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.