Objection 1: It seems that it is unlawful to kill any living being. For the Apostle says (Romans 13:2): „Whoever resists the ordinance of God brings judgment upon himself.“ Now, divine providence has ordained that all living beings should be preserved, according to Psalm 146:8-9: „He makes grass grow on the mountains… He gives food to the cattle.“ Therefore, it seems unlawful to take the life of any living being.
Objection 2: Further, murder is a sin because it deprives a man of life. Now, life is common to all animals and plants. Therefore, for the same reason, it would apparently be a sin to kill irrational animals and plants.
Objection 3: Further, in divine law, a special punishment is appointed only for a sin. Now, according to divine law, a special punishment had to be imposed on one who killed another’s ox or sheep (Exodus 22:1). Therefore, killing irrational animals is a sin.
On the contrary, Augustine says (De Civitate Dei I, 20): „When we hear, ‚Thou shalt not kill,‘ we do not apply this to trees, for they have no sensation, nor to irrational animals, because they have no fellowship with us. Hence, the words ‚Thou shalt not kill‘ refer to the killing of a man.“
I answer that there is no sin in using a thing for the purpose for which it is intended. Now, the order of things is such that the imperfect exists for the sake of the perfect, just as in the process of generation nature proceeds from the imperfect to the perfect. Thus, just as in the generation of a man, first there is a living being, then an animal, and finally a man, so also things like plants, which have only life, all exist for the sake of animals, and all animals for the sake of man. Therefore, it is not unlawful for man to make use of plants for the benefit of animals, and animals for the benefit of man, as the Philosopher states (Politics I, 3).
Now, it seems that the most necessary use is that animals use plants and men use animals for food, and this cannot be done without depriving them of life. Therefore, it is permitted to take the life of plants for the benefit of animals, and of animals for the benefit of men. Indeed, this is in accordance with the very command of God; for it is written (Genesis 1:29-30): „Behold, I have given you every plant… and every tree… for your food, and to all the animals of the earth“; and again (Genesis 9:3): „Every moving thing that lives shall be food for you.“
Reply to Objection 1: According to the divine ordinance, the life of animals and plants is preserved not for their own sake, but for the sake of man. Therefore, as Augustine says (De Civitate Dei I, 20), „by a most just ordinance of the Creator, both their life and their death are subject to our use.“
Reply to Objection 2: Irrational animals and plants lack the life of reason, by which they could move themselves; rather, they are, as it were, moved by another, by a kind of natural impulse, a sign of which is that by nature they are enslaved and adapted to the use of others.
Reply to Objection 3: Whoever kills another’s ox does not sin by killing the ox, but by harming another man in his property. Therefore, this does not belong to the genus of the sin of murder, but to the sin of theft or robbery.