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.