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.
[1] The concept of a mixed government was studied during the Renaissance and the Age of Reason by Tomás Fernández de Medrano, Niccolò Machiavelli, Giambattista Vico, Immanuel Kant, Thomas Hobbes and others.
Polybius was also rediscovered and the positive view of mixed governments became a central aspect of Renaissance political science integrated into the developing notion of republicanism.
[3] Mixed government theories became extremely popular in the Enlightenment and were discussed in detail by Thomas Hobbes, John Locke, Giambattista Vico, Montesquieu, Jean-Jacques Rousseau and Immanuel Kant.
[4] According to some scholars, for example, Heinrich August Winkler, the notion also influenced the writers of the United States Constitution who based the idea of checks and balances, in part, upon the ancient theory.
It "described two sorts of threats to political freedom: a general decay of the people which would invite the intrusion of evil and despotic rulers, and the encroachment of executive authority upon the legislature, the attempt that power always made to subdue the liberty protected by mixed government.
[...] Radical Whig perceptions of politics attracted widespread support in America because they revived the traditional concerns of a Protestant culture that had always verged on Puritanism.
[7] 18th-century Whigs, or commonwealthmen, such as John Trenchard, Thomas Gordon and Benjamin Hoadly "praised the mixed constitution of monarchy, aristocracy, and democracy, and they attributed English liberty to it; and like Locke they postulated a state of nature from which rights arose which the civil polity, created by mutual consent, guaranteed; they argued that a contract formed government and sovereignty resided in the people".
[13] Another school of thought in the U.S. says the Supreme Court has taken on the role of "The Best" in recent decades, ensuring a continuing separation of authority by offsetting the direct election of senators and preserving the mixing of democracy, aristocracy and monarchy.