Objection 1. It would seem that in the state of innocence man would not have been master over man. For Augustine says (De Civ. Dei xix, 15): "God willed that man, who was endowed with reason and made to His image, should rule over none but irrational creatures; not over men, but over cattle."
Objection 2. Further, what came into the world as a penalty for sin would not have existed in the state of innocence. But man was made subject to man as a penalty; for after sin it was said to the woman (Genesis 3:16): "Thou shalt be under thy husband's power." Therefore in the state of innocence man would not have been subject to man.
Objection 3. Further, subjection is opposed to liberty. But liberty is one of the chief blessings, and would not have been lacking in the state of innocence, "where nothing was wanting that man's good-will could desire," as Augustine says (De Civ. Dei xiv, 10). Therefore man would not have been master over man in the state of innocence.
On the contrary, The condition of man in the state of innocence was not more exalted than the condition of the angels. But among the angels some rule over others; and so one order is called that of "Dominations." Therefore it was not beneath the dignity of the state of innocence that one man should be subject to another.
I answer that, Mastership has a twofold meaning.
First, as opposed to slavery, in which sense a master means one to whom another is subject as a slave. In another sense mastership is referred in a general sense to any kind of subject; and in this sense even he who has the office of governing and directing free men, can be called a master. In the state of innocence man could have been a master of men, not in the former but in the latter sense. 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. So that one man is master of another as his slave when he refers the one whose master he is, to his own--namely the master's use. And since every man's proper good is desirable to himself, and consequently it is a grievous matter to anyone to yield to another what ought to be one's own, therefore such dominion implies of necessity a pain inflicted on the subject; and consequently in the state of innocence such a mastership could not have existed between man and man.
But a man is the master of a free subject, by directing him either towards his proper welfare, or to the common good. Such a kind of mastership would have existed in the state of innocence between man and man, for two reasons.
First, because man is naturally a social being, and so in the state of innocence he would have led a social life. Now a social life cannot exist among a number of people unless under the presidency of one to look after the common good; for many, as such, seek many things, whereas one attends only to one. Wherefore the Philosopher says, in the beginning of the Politics, that wherever many things are directed to one, we shall always find one at the head directing them.
Secondly, if one man surpassed another in knowledge and virtue, this would not have been fitting unless these gifts conduced to the benefit of others, according to 1 Peter 4:10, "As every man hath received grace, ministering the same one to another." Wherefore Augustine says (De Civ. Dei xix, 14): "Just men command not by the love of domineering, but by the service of counsel": and (De Civ. Dei xix, 15): "The natural order of things requires this; and thus did God make man."
From this appear the replies to the objections which are founded on the first-mentioned mode of mastership.
Source: Summa Theologica (New Advent)