On the Fifth Precept of the Decalogue

Thou shalt not kill. - Exod. xx, 13.

This precept is expressly negative and virtually positive. Insofar as it is positive, it commands the preservation of life; and insofar as it is negative, it prohibits primarily the unjust killing of a man, and secondarily every unjust lesion, wounding, or mutilation of the body, whether one's own or another's. But those things which will be established concerning killing, whether licit or illicit, will have to be applied to mutilation or to any bodily lesion, with due proportion preserved.