The new branch of government needs a firm basis on which to rest its legitimacy. Otherwise its role can be constantly challenged. Politicians will be tempted to interfere from the one side, the judiciary from the other. Conversely, it can itself encroach on the legitimate roles of the other branches of government. A means by which the unelected bodies can be held to account also has to be provided. The new institutions can abuse their powers and the trust placed in them. They too can perform badly and require a means through which poor performance can be corrected.

In the classic form of separation of powers each branch of government rested on its own form of legitimacy. By contrast, each of the conventional attempts discussed earlier, that failed to acknowledge the emergence of a new branch, proposed that the new bodies should rely on a legitimacy derived from the other branches. For those who hold that the unelected bodies operate under some kind of democratic overhead, it is the elected bodies that confer legitimacy. For those who see the unelected bodies operating under the authority of a constitution as part of the rule of law, it is the constitution that confers legitimacy. Only in the case of the ‘pragmatic’ account of unelected bodies is there an attempt to go beyond a derived legitimacy. This account emphasises the acceptability to the public of the practical functions these bodies perform.