Separation of powers refers to the division of a state's government into "branches", each with separate, independent powers and responsibilities, so that the powers of one branch are not in conflict with those of the other branches. The typical division into three branches of government, sometimes called the trias politica model, includes a legislature, an executive, and a judiciary. It can be contrasted with the fusion of powers in parliamentary and semi-presidential systems where there can be overlap in membership and functions between different branches, especially the executive and legislative. In most non-authoritarian jurisdictions, however, the judiciary almost never overlaps with the other branches, whether powers in the jurisdiction are separated or fused.
The intention behind a system of separated powers is to prevent the concentration of power by providing for checks and balances. The separation of powers model is often imprecisely and metonymically used interchangeably with the trias politica principle. While the trias politica model is a common type of separation, there are governments that have more or fewer than three branches.
Aristotle first mentioned the idea of a "mixed government" or hybrid government in his work Politics, where he drew upon many of the constitutional forms in the city-states of Ancient Greece. In the Roman Republic, the Roman Senate, Consuls and the Assemblies showed an example of a mixed government according to Polybius (Histories, Book 6, 11–13). It was Polybius who described and explained the system of checks and balances in detail, crediting Lycurgus of Sparta with the first government of this kind.
John Calvin (1509–1564) favoured a system of government that divided political power between democracy and aristocracy (mixed government). Calvin appreciated the advantages of democracy, stating: "It is an invaluable gift if God allows a people to elect its own government and magistrates.
