What is it?

Everybody knows that the separation of powers is a good thing. It is less easy to say how it came to be this way. In 1748, when Montesquieu commended England as a model of the separation of powers in a democracy, judges still sat in Cabinet and the final court of appeal was the upper chamber of the legislature. But the myth took hold, as myths do. Within fifty years it had become solid doctrine in the French and American revolutions: unless the state's principal powers were in separate hands, there would be (in Montesquieu's phrase) an end of everything.

First, what does the separation of powers mean? In its platonic form it means that the legislative, executive and judicial functions of the state are carried out independently of one another. Plainly this is unreal: the judiciary has to be appointed and paid by a body other than itself; the law it interprets and applies has to be made in substantial part by a legislature; the legislature is largely dependent for its own functioning on the executive and its political heads – ministers who are almost invariably parliamentarians; the executive requires a parliamentary mandate for much of what it does; and the courts have to be able to say whether the executive is acting within its mandate.

There is nothing original about this notion of symbiosis. James Madison wrote in 1788 about the three core functions: