Nicene and Post-Nicene Fathers, Vol. II:
City of God: Chapter 16

Chapter 16.—Of the Ranks and Differences of the Creatures, Estimated by Their Utility, or According to the Natural Gradations of Being.

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.  Thus the reason of one contemplating nature prompts very different judgments from those dictated by the necessity of the needy, or the desire of the voluptuous; for the former considers what value a thing in itself has in the scale of creation, while necessity considers how it meets its need; reason looks for what the mental light will judge to be true, while pleasure looks for what pleasantly titilates the bodily sense.  But of such consequence in rational natures is the weight, so to speak, of will and of love, that though in the order of nature angels rank above men, yet, by the scale of justice, good men are of greater value than bad angels.