Mixed government (or a mixed constitution) is a form of government that combines elements of democracy, aristocracy and monarchy, ostensibly making impossible their respective degenerations which are conceived as anarchy, oligarchy and tyranny. The idea was popularized during classical antiquity in order to describe the stability, the innovation and the success of the republic as a form of government developed under the Roman constitution.
Unlike classical democracy, aristocracy or monarchy, under a mixed government rulers are elected by citizens rather than acquiring their positions by inheritance or sortition (at the Greco-Roman time, sortition was conventionally regarded as the principal characteristic of classical democracy).
The concept of a mixed government, including monarchic, aristocratic, and democratic aspects, was proposed by Thomas Aquinas as the ideal form of governance. It was later studied during the Renaissance and the Age of Reason by Niccolò Machiavelli, Giambattista Vico, Immanuel Kant, Thomas Hobbes and others. It was and still is a very important theory among supporters of republicanism. Various schools have described modern polities, such as the European Union and the United States, as possessing mixed constitutions.
Plato in his book The Republic divided governments into five basic types (four being existing forms and one being Plato's ideal form, which exists "only in speech"):
democracy: government by the many
oligarchy: government by the few
timocracy: government by the honored or valued
tyranny: government by one for himself
aristocracy: government by the best (Plato's ideal form of government)
Plato found flaws with all existing forms of government and thus concluded that aristocracy, which emphasizes virtue and wisdom, is the purest form of government. Aristotle largely embraced Plato's ideas and in his Politics three types (excluding timocracy) are discussed in detail.