor but through his misfortune. There is a collision where both rights cannot be exercised at the same time, and there is no reason obliging the defendant to forego his own right.
We may defend another against an unjust aggressor because we can assume that the attacked person communicates to us the use of his own coactive right. If the aggressor is our own father, mother, son, or daughter, or in general any one to whom charity obliges us more than to the person attacked, we are not permitted to kill our own kin because charity does not oblige us to prefer the good of an alien to the good of one of our blood. Ordinarily we are not obliged in justice or charity to defend another at the risk of our own life.
We may kill an unjust aggressor, servatis servandis, in defence of good equivalent in value to life: for example, to prevent life imprisonment, the loss of reason, a mutilation which would render us useless, the loss of a woman's chastity.
There are cases of accidental homicide, in medicine and elsewhere, which have an element of guilt in them. If a death follows accidentally upon an act which in itself is licit, and the agent uses all proper precautions, he is not morally guilty in case of an accidental death following his act. This is true even if the agent foresaw a probable death but did not intend it. If, however, the