For a political system to be stable, the possessors of power need to be balanced against each other.
"Power tends to corrupt and absolute power corrupts absolutely". .
Concentration of government authority inevitably leads to abuse; separation serves as a check on abuse. By preserving a balance of power between various political forces, power is prevented from being concentrated in the hands of people who would be tempted to misuse it.
The concept of separation of powers is not strictly restricted to modern democracies. Even the idea that the most efficient separation of a governing body is into three discrete parts is an old one. The Greek philosopher, Aristotle wrote that the fairest political system would be the one in which power was shared among the monarchy, the aristocracy, and the common people. John Locke described a kind of a more modern proposition: the government should be divided between an executive and a legislative branch. The role of the executive would be to govern the country and implement law, while the legislature would be responsible for creating and managing the law itself. The underline problem with Locke's political system is that there is nothing to prevent one branch of the government encroaching upon the rights of the other. .
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Modern ideas of separation of powers are mostly derived from the writings of French political theorist, Charles de Montesquieu. In his book The Spirit of the Law, Montesquieu describes a political system where governmental power is shared between an executive, a legislature, and a judiciary. According to Montesquieu, the law which is created by the legislature is applied by the executive, and the judiciary regulates the way in which the executive applies the law. The judiciary would be limited by the legislature, since only the legislature could create new laws. The legislature could create law, but had no direct powers to apply it, since only executive can do so.