2. It is, however, lawful _indirectly_ to kill a man provided this man is an unjust aggressor. Cardinal de Lugo (_De Just. et Jure_, 10, 149) and others hold you may even _directly_ kill an unjust aggressor. _Indirectly_ here means incidentally. An effect happens indirectly when it is neither intended as an end nor a means, but happens as a circumstance unavoidably attached to the end or means intended. We may not, however, kill an innocent man even indirectly, because no end is proportionate to the sacrifice of an innocent man's life, but the case of an unjust aggressor differs from that of an innocent man. By an unjust aggressor is meant some one that outside the due course of law threatens your life or the equivalent of your life, or the life of some one you should or may protect. You may stop such an aggressor, and if you happen to kill him while trying to stop him, there is no moral wrong involved. This aggressor may be formally or only materially unjust: he may be a normal man with a formal intention to kill you or your ward, or a murderous lunatic that tries to kill you or your ward, but he must be _unjust_ either formally or materially. It is natural for every being to maintain itself in existence, to resist destruction. This is a primary law of nature. As Father Holaind well said (_Amer. Eccl. Rev._, January, 1894): "The ethical foundation of self-defence is this: Justice requires a sort of moral equation, and if a right prevails it must be superior to the right which it holds in abeyance. At the outset both the aggressor and his intended victim have equal rights to life, but the fact of the former using his own life for the destruction of a fellow man places him in a condition of juridic inferiority with regard to the latter. If we may be {19} allowed so to express it, the moral power of the aggressor is equal to his inborn right to life, less the unrighteous use which he makes of it, whilst the moral power of the intended victim remains in its integrity and has consequently a higher juridic value. When the person assailed cannot defend himself, his right _can_ and sometimes _must_ be exercised by those who are bound in justice or charity to protect the innocent. At the dawn of human life the physician or surgeon stands as the natural protector both of the mother and of the child; he is beholden to both. "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 and in itself; it is yet an unjust aggression, and in the conflict, the life assailed has yet a superior juridic value. The right of killing in self-defence is not based on the ill will of the aggressor but on the illegitimate character of the aggression. Now, an aggressor is _at least materially unjust_ whenever he perpetrates an act destructive of the right of another."