Short answers, pulled from the story.
Mixed government, also called mixed constitution, is a form of government that combines elements of monarchy, aristocracy, and democracy. The theory holds that blending all three prevents each form's characteristic degeneration: tyranny, oligarchy, and anarchy respectively.
The foundations were laid by Plato and Aristotle in ancient Greece. Aristotle named the cycle of governmental decay anacyclosis and argued in his Politics that a constitutional government mixing oligarchy and democracy under law was the closest to an ideal. Polybius later extended the theory and applied it to the Roman Republic.
Polybius identified the Roman Republic as the clearest example of mixed government in practice. He saw the consuls as the monarchical element, the Senate as the aristocratic element, and the popular assemblies and elections as the democratic element, with each institution checking and complementing the others.
James Madison stated in Federalist Paper No. 40 that the Constitutional Convention of 1787 created a mixed constitution, and he cited Polybius in Federalist Paper No. 63. Scholars such as Heinrich August Winkler have also argued that the system of checks and balances was based in part on the ancient theory.
John Calvin advocated a mixture of aristocracy and democracy as the best form of government, praising democratic participation as an invaluable gift when God allows a people to elect its own leaders. He also favored distributing power across several political institutions as a safeguard for ordinary people's rights and liberties.
Some scholars describe the European Union as a mixed constitution in which the Commission President represents rule by one, the Commission represents the aristocratic dimension, and the European Parliament represents the democratic dimension.