Thomas Aquinas

“ I answer that, Man in a certain sense contains all things; and so according as he is master of what is within himself, in the same way he can have mastership over other things. Now we may consider four things in man: his "reason," which makes him like to the angels'; his "sensitive powers," whereby he is like the animals; his "natural forces," which liken him to the plants; and "the body itself," wherein he is like to inanimate things. Now in man reason has the position of a master and not of a subject. Wherefore man had no mastership over the angels in the primitive state ”

“ This distinction is founded on the reason that a slave differs from a free man in that the latter has the disposal of himself, as is stated in the beginning of the Metaphysics, whereas a slave is ordered to another. ”

“ First, from the order observed by nature; for just as in the generation of things we perceive a certain order of procession of the perfect from the imperfect (thus matter is for the sake of form; and the imperfect form, for the sake of the perfect) , so also is there order in the use of natural things; thus the imperfect are for the use of the perfect; as the plants make use of the earth for their nourishment, and animals make use of plants, and man makes use of both plants and animals. Therefore it is in keeping with the order of nature, that man should be master over animals. ”

“ For the human body was not entirely exempt from the laws of nature, so as not to receive from exterior sources more or less advantage and help: since indeed it was dependent on food wherewith to sustain life. ”

“ Wherefore, as man, being made to the image of God, is above other animals, these are rightly subject to his government. Thirdly, this is proved from a property of man and of other animals. For we see in the latter a certain participated prudence of natural instinct, in regard to certain particular acts; whereas man possesses a universal prudence as regards all practical matters. Now whatever is participated is subject to what is essential and universal. Therefore the subjection of other animals to man is proved to be natural. ”

“ A higher power can do many things that an inferior power cannot do to those which are subject to them. Now an angel is naturally higher than man. Therefore certain things in regard to animals could be done by angels, which could not be done by man; for instance, the rapid gathering together of all the animals. ”

“ By those words Gregory means to exclude such inequality as exists between virtue and vice; the result of which is that some are placed in subjection to others as a penalty.
Reply to Objection 2: Equality is the cause of equality in mutual love. Yet between those who are unequal there can be a greater love than between equals; although there be not an equal response: for a father naturally loves his son more than a brother loves his brother; although the son does not love his father as much as he is loved by him. ”

“ So we may say that, according to the climate, or the movement of the stars, some would have been born more robust in body than others, and also greater, and more beautiful, and all ways better disposed ”