Affection is a strange basis for a system of government.

Most politicians would dearly love to be loved, but few of us pick a candidate because of fondness. We don't bother, either, with nostalgia, or the fiction that an office-holder is the embodiment of the nation. That would be fatuous.

In the realms of reality, meanwhile, few of us have much time for the hereditary principle. It's fine for the family firm: what's your business is your business; your property is your property. But it is not often done, these days, to think of a country in those terms. It feels a little off, let's say. Parentage, in any case, is a poor guide to aptitude.

Fans of the House of Windsor will tell you that the role of constitutional monarch is not just any old career. The correct response would be that, where Britain is concerned, this is exactly the problem. Set aside any sentiment bestowed on the royal house and what's left is, in fact, a job of work, namely that of head of state. We are not well-served.

Nor are we republicans, in the main. The most recent British Social Attitudes survey found that in the event of independence 62 per cent would choose to "keep" the Queen, with just one-third preferring a modern state. A Wings Over Scotland/Panelbase poll last October was a little more precise: 48 per cent "in favour" of monarchy; 32 per cent against; and 20 per cent "don't know". In both cases, roughly one-third was as good as it got for republicanism.

Considering that a poll in 2008 found 45 per cent for a republic and 39 per cent for the monarch, the sans-culottes don't seem to be making much progress, then. You would have to say, in fact, that belief in a country without royalty is in decline. That happy monarchist thought misses the point.

"Only" one-third is better understood as a hefty share of a vote no-one dares to call. It arises, moreover, with no real aid from the press and no support whatever from broadcasters who define it as their duty to ignore the third entirely and promote royalty relentlessly. A large minority, to put it no higher, are marginalised utterly.

One-third of Scots would meanwhile vote for a republic with no encouragement from any "major" party. The Scottish Socialists would retire the Windsors, as would many Scottish Greens. Beyond that there is the usual deference, the familiar silence. Parties that otherwise strain every sinew to discover what voters think and what they want are happy, even determined, to ignore a political reality. But then, if Panelbase can find 32 per cent for a republic, with a further 20 per cent who are presumably open to persuasion, who knows where loose talk might lead?

There are a number of ways to argue over royalty. Affection, continuity and "uniting the nation" are favourites among proponents. Anachronistic practices, residual powers, the hereditary principle and sundry insults to democracy animate republicans. There is, however, another way to look at the matter. The Queen is head of state. How is that working out for us?

The job comes in many varieties, ranging from the merely decorative to the fully executive. Joachim Gauck of Germany stands at one end of the scale, Barack Obama of the United States at the other. The Queen leans more to a German style, so to speak, where the work at hand is concerned, but no-one still believes she is bereft of power. Her right to appoint prime ministers, approve legislation and - above all - "advise and warn" ministers is guarded jealously.

It is claimed, nevertheless, that Britain's constitution depends on this constitutional monarch. In some countries that would involve the head of state acting as guardian of the nation's founding principles. It might mean telling the government of the day that its legislation was unconstitutional, that rights were being put at risk, that its schemes for war (for example) were illegal under the constitution. Britain is spared these complications.

The Queen governs - that's the word they use on the Buck House website - according to a constitution that is not written down. This is handy for governments who can always find a useful ghostly convention, but handier still for the monarch. Nothing is spelled out. She is not, in that event, a "prisoner of the constitution", but neither is she bound to any useful duties in relation to the executive, the legislature and the people.

In terms of heads of state, in other words, we get a raw deal. Rather than a system of checks and balances, we wind up with systemic failure and pretty ceremonies. The monarchy thus provides vast scope for abuse. How did we wind up in a shooting war in the Falklands? Nothing to do with Parliament; nothing to do with the people; precious little to do with the Cabinet. Margaret Thatcher used the royal prerogative: war by executive fiat. If our head of state advised on the constitutional meaning of such behaviour, she kept the advice quiet.

Were you starting from scratch, this is not the system you would sketch on the back of a garden party invitation. If you were drafting a skeleton constitution for a country recovering its independence, indeed, you would want to look quite hard at what the job of head of state can or should mean. The Scottish National Party, though it awaits comment on its proposals, has declined the opportunity.

Come independence, the people will be sovereign: a good start. In contrast with the murk and mystery of the Westminster system, the declaration is clear and unambiguous. It revives (allowing for a good deal of intervening history) the essential principle underpinning the Declaration of Arbroath. Nothing and no-one can be above the people.

By what logic, then, does the SNP propose that the Queen and any heirs who happen along should persist as sovereign and, perpetuating Westminster, as head of state? In Britain, royalty does not defend the constitution. The constitution exists to defend royalty. Why would anyone import such an arrangement, especially when you get a poor excuse for a head of state for your pains?

Those opinion polls no doubt provide an answer. In 2002, the SNP were still proposing a referendum on the monarchy at the "earliest opportunity" after independence. Now the party leadership would rather not hear of the republican "distraction", preferring to talk instead of an invented figure called Queen of Scots. So one-third of actual Scots are once again ignored.

That's a mistake. Better than most, the SNP have attempted to frame the case for a progressive country, one capable of addressing the issue of inequality. Missing from that argument is an understanding of the role the House of Windsor plays in embedding hierarchies in little Britain. Royalty allows and encourages the fantastic notion that there is a natural order to our society. When that is tolerated, inequality is tolerated.

You can have a sovereign or a sovereign people; you can't have both. Anyone who imagines otherwise has probably earned a peerage.