Justice and order must prevail; that is a fundamental natural law to which all other laws are subordinate. Justice, moreover, is a moral equation, and whenever one right transcends another it must be superior to the right it holds in abeyance. The right an innocent human being has to his life, however, is so great that no other human right can be superior to it while he remains innocent. Subversion of this right by creatures is intrinsically evil, as blasphemy and perjury are evil, although not in exactly the same degree.
There are occasions upon which it is permissible to kill, _indirectly_, innocent persons. An effect is brought about indirectly when it is neither intended as an end for its own sake, nor chosen as means toward an end, but is attached as a circumstance to the end or the means. Means help to an end, circumstances often do not, although they may affect the morality of an act.
Suppose two swimmers, Peter and Paul, are trying to save Thomas, who dies in the water; as he dies Thomas grips Peter and Paul so tightly that they cannot shake the corpse off. Peter is weak, and he will soon sink and drown, owing to his weakness and the weight of the corpse; Paul also will go down later, owing to the weight of Peter and Thomas. Peter, however, cuts his own clothing loose from the grip of the corpse and is saved; but Paul immediately is drowned, owing to the fact that the full weight of the corpse comes upon him. Is Peter justified in cutting himself loose? Certainly he is. This is an example of indirect killing, a case of double effect, one good, the saving of Peter's life, the other evil, the loss of Paul's life, both proceeding immediately and equally from the causal act, the cutting loose of the clothing. The good effect is intended, the bad effect is reluctantly permitted.
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.