When the legislative and executive powers are united in the same person, or in the same body of magistracy, there can be then no liberty … Again, there is no liberty, if the power of judging be not separated from the legislative and executive powers. Were it joined with the legislative, the life and liberty of the subject would be exposed to arbitrary control; for the judge would be then the legislator. Were it joined to the executive power, the judge might behave with all the violence of an oppressor.
Miserable indeed would be the case, were the same man, or the same body, whether of the nobles or of the people, to exercise those three powers, that of enacting laws, that of executing the public resolutions, and that of judging the crimes or differences of individuals … Amongst the Turks, where these three powers are united in the Sultan, the subjects groan under absolute despotism.
—Montesquieu. The Spirit of the Laws, 1748.
Separating the functions of the legislative, judicial, executive, and bureaucratic branches, and prohibiting the same persons from concurrently holding offices in different branches, is an essential safeguard. It is like a bank vault that requires two people to simultaneously turn two keys before it will open. Together the key-holders can achieve their legitimate function, but neither by his own misbehavior can successfully abuse the trust.
The Westminster system breaches this principle by appointing the executive from within the legislature. This leads to laws that grant ever wider discretion to the executive. By contrast, where the executive and legislature are separate, the legislators jealously guard their power from executive encroachments. As John Trenchard noted:
For all wise governments endeavor as much as possible to keep the Legislative and Executive parts asunder, that they may be a check upon one another … One part of the duty of the House of Commons is to … redress the grievances occasioned by the Executive part of the government; and how can that be done if they should happen to be the same persons, unless they would be public-spirited enough to … drown themselves?. (A short history of standing armies in England, 1698.)