Separation of powers, therefore, refers to the division of responsibilities into distinct branches to limit any one branch from exercising the core functions of another. The intent of separation of powers is to prevent the concentration of unchecked power and to provide for checks and balances to avoid autocracy or inefficiencies.
The separation of powers model is often imprecisely and metonymically used interchangeably with the trias politica principle. While the trias politica is a common type of this form, it is not its only type.
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).
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." In order to reduce the danger of misuse of political power, Calvin suggested setting up several political institutions which should complement and control each other in a system of checks and balances.
In this way, Calvin and his followers resisted political absolutism and furthered the growth of democracy. Calvin aimed to protect the rights and the well-being of ordinary people.[need quotation to verify] In 1620, a group of English separatist Congregationalists and Anglicans (later known as the Pilgrim Fathers) founded Plymouth Colony in North America. Enjoying self-rule, they established a bipartite democratic system of government. The "freemen" elected the General Court, which functioned as legislature and judiciary and which in turn elected a governor, who together with his seven "assistants" served in the functional role of providing executive power. Massachusetts Bay Colony (founded 1628), Rhode Island (1636), Connecticut (1636), New Jersey, and Pennsylvania had similar constitutions – they all separated political powers. (Except for Plymouth Colony and Massachusetts Bay Colony, these English outposts added religious freedom to their democratic systems, an important step towards the development of human rights.) Books like William Bradford's History of Plymoth Plantation (written between 1630 and 1651) were widely read in England. So the form of government in the colonies was well known in the mother country, including to the philosopher John Locke (1632–1704). He deduced from a study of the English constitutional system the advantages of dividing political power into the legislative (which should be distributed among several bodies, for example, the House of Lords and the House of Commons), on the one hand, and the executive and federative power, responsible for the protection of the country and prerogative of the monarch, on the other hand. (The Kingdom of England had no written constitution.)[need quotation to verify]
The term "tripartite system" is commonly ascribed to French Enlightenment political philosopher Baron de Montesquieu, although he did not use such a term. In reality he referred to "distribution" of powers. In The Spirit of the Laws (1748), Montesquieu described the various forms of distribution of political power among a legislature, an executive, and a judiciary. Montesquieu's approach was to present and defend a form of government which was not excessively centralized in all its powers to a single monarch or similar ruler, form of government known then as "aristocracy". He based this model on the Constitution of the Roman Republic and the British constitutional system. Montesquieu took the view that the Roman Republic had powers separated so that no one could usurp complete power. In the British constitutional system, Montesquieu discerned a separation of powers among the monarch, Parliament, and the courts of law.
In every government there are three sorts of power: the legislative; the executive in respect to things dependent on the law of nations; and the executive in regard to matters that depend on the civil law.