Montesquieu never used the phrase 'tripartite system' so often credited to him
The French philosopher Montesquieu is routinely credited with the three-branch model of government, yet in The Spirit of Law, published in 1748, he spoke of a distribution of powers among legislature, executive and judiciary. He drew his model from the Roman Republic and from what he saw in Britain's monarch, Parliament and courts.
His core argument was that each power must stick to its own function. If one person or body both made and enforced the laws, it could pass tyrannical laws and then carry them out; combining judging with lawmaking would bring arbitrary rule, and judging with executing would bring oppression. He also identified an executive role over war, peace and embassies, separate from the power to punish crimes and settle disputes, which he called judicial.
Others had been working toward the idea. Polybius described Rome as a mixed government balancing Senate, consuls and assemblies, and credited Lycurgus of Sparta with the first such system. During the English Civil War, parliamentarians saw three branches in the King, the Lords and the Commons. In 1653 General John Lambert drafted the Instrument of Government, briefly England's constitution under the Protectorate, which created a Parliament and two elected executive branches, a Council of State and a Lord Protector chosen for life, each checking the others. After the Restoration, the Crown's use of the courts against opposition leaders in the 1680s pushed English thinkers to want judges independent of the executive.
John Locke's Two Treatises of Government of 1690 split power differently, into legislative, executive and federative, the last covering war, peace and alliances. Locke held that the legislature was supreme because whoever gives laws must be superior, but insisted it could not rule arbitrarily, tax or seize property without consent, or hand its lawmaking power to anyone else.
Most modern constitutions blend the doctrine with checks and balances, adopted in the 1787 United States Constitution. James Madison, in Federalist No. 51, reasoned that angels would need no government, but men governing men must make government control itself. Immanuel Kant went further, arguing that even a nation of devils could build a working state with a constitution that set factions against one another. The cost is gridlock when veto players in different branches disagree.
Source: Separation of powers
