For, among those beings which exist, and which are not of God the Creator’s essence, those which have life are ranked above those which have none; those that have the power of generation, or even of desiring, above those which want this faculty. And, among things that have life, the sentient are higher than those which have no sensation, as animals are ranked above trees. And, among the sentient, the intelligent are above those that have not intelligence,—men, e.g., above cattle. And, among the intelligent, the immortal such as the angels, above the mortal, such as men.
These are the gradations according to the order of nature; but according to the utility each man finds in a thing, there are various standards of value, so that it comes to pass that we prefer some things that have no sensation to some sentient beings. And so strong is this preference, that, had we the power, we would abolish the latter from nature altogether, whether in ignorance of the place they hold in nature, or, though we know it, sacrificing them to our own convenience. Who, e.g., would not rather have bread in his house than mice, gold than fleas? But there is little to wonder at in this, seeing that even when valued by men themselves (whose nature is certainly of the highest dignity), more is often given for a horse than for a slave, for a jewel than for a maid.
— City of God XI.16