“[We] must now inquire what is better for a province or a city: whether to be ruled by one man or by many…
Now it is manifest that what is itself one can more efficaciously bring about unity than several… several men, for instance, could not pull a ship in one direction unless joined together in some fashion. Now several are said to be united according as they come closer to being one. So one man rules better than several who come near being one.
Again, whatever is in accord with nature is best, for in all things nature does what is best. Now, every natural governance is governance by one…Wherefore, if artificial things are an imitation of natural things and a work of art is better according as it attains a closer likeness to what is in nature, it follows that it is best for a human multitude to be ruled by one person.
This is also evident from experience. For provinces or cities which are not ruled by one person are torn with dissensions and tossed about without peace, so that the complaint seems to be fulfilled which the Lord uttered through the Prophet [Jer 12:10]: “Many pastors have destroyed my vineyard.” On the other hand, provinces and cities which are ruled under one king enjoy peace, flourish in justice, and delight in prosperity.”
~ St. Thomas Aquinas
The arguments of St. Thomas are helpful not only because of their simplicity (anyone could memorize them in a minute) and cogency (they would be difficult to directly refute) but because they convey very well the traditional modes of reasoning.
For example, the doctor bases his arguments on nature, which is to say, concrete reality as it is. He does not begin in an abstract ideal which he then attempts to realize, as must be done with democracy. He also insists on rationality, and then follows with historical experience. Also, to get a further idea of the medieval mind, he stresses the need for unity, which is a specifically traditional principle, as opposed to the liberal belief in ‘progress by competition’. Furthermore, he argues from nature and its mode of governance. He does this because natural laws are derived from the eternal law, which is nothing other than the mind of God. Therefore, it can be assumed that nature has educational value. If natural things are governed by one, then the artificial things of man can be considered superior if they follow its structure. This does not stop at manufacture but extends even into politics because statesmanship is an art, and “Art is the imitation of Nature in her manner of operation.”
 De Regno, Book 1, paragraph 3.
 Summa Theologica, 1.117.1.