The ancient Greeks devised systems for the classification and evaluation of types of political governments. Many of the terms in the U.S.'s basic political lexicon -- democracy, aristocracy, monarchy, oligarchy, tyranny, not to mention politics itself -- are of Greek provenance and come from the efforts of the ancient Greeks to classify different forms of government.
In order to understand how the ancient Greeks classified governments, it is important to understand the ancient Greek concept of a constitution. Today, a constitution is described as a document that sets out the fundamental rights of citizens -- a bill of rights -- and the basic structure of government: the separation of powers and the authority of different branches, for example. For the Greeks, a constitution meant something very different; it was a form of society, determining who was a citizen and who governed. A classification of governments meant a classification of constitutions, organized around a single question: Who governs?
One of the most influential methods of classifying governments can be found in Book IX of Plato’s "Republic." According to Plato (428 B.C.-348 B.C.), the best possible kind of state is a "kallipolis," a type of government ruled by philosophers. If this government is corrupted, the next best is a timocracy, in which rulers are motivated by honor and glory. A corruption of timocracy produces an oligarchy, which is rule by the rich motivated by their own financial gain. According to Plato, the corruption of oligarchy produces a democracy, by which he meant a form of mob rule. Finally, a democracy that becomes disorderly and chaotic produces a tyranny, in which an individual rules over an entire society for his own private gain.
Another influential way of classifying governments comes from Plato’s student, Aristotle (384 B.C.-322 B.C.). Aristotle identified three basic forms of government: rule by one person, the few or many. He identified the monarchic, aristocratic and constitutional as types of governments that ruled for the common good, whereas tyrannies, oligarchies and democracies were primarily interested in benefitting the rulers themselves.
The Greek philosopher Polybius (200 B.C.-118 B.C.) developed yet another classification system that led to the theory of a "mixed" form of government. Influenced by Aristotle, Polybius adopted the basic distinction between the one, the few and the many as possible types of governance. However, he found that not all governments fit neatly into any of these single categories. Rather, some constitutions were “mixed," containing the aristocratic institution of the senate through which “the few” ruled, while at the same time including popular assemblies through which “the many” could rule as well. For Polybius, this “mixed” constitution was the best and most stable, since it included the advantages of both aristocracy and democracy.