Queen Elizabeth I's secretary of state, Sir Thomas Smith, wrote: "To be short, the prince is the life, the head, and the authority of all things that be done in the realm of England." (Sir Thomas Smith, Republica). By the 19th century, Bagehot had described the monarchy as the dignified part of the constitution.
Yet, shorn of any personal, despotic power, Smith's definition remains an uncannily accurate description of the monarchy in our own times. It is still "the head and authority of all things", albeit circumscribed in that function by convention and parliamentary accountability.
The royal cipher on government documents, on pillarboxes; the need for all legislation to be touched by the royal sceptre - this surviving panoply of authority testifies to the British genius for adaptation of ancient institutions to contemporary needs.
The monarchy's most important constitutional function is simply to be there: by occupying the constitutional high ground, it denies access to more sinister forces; to a partisan or corrupt president, divisive of the nation; or even to a dictator. The Queen's powers are a vital safeguard of democracy and liberty.
The critics of the monarchy belong to two categories. The first, and more rarefied, are those belonging to the Tony Benn school, who agitate against the surviving pockets of the royal prerogative as undemocratic and inimical to open government.
The second category of anti-royalists - much more in evidence today - are the self- conscious modernisers. Jewelled crowns and robes are old- fashioned; titles and ceremonial look more like historical pageant than contemporary life; gorgeously uniformed soldiers on horseback are anachronistic, with uncomfortable undertones of hierarchy and privilege.
The real motive of the reformers is a drab puritanism, a distaste for pageantry, on the misapprehension that it somehow affronts democracy when, in reality, it glorifies the nation, personified by the sovereign.
Is the monarchy, then, so perfectly constituted that, as Wellington claimed of the pre-1832 parliamentary system, it is not susceptible of any improvement whatsoever? On close scrutiny, apart from details of the royal household's organisation and domestic economy too minor to justify the term "constitutional", the monarchy at present is disfigured by only one serious flaw.
It is astonishing that a government that has concerned itself with the number of heralds in the procession at the State Opening of Parliament and which has endlessly preached the doctrine of an "inclusive" society, has not been moved to amend the Act of Settlement. This statute, couched in offensive 18th-century language, excludes Roman Catholics from the throne, or from marrying the monarch.
The Act of Settlement was passed in 1701, after the death of the Duke of Gloucester, the last surviving son of Princess (later Queen) Anne. Ironically entitled "An Act for the further Limitation of the Crown and better securing the Rights and Liberties of the Subject", it amplified the already violently anti-Catholic provisions of the Bill of Rights of 1689, excluding from succession to the throne all those who "are or shall be reconciled to or shall hold Communion with the See or Church of Rome or shall profess the Popish Religion or shall marry a Papist".
Even in the harshly sectarian climate of the early 18th century it scraped through the House of Commons only by a majority of one. Why retain the Act of Settlement, which enshrines at the heart of the constitution the doctrine that some 10 per cent of the Queen's subjects are to be treated as second-class citizens?
The Act is deeply discriminatory. It does not constructively prescribe that the sovereign's consort must belong to the Church of England: it is perfectly legal for the monarch to marry a Buddhist, a Hindu, or even a Moonie - but not a Roman Catholic. Nor does the Act represent some dusty detritus, like an unrepealed statute prohibiting snuff-taking.
The Act of Settlement is the British constitution's grubby little secret: nobody wants to tackle it. Apart from the Act of Settlement, the monarchy requires no refashioning. It is strong in its essence and in its place in the hearts and confidence of the British people. It is our greatest guarantor of stability.