On the contrary, Augustine says (De Civ. Dei i, 20): "When we hear it said, 'Thou shalt not kill,' we do not take it as referring to trees, for they have no sense, nor to irrational animals, because they have no fellowship with us. Hence it follows that the words, 'Thou shalt not kill' refer to the killing of a man."

ii
I answer that, There is no sin in using a thing for the purpose for which it is. Now the order of things is such that the imperfect are for the perfect, even as in the process of generation nature proceeds from imperfection to perfection. Hence it is that just as in the generation of a man there is first a living thing, then an animal, and lastly a man, so too things, like the plants, which merely have life, are all alike for animals, and all animals are for man. Wherefore it is not unlawful if man use plants for the good of animals, and animals for the good of man, as the Philosopher states (Polit. i, 3).

ii
Now the most necessary use would seem to consist in the fact that animals use plants, and men use animals, for food, and this cannot be done unless these be deprived of life: wherefore it is lawful both to take life from plants for the use of animals, and from animals for the use of men. In fact this is in keeping with the commandment of God Himself: for it is written (Gn. 1:29,30): "Behold I have given you every herb . . . and all trees . . . to be your meat, and to all beasts of the earth": and again (Gn. 9:3): "Everything that moveth and liveth shall be meat to you."

ii
Reply to Objection 1: According to the Divine ordinance the life of animals and plants is preserved not for themselves but for man. Hence, as Augustine says (De Civ. Dei i, 20), "by a most just ordinance of the Creator, both their life and their death are subject to our use."