On the morning of September 19 the Scots and the rest of the British may well wake up as foreigners to each other. For millions of Scots this will be a reason to rejoice; for even more millions, on both sides of the Tweed, a moment of incredulous sorrow at the loss of our common home, a catastrophe that somehow came about in our political sleep.

Our shrunken country will henceforth be divided by borders, barriers, perhaps passports. A psychological wound will open that is unlikely to heal for a very long time. Something precious, to this historian at any rate, will have been irreparably destroyed: a nation state whose glory over the centuries has been precisely that it does not correspond with some imagined romance of tribal singularity but has been made up of many peoples, languages, customs, all jumbled together within the expansive, inclusive British home.

Robert the Bruce may have been the victor of Bannockburn, destroying the English forces of Edward II; but he was also lord of the manor in Tottenham and his grandfather, the first Robert Bruce of Annandale, had been constable of Carlisle castle for Henry III. But the rewriting of history in the service of opposed identities seldom allows for human ambiguities.

Instead, our splendid mess of a nation will have been separated out into its national parts. Three hundred years of shared experience, in war and peace, hard times and good, will have been thrown into the dustbin of history – and for what, exactly? So that Scots may be relieved of the bedroom tax and the Trident nuclear missile? So that Scotland can take its place in the councils of Brussels alongside Slovenia and Slovakia?

This will be a bitter irony, for Great Britain was, in the first instance, a Scottish enterprise. The great English Tudor antiquarian, William Camden, published his county-by-county chronicle, Britannia, in 1586, covering the entirety of the two kingdoms. But this was a Latin edition for the learned and, by the time the English edition appeared in 1610, a Scottish king – James Stuart – was on the throne of what he insisted should be called Magna Britannia.

By the time James I had reached Newcastle on his progress south into England in 1603, he had already issued coinage bearing the new designation of Great Britain. The erstwhile Bishop of Galloway, John Gordon, whom James would appoint Dean of Salisbury, was so taken with godly destiny of the united realm that he imagined the word “Britannia” to be Hebrew, in which case it read as “covenant (Brit) with God (Yah)”. This was a sure sign the British were destined to be the new chosen people. It became a commonplace to refer to Scotland and England as Israel and Judah, whose disunion would bode ill and whose reunion was a holy moment. James himself features in the Rubens ceiling paintings in Whitehall Banqueting House as wise Solomon, intervening to spare the infant Britain from being sliced in two to satisfy those arguing mothers, Anglia and Caledonia.

In depth Future of the union Scotland will decide in a referendum to be held on September 18 2014 whether or not to end the 307-year-old union with England

Things did not go exactly as planned. Instead of harmonising the pieces of Britain, the Stuart devotion to divine-right monarchy succeeded in breaking it apart in the civil wars, which – it should always be remembered – were fought between Scots and Scots, and Irish and Irish, as well as English and English. The first sovereign of a truly united Britain was Oliver Cromwell, and he imposed that unity by terrible force. Slaughter in the name of dynasty and religion would go on and on until the tragic consummation in the Jacobite rising of 1745, invading England as far as the Midlands before turning back north to meet its fate at Culloden.

But, while the epic of the clans was being enacted, another Scotland was coming into its own. Its battles were of the mind not the heart; its victories enacted in the clubs, universities and libraries of Edinburgh and Glasgow were those of philosophical understanding, scientific invention and humane knowledge. Encyclopedia Britannica was a concept conceived in 1768 in the head of a Scot: William Smellie (with much content borrowed from Alexander Pope and Samuel Johnson). However, as its title declared, it was meant for the entire nation.

Some of Scotland’s heroes – Adam Smith and the great philosopher, historian and unrepentant atheist David Hume, and the founders, respectively, of sociology and comparative linguistics, Adam Ferguson and Lord Monboddo – may have resided in Edinburgh and Glasgow but they flourished in a culturally borderless Britain.

The great flowering of a culture that overrode ancient hatreds and battles was sustained over centuries

The next time Prime Minister David Cameron and George Osborne, his chancellor of the exchequer, emerge from their lairs they might just take a short stroll along Downing Street, where they will encounter the ghost of their neighbour, Tobias Smollett – among other things the Scottish editor of pro-union drumbeating journal, The Briton; author, like Hume, of a History of England; and resident of Downing Street in the 1740s. If they turn left up Whitehall and cross over Trafalgar Square they can walk down John Adam Street, named after one of the two geniuses of neoclassical architecture – the other being his brother Robert, the King’s architect – who built the Adelphi, perhaps the most elegant apartment block ever created (no longer, alas, preserved in its elegant Adamite form but with some of the graceful interiors intact).

The Adam family firm – which changed architecture on both sides of the Atlantic – was indivisibly British, with as many English masterworks as Scottish: the Pulteney Bridge in Bath; Syon House in west London; Kenwood House in north London; as well as Dumfries House in southwest Scotland. The same was true of their friend, Allan Ramsay, son of a Scottish poet, then painter in ordinary to King George III and prolific portraitist.

The great organising principle of this exuberantly inventive new nation was friendship, the affinity of minds – James Boswell, the Scottish diarist, and Samuel Johnson, the English author and lexicographer.

This great flowering in a British culture that overrode ancient hatreds and battles was sustained over the next two centuries. Scots stamped their imprint on the Victorian world, disproportionately manning the government of the empire, civilian and military, while their banks and businesses were powerful contributors to Britain’s global economic pre-eminence. But in the life and writings of David Livingstone, the explorer and missionary, and Thomas Carlyle, was also embodied the fierceness of the anti-materialist Christian conscience. It is a legacy that lives on – not least in administrative and political life: five of the UK’s 13 postwar prime ministers are Scots or from Scottish families, including Mr Cameron.

This all owed something to what Adam Smith, in The Theory of Moral Sentiments, described as “sympathy”. For Smith, the capacity to enter into the experience of someone not necessarily like you was the fundamental principle around which just societies, as well as rich ones, evolved.

Smith and David Hume would, I am convinced, be No voters for they would plainly see that the world is now divided.

On the one hand, the legions of die-stamp patriots – whether Nigel Farage of the UK Independence party, Russian President Vladimir Putin or Scotland’s own first minister, Alex Salmond – for whom similarity is identity. And on the other, those who feel enriched by sharing a national home with people who do not necessarily look, sound or pray like themselves but nonetheless manage to live in neighbourly sympathy.

All enduring nations in the end are communities of shared sentiment and ours might be said, without making light of our many conflicts, to be a sustained exercise in the practice of Smith’s sympathy. To write the prospective requiem of this sympathetic country is hard enough. To live through its demise will be unbearable.

The writer is an FT contributing editor

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Letters in response to this article:

Old view of Scots as semi-barbarous / From Mr Conor Magill

Perhaps there isn’t a coherent case for defending the union / From Mr Paul Davis

Ties that link the UK’s nations are close – and precious / From Lord Radice

Scotland symbolises an unbalanced kingdom / From Mr James Hanshaw

Irritation at centralisation is not confined to the Scots / From Emeritus Prof Richard Batley

Splendid union fell at first hurdle / From Mr Alex Chisholm

Opposing camps bear same support / From Mr Stephen Myers

Westminster policies have surely diminished shared values / From Cllr Alasdair Rankin