It is significant that in Holy Scripture no passage can be found enjoining or permitting suicide either in order to hasten our entry into immortality or to void or avoid temporal evils. God’s command, ‘Thou shalt not kill,’ is to be taken as forbidding self-destruction, especially as it does not add ‘thy neighbor,’ as it does when it forbids false witness, ‘Thou shalt not bear false witness against thy neighbor.’ However, no one should think he is guiltless when he bears false witness against himself, since the duty to love one’s neighbor is measured by the love of oneself, as it is written, ‘Thou shalt love thy neighbor as thyself.’
To be sure, the commandment forbidding false witness has another directly in view, and by misunderstanding the matter some may judge that no one is obliged to be truthful to himself. But, the fact is that a man who lies against himself is no less guilty of false witness than if he lied against another. All the more must we realize that no man may take his own life, for, in the command, ‘Thou shalt not kill,’ there are no limitations; hence, no one, not even the one who is commanded, is to be excepted.
Indeed, some people try to stretch the prohibition to cover beasts and cattle, and make it unlawful to kill any such animals. But, then, why not include plants and anything rooted in and feeding on the soil? After all, things like this, though devoid of feeling, are said to have life, and, therefore, can die, and so be killed by violent treatment. St. Paul himself, speaking of seeds, says, ‘That which thou sowest is not quickened, except it die first,’ while the Psalmist writes: ‘And he destroyed their vineyards with hail.’ Must we, then, when we read, ‘Thou shalt not kill,’ understand that it is a crime to pull up a shrub, and foolishly subscribe to the error of the Manichaeans?
Putting this nonsense aside, we do not apply ‘Thou shalt not kill’ to plants, because they have no sensation; or to irrational animals that fly, swim, walk, or creep, because they are linked to us by no association or common bond. By the Creator’s wise ordinance they are meant for our use, dead or alive. It only remains for us to apply the commandment, ‘Thou shalt not kill,’ to man alone, oneself and others. And, of course, one who kills himself kills a man.
Augustine of Hippo, The City of God, Books I–VII