The doctrine of the separation of powers ensures that the liberty of the individual is secure only if the three primary functions of the state (legislative, executive, and judicial) are exercised by distinct and independent organs. The doctrine was propounded by Montesquieu (De l"Esprit des Lois, 1748), who regarded it as a feature of the British constitution. It is a concept which has a superficial simplicity, but at the same times a rather deep complexity. It contains both a descriptive and prescriptive element; the concept has a long history in political thought, right back to the time of Aristotle. The conscious adaptation of the "separation of powers" principles by the framers of the American constitution in the late 18th century ensured its importance in subsequent constitution making.
It is based on the idea that there are three classes of governmental function, each carried out by a distinctive organ of government. In descriptive terms these are;.
1.) Executive function, carried out by the executive (government),.
2.) Legislative function, carried out by the legislature (parliament, assembly), and.
3.) Judicial function, carried out by the judiciary (courts).
The prescriptive or normative element has two parts in it;.
1.) the three functions should be operated by three organs of government, and.
2.) to allow some (or, on some views, any) mixing of the three functions and, in particular, the three organs is a threat to liberty.
There are wide ranges of ways in which the doctrine can be applied, or indeed not applied as the case may be in the context of the constitution. At one extreme there could be a complete mixing of all of the functions in one organ, or indeed one person such as a dictator or absolute monarch. At the other extreme is the idea of absolute separation, which has absolutely no overlapping of function or persons.