Neque enim frustra in sanctis canonicis libris nusquam nobis divinitus praeceptum permissumve reperiri potest, ut vel ipsius adipiscendae inmortalitatis vel ullius cavendi carendive mali causa nobismet ipsis necem inferamus. nam et prohibitos nos esse intellegendum est, ubi lex ait: non occides, praesertim quia non addidit: “proximum tuum”, sicut falsum testimonium cum vetaret: falsum, inquit, testimonium non dices adversus proximum tuum. nec ideo tamen si adversus se ipsum quisquam falsum testimonium dixerit, ab hoc crimine se putaverit alienum, quoniam regulam diligendi proximum a semet ipso dilector accepit, quando quidem scriptum est: diliges proximum tuum tamquam te ipsum. porro si falsi testimonii non minus reus est qui de se ipso falsum fatetur, quam si adversus proximum hoc faceret, cum in eo praecepto, quo falsum testimonium prohibetur, adversus proximum prohibeatur possitque non recte intellegentibus videri non esse prohibitum, ut adversus se ipsum quisque falsus testis adsistat: quanto magis intellegendum est non licere homini se ipsum occidere, cum in eo, quod scriptum est: non occides, nihilo deinde addito nullus, nec ipse utique, cui praecipitur, intellegatur exceptus! unde quidam hoc praeceptum etiam in bestias ac pecora conantur extendere, ut ex hoc nullum etiam illorum liceat occidere. cur non ergo et herbas et quidquid humo radicitus alitur ac figitur? nam et hoc genus rerum, quamvis non sentiat, dicitur vivere ac per hoc potest et mori, proinde etiam, cum vis adhibetur, occidi. unde et apostolus, cum de huius modi seminibus loqueretur: tu, inquit, quod seminas non vivificatur, nisi moriatur; et in psalmo scriptum est: occidit vites eorum in grandine. num igitur ob hoc, cum audimus: non occides, virgultum vellere nefas ducimus et Manichaeorum errori insanissime adquescimus? his igitur deliramentis remotis cum legimus: non occides, si propterea non accipimus hoc dictum de frutectis esse, quia nullus eis sensus est, nec de irrationalibus animantibus, volatilibus natatilibus, ambulatilibus reptilibus, quia nulla nobis ratione sociantur, quam non eis datum est nobiscum habere communem (unde iustissima ordinatione creatoris et vita et mors eorum nostris usibus subditur): restat ut de homine intellegamus, quod dictum est: non occides, nec alterum ergo nec te. neque enim qui se occidit aliud quam hominem occidit.
(Augustine, Civ. Dei 1.20)
It is not without significance, that in no passage of the holy canonical books there can be found either divine precept or permission to take away our own life, whether for the sake of entering on the enjoyment of immortality, or of shunning, or ridding ourselves of anything whatever. Nay, the law, rightly interpreted, even prohibits suicide, where it says, “Thou shalt not kill.” This is proved especially by the omission of the words “thy neighbor,” which are inserted when false witness is forbidden: “Thou shalt not bear false witness against thy neighbor.” Nor yet should any one on this account suppose he has not broken this commandment if he has borne false witness only against himself. For the love of our neighbor is regulated by the love of ourselves, as it is written, “Thou shalt love thy neighbor as thyself.” If, then, he who makes false statements about himself is not less guilty of bearing false witness than if he had made them to the injury of his neighbor; although in the commandment prohibiting false witness only his neighbor is mentioned, and persons taking no pains to understand it might suppose that a man was allowed to be a false witness to his own hurt; how much greater reason have we to understand that a man may not kill himself, since in the commandment,” Thou shalt not kill,” there is no limitation added nor any exception made in favor of any one, and least of all in favor of him on whom the command is laid! And so some attempt to extend this command even to beasts and cattle, as if it forbade us to take life from any creature. But if so, why not extend it also to the plants, and all that is rooted in and nourished by the earth? For though this class of creatures have no sensation, yet they also are said to live, and consequently they can die; and therefore, if violence be done them, can be killed. So, too, the apostle, when speaking of the seeds of such things as these, says, “That which thou sowest is not quickened except it die;” and in the Psalm it is said, “He killed their vines with hail.” Must we therefore reckon it a breaking of this commandment, “Thou shalt not kill,” to pull a flower? Are we thus insanely to countenance the foolish error of the Manichaeans? Putting aside, then, these ravings, if, when we say, Thou shalt not kill, we do not understand this of the plants, since they have no sensation, nor of the irrational animals that fly, swim, walk, or creep, since they are dissociated from us by their want of reason, and are therefore by the just appointment of the Creator subjected to us to kill or keep alive for our own uses; if so, then it remains that we understand that commandment simply of man. The commandment is, “Thou shall not kill man;” therefore neither another nor yourself, for he who kills himself still kills nothing else than man. (tr. Marcus Dods)
Crambe Repetita 