St. Thomas Aquinas takes many of Aristotle's ideas from The Politics in order to create his idea of the best regime. He revisits the good and bad forms of each type of government Aristotle introduced, and then makes his decision that the best regime is a type of monarchy that he calls kingship. This decision stems from his definition of a king as "one who rules over the people of a city or province for the common good" (17).
Kingship is beneficial because it is the rule of one person. Aquinas states that the correct and most useful way to carry out an objective is "when it is lead to its appropriate end" (15). The incorrect way would be the opposite--to lead something to an inappropriate end, or not to lead it to an end at all. In light of this definition, the most effective government would lead the people to their appropriate end, which Aquinas believes is unity.
In this sense, Aquinas believes that obviously something that "is itself one can promote unity better than that which is a plurality" (17). This may not seem quite so obvious to anyone else, and his analogy between unity and heat may seem a little vague, but Aquinas still makes a valid point in that creating a government promoting unity is more difficult when more people are involved. This is simply because of the number of ideas and interpretations present within a group that are not present under the rule of one.
Aquinas also argues that kingship, or the good, just monarchy, is preferable because it is present in nature. He likens the king to God, because naturally God is the "Ruler over all" (17). It is therefore natural for one man to rule many, as long as he is leading the...