...People also have a goal toward which all the actions of their lives are directed, since all intelligent things act for a goal. Yet the diversity of people's pursuits and activities entails that different people move towards their intended goals in different ways. Therefore, people need someone to direct them toward their goal...If people were made to live alone as do many other animals, then there would be no need for anyone to direct them towards their goal, since each person would be his own king under God, the highest king, and the light of reason given to him from above would empower him to act on his own. But humanity is, by nature, a political and a social animal...

Therefore, if it is natural for people to live in association with others, there must be some way for them to be governed. After all, if many people were to live together and each provided only what was convenient for himself, then the group would fall apart unless one of them had the responsibility for the good of the group...Private concerns divide the community, while common concerns unite it. Those differences exist for different purposes. And so, in addition to what moves each person to his own private good, there must also be something that moves everyone to their common good...

If a government is under one man who seeks his own benefit and not the good of all those under him, then that ruler is called a "tyrant"...But if an unjust government is not exercised by just one, but by more than one (though still only a few), then it is called an "oligarchy"...An unjust government exercised by many is called a "democracy"...and occurs when the common people use their force in numbers in order to oppress the wealthy. In this case it is the whole people who act like a tyrant.

People must live together because they cannot acquire what is needed to live if they stay by themselves, and so a social group is more perfect if it provides better for the necessities of life. A family in a single household provides adequately for some of the needs of life such as the natural acts of nourishment and the procreation of children, and so on. In a single location you may find self-sufficiency in a particular kind of manufacturing. But a city which is a complete community has within it whatever it needs for commun defense and mutual aid against its enemies. Therefore, the correct name for someone who rules such a complete community, whether a city or a province, is "king"...

Chapter 3

...when a ruler departs from law there is no security and everything is uncertain...He threatens not only the bodies of those under him, but also their spiritual welfare, since they try to use those under them--rather than to be of use to them--and thus oppose any progress by their subjects since they suspect that any excellence among them is a threat to their unjust rule. Tyrants always suspect the good rather than the evil and are always afraid of virtue. They seek to prevent those under them from becoming virtuous and developing a sense of public spirit which would not tolerate the tyrant's unjust domination. Tyrants prevent the bond of friendship from growing among those under them or the enjoyment of mutual peace, since as long as there is mutual mistrust, no serious attempt can be made to overthrow their tyranny...

Chapter 6

The rule of one man is to be preferred since it is the best form of government, but since it can transform into a tyranny (the worst form of government), we must make a concerted effort to make sure that the community will have a king who shall not become a tyrant. In the first place, those who have the duty to elect the king should only raise up someone to that office who is of such character that it is unlikely that he will become a tyrant...Then, once the king is established, the form of the kingdom's government should be constructed in such a way that it provides no opportunity for the king to become a tyrant. The king's power should be limited in a way that he cannot easily fall into tyranny...

Aquinas intended to discuss these limitations upon power in On Kingship, but since he never completed it, those limitations were not outlined there. Nevertheless, a similar discussion occurs in his Summa Theologica I-II, Question 105, Article 1, from which the following exceprt is taken:

Continuing now with On Kingship, Chapter 6:

It seems that the solution for the evils of tyranny does not rest in the private decision of a few, but by going through a public authority. First of all, if a particular community has the right to appoint its ruler, then it is not unjust for that community to depose its king or to restrict his power if he were to abuse it by becoming a tyrant. The community should not be accused of disloyalty if it deposes a tyrant even if it had previously agreed to obey him forever, since the ruler did not govern the community as the office of king requires and thus deserved to have those under him nullify their agreement...

If, on the other hand, some higher authority has the right to appoint a king over a particular community, then the remedy for the wickedness of the tyrant is to be sought from that authority...

If, however, no human aid is possible against the tyrant, recourse is to be made to God, the king of all, who is the help of those in tribulation...But for a people to merit such a benefit from God, they must abstain from sin, since God allows the impious to rule as a punishment for sin...