British prime ministers do not offer addresses on the state of the Union. For America's presidents, the job is compulsory, and a big deal. For the occupants of Downing Street, the idea doesn't arise. This is probably just as well. Least said, perhaps, soonest mended.

The omission might strike outsiders as odd, nevertheless. They can get their heads around the idea that Britain's constitution is, famously and usefully, "unwritten", but some still note that the 28 articles of the 1707 treaty amount to a founding document. The piece of paper exists.

Without that bill of sale, in fact, there is no Great Britain, no United Kingdom. Yet, thanks to those 28 clauses, the misty notion of a Britannic constitution born of convention, convenience and a fictive "Crown-in-Parliament" looks dubious. The state we're in depends on pens put to paper three centuries ago today.

Prime ministers, for all that, are not obliged to report on the health of the arrangement.

History reminds us why. In the very earliest years the monarch's leading adviser might have been obliged to mention that the deal was close to collapse, thanks to London's failures to keep its bargains. Half a century after 1707, he might have been reporting, with regret, that Scotland had not prospered much and that most Scots were not reconciled to occupancy of "North Britain".

Later, industrialisation and imperial trade helped matters, at least for those in a position to benefit. The Enlightenment, with its pre-Union roots in a progressive education system, displayed several of Scotland's advantages to the world. Old trading habits, freed from England's tariff barriers, reasserted themselves. The Scots of the nineteenth century became as fully British as they have ever been, or are ever likely to be.

The defence of the Union today is born of a nostalgia for that Victorian age. It represents both the Britain for which some still hanker, and a problem for nationalists. If no-one in the later nineteenth century talked seriously of dissolving the Union, when all the maps were pink, why not return to that happy state, "stronger together", as new Labour likes to say, "than apart"? If the vast majority could subscribe to the British ideal when Britain's empire was triumphant, why not now?

For some, the question answers itself. Imagine, though, if Prime Minister Blair did decide to address the state of the Union this spring. What might he say? (Let's presume that he would speak honestly.) No doubt he would tell us that the Union signifies one the largest, and currently most successful, economies in the world. He might then assert, as he has before, that Scots would be foolish to disdain a share in such an enterprise.

But then he would have to explain why Scotland is also pathetically incapable, economically, of autonomy.

Anyone searching for pressures on the Union at the tercentenary will find perhaps the largest in that paradox. The Prime Minister would claim that three centuries of incorporating Union have been a boon to Scotland. But he would then have to tell us why, after 300 years, the junior partner remains unfit to face the world unaided. Boon or bust?

Having spent time lecturing Muslims and others on the nature of Britishness, Blair would then have to deal with contrariness, otherwise known as the "politics of identity". After three long centuries, what Robert Louis Stevenson called the Scottish "accent of the mind" persists. Like David Hume on his deathbed, we have yet to rid ourselves of our Scotticisms. British, for many, remains a "perhaps", an optional extra, and this counts as remarkable. Assimilation has failed.

Sophisticated Unionists will tell you this was never the idea. After all, the treaty protected Scotland's rights in matters of Presbyterian religion and education. A Cabinet stuffed with Scots - though Blair declines the honour - has a well-modulated, adaptable notion of national identity. Its Scots are "Scots-and-British", "without contradiction", "comfortably" and doubly patriotic with it. Famous figures in the London media take the same line, as they and the politicians have done for centuries. Their compatriots prefer something less complicated.

The cultural independence of Scotland has never been ceded. Nor have the many demands for assimilation, implicit and explicit, succeeded. The poets tell that story. The deaths of Scotland's languages have been predicted many times over, but something, ever-changing, persists. Above all, the sense of the nation is ineradicable.

For Unionists, this is not a political fact. If anything it is a phenomenon, in their ideology, for which the Union was designed; a peculiarity to be accommodated. But should a prime minister ever speak to the state of the Union, he should speak to that truth: the Scots go on being Scots. Not out of habit, not merely for the sake of football: after three centuries, they still accept Britain, if at all, with reservations. If that is not a political fact, nothing is.

It means the metamorphosis of Union did not quite take. It means that the treaty, like any treaty, can be revoked. It suggests that Unionism, not nationalism, is the aberration. Why are we not more contentedly British? Perhaps, very simply, because the larger partner in the deal claimed Britain for itself; perhaps because the prior claim of identity is stronger. Strong enough, at least, to survive through three centuries.

A good state of the Union speech would admit that there is unrest, and that the unrest grows. Devolution was supposed to satisfy all demands, but that settlement now appears inadequate, even patronising. The latest polls say the largest portion of Scots will reject independence if Holyrood is granted more powers. The effective difference, a fine one, is between what Tom Nairn has called de facto and de jure autonomy. A return to the status quo ante, as a previous imperial language would have it, is not desired.

What is the state of the Union, Mr Blair? The prognosis is not encouraging. For one, important, thing, England has begun to realise that "asymmetrical devolution" is unfair devolution. The complacent belief that the Scots would simply go away, after home rule, has not been borne out. England has begun to notice disjunctions and discrepancies. It has begun to realise that the affairs of the north affect its own governance. That cannot last. And who presumes that only Scots are entitled to revoke a treaty?

Scottishness, never banished, reasserts itself. That doesn't mean independence this year, or next. The Scottish National Party has a road to travel before it can hope to win a referendum, even if it can engineer a coalition of the willing. In a sense, that doesn't matter. Objectively, even a minority SNP administration would count as a judgment on the Union. The judgment would say this: 300 years on, the essential problems of incorporation remain unresolved.

Is Scotland thriving, or hobbled? Does it feel more or less British? Has it a real voice in matters of war and peace? Is its economy enhanced or enfeebled by the centripetal force of London and ultimate control over the nation's natural resources? By the way, who do the Scots think they are?

Political opinions and psychological conditioning are never far apart. An honest prime minister would accept that once-reliable institutions no longer worry over the continuance of the British state. From a prince of the Roman Catholic Church to the Historiographer-Royal; from Sir Tom Farmer, a very rich man, to formerly Tory thinkers, football players or pop stars; from trade unions to the comical owner of Harrods: consensus evaporates. In a modern world, the Union feels like a time-worn formality.

Had Scotland's magnates accepted the Hanoverian succession late in the seventeenth century, things could have been different. The deal offered then involved a properly federal union, a matter of trade and mutual aid that did not attempt to subsume one nation within the institutions of another. It might not have worked. But, three centuries later, it begins to sound far-sighted.

Blair's imaginary address would be a gloomy affair for Unionists. It would speak of problems, impulses and inexplicable discontents. It would concede that his own administration has created dissent, deep and wide, in Scotland. It might even say that, in 2007, Britain remains of questionable legitimacy.

Its real point would be simple, if baffling to the speaker: 300 years and the Scots go on being Scots. How come?