- Monarchy: rule by one man/king/queen.
Aristocracy: rule by the best group of people (i.e the wealthiest class/the wisest class)
Democracy: rule by the people.
The logic goes like the following:
Starting with Monarchy, it is not stable by its nature, as it will naturally default-the King or Queen will die. It could also turn into Tyranny because even the first king had made good policies and passed the kingdom to his son, his son could forget everything.
Then the society starts to discuss and lead to the rise of the aristocracy. Yet, it is not stable by its nature too, because different groups or factions will compete with each other. As a result, there will be oligarchy-ruled by a few who rule in their respective Tyranny.
Subsequently, democracy rises up. Polybius believes that this form is the most unstable on the grounds that people by themselves could form the institutions to constraint every citizen. Sometimes, democracy defaults into ochlocracy in which there is an absence of government.
Later, there will be a leader to revolutionise and the monarchy rises up again.
From the above logic, Polybius proposed that when there is a balanced mixture of the Monarchy, Aristocracy and Democracy, will the society then become stable.