ONCE upon a time, I was trying to write a piece about Scotland for a London-based magazine. To describe the Scottish fiction writers of the 1980s and 1990s, I flourished a brilliant phrase: “valedictory realism”.

It was meant to describe the way those novelists evoked the vanishing industrial world that was being purposefully demolished by Margaret Thatcher. There was a clarity of vision that came with the awareness that a whole way of life was being extinguished.

I was very pleased with the formulation but the editor who had commissioned the piece told me he didn’t like it and cut it out. Imagine my surprise, though, when that issue of the magazine appeared with the editor’s own introductory piece at the front. It was headlined: Valedictory Realism.

My dudgeon was high and I was about to protest that it was not OK to cut a bit from my copy and paste it into his own. But something was nagging at the foggy backend of my brain.

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And before I made a complete fool of myself I realised that while it was indeed likely that the editor had pilfered the phrase from me, I in turn had swiped it from – who else but Tom Nairn? It was from a dazzling essay he had written about Walter Scott.

This was the thing about him. Only the greatest public intellectuals seep into public consciousness so thoroughly that even those of us who are supposedly in the same trade forget how much we are influenced by them. Nairn was one of those quiet titans.

For another mark of great public intellectuals is that you don’t have to agree with them – but you do have to reckon with them.

It was impossible for anyone who thought seriously about the future of the British and Irish archipelago to avoid a reckoning with Nairn’s writing. He changed the terms of debate. He did this by challenging what had appeared to be a natural binary – nationalism versus internationalism.

This opposition was as potent 50 years ago as it has become again, except that it was then as germane to the left as it is now to the “anti-globalist” right. It seemed obvious that the choice was between the nation-state on the one side and the European Union on the other.

What made this binary so powerful was that people on opposite sides of the debate seemed to accept it. To be pro-European was to assume that the nation-state would wither away into glorious irrelevance. To be anti-European was to treat the idea that the EU would indeed destroy the nation-state as a threat rather than a promise.

Nairn was wonderfully non-binary. He understood that a sense of national belonging did not have to be inward-looking and reactionary and that an ideal of European belonging neither demanded nor made practically possible the dissolution of nation-states.

Time has proved him to be entirely right about this: the more the EU has expanded, the further it has had to move from the delusion that it would replace rather than enhance national sovereignty.

My own country, Ireland, is one of the most striking of test cases. Its hard-won independence was a failure when it was understood to mean, in the English translation of Sinn Fein, ourselves alone.

Independence became real only when it took the shape of interdependence. It was having a seat at the top tables of the EU that made Irish sovereignty more than a rhetorical construct. Nairn could have told us this, but we had to go through long periods of misery before we learned it from bitter experience.

Nairn understood that exit and re-entry had to go hand-in-hand. This is what made him, too, a great valedictory realist. The valediction he imagined so brilliantly was the long goodbye to the political construct he so memorably called Ukania.

This is what made him not just a vital Scottish intellectual but – paradoxically – a great British thinker too. For the realist bit of his project was a forensic diagnosis of the nature, history, functioning and malfunctioning of the UK state. It is not accidental that The Enchanted Glass remains the essential study of the British monarchy, not just as an institution in itself but as a hall of mirrors in which the whole state is reflected in distorted forms.

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Nairn’s explanatory power has continued to grow. It is hard for any serious person of any political persuasion to deny that Ukania is in its death throes.

The morbid symptoms are all too obvious: the rise and fall of the Brexit project; the rapid succession of failed prime ministers whose main achievements seem to be the constant redefinition of the word “worst”; the deliberate creation of cruel destitution; the crumbling of physical infrastructure; the collapse of basic governance; the palpable sense of despair.

And re-entry has now become the key to the exit door. There is no viable political project for any part of the UK that does not involve a return to the EU. The revival of Ukania as a post-imperial version of Ourselves Alone is not feasible – even if it were desirable. Its ramshackle structures, built without the democratic design of a written constitution and not even buttressed by an electoral system that any European democracy would tolerate, sit on foundations that have already shifted and become chronically unstable.

Brexit was a symptom of this existential crisis. It drew on a recognition that there is indeed something fundamentally awry with the way sovereignty works in the UK, but twisted that truth into a bigger set of lies.

The rapidity with which Brexit became a zombie endeavour, devoid of political life but still stalking the land, may seem astonishing. But to anyone who had read Nairn it is not so surprising: it was an exit, not into the open air but into a cold, dark room.

Valedictory realism is the only way forward. The long farewell to Ukania has to be accompanied by a realistic sense of commonality: within Britain, around our archipelago, and across Europe. As Nairn understood so well, the only point of independence is to create a more solid ground for the operation of interdependence.

Fintan O’Toole is a columnist with the Irish Times. Tickets for the Break-Up of Britain conference on November 18 honouring Tom Nairn can be found at thebreakupofbritain.net