So far as we know, only two former British prime ministers have favoured a united Ireland. These were Alec Douglas-Home (pronounced Hume) and Harold Wilson. The paucity of such champions of severing the Union at the highest level of political life suggests that this is almost an eccentric idea that great political minds baulk at.
Logical as it might seem to some that Britain should just be rid of the burden and embarrassment of governing Northern Ireland, whenever the idea occurs to a senior politician there are always plenty of others around to kill it off.
In 1998, during the talks towards the Good Friday Agreement, Sinn Fein tried to insist that Britain should "act as a persuader" for Irish unity and Tony Blair refused. British prime ministers might not have cherished the Union, but few have considered it even possible to dispense with it.
One was Harold Wilson, who thought in 1971 that the two parts of Ireland might be given 15 years to get used to the idea and that, in the interim, Britain would ease them towards concluding arrangements.
When back in office three years later, he berated the Ulster loyalists as "spongers", but he made no further move to promote a united Ireland. Perhaps he realised that he simply couldn't and there was no point in trying.
A modern European nation can't simply divest itself of a chunk of territory and its population if only a minority portion of that population wants it to.
Douglas-Home urged Ted Heath in 1972 not to impose direct rule without a clear programme for nudging us into a united Ireland and the timeframe he had in mind seems to have been months rather than years.
Both former prime ministers had, of course, been motivated by the apparently intractable challenge of bringing violence to an end through agreement.
Northern Ireland looked like a problem that was never going to go away. This seemed even more pressing after Bloody Sunday, which nearly wrote off any prospect of the wider Catholic population, which did not support the Provos, ever again putting faith in Britain's ability to manage the crisis.
Until then, the demand of the civil rights movement and the SDLP had been that Britain should intervene and teach the unionists to govern inclusively.
The premiss at the heart of such a demand was that Britain was politically more mature and more able than the Unionist Party. That idea took a total trashing when paratroopers murdered civilians in Derry and British ministers and diplomats rushed to provide them with cover. It had seemed for a time, as John Hume observed, that for many a united Ireland was now the only conceivable solution. But that horrific level of disaffection was not sustained.
The Irish government after direct rule urged the British to make a united Ireland the goal of policy and the British refused, but, even so, northern nationalists in the SDLP - the largest nationalist party then - were willing to enter negotiations on a solution short of that.
Had the majority really thought that it was "a united Ireland or nothing", Gerry Fitt, Paddy Devlin and John Hume would have had to represent that position and would have had Irish government support in doing so.
If prime ministers find breaking the Union difficult, but not always impossible to contemplate, then it remains possible that a current or future holder of that office might return to the idea, if the old problems of division and dissension here seem ultimately intractable.
And the last week has made it seem so again.
A recent article by Kevin Rooney in Spiked argues that Northern Ireland is a vulnerability that hinders Britain from getting the fullest advantage from Brexit and exercising its sovereignty.
He writes: "Brexit has demonstrated that the Union with Northern Ireland is not only a denial of national sovereignty to the Irish people, but also a limit to the sovereignty of the British people."
Now, what if that idea occurred to Boris Johnson? It would enable him to pass off a severance of Northern Ireland from the Union as an act of generosity, or tough love, and, at the same time, a more robust underpinning of Brexit and sovereignty.
If any prime minister was to be so reckless then surely the most likely is the tousle-mopped chappie who holds that office today.
Johnson is, however, Minister of the Union. It's likely that he only adopted that title to impress the DUP with a level of commitment which he has, from their perspective, already undermined.
He would have to find a mechanism for breaking the Union. An all-UK referendum? After all, if Scotland and Northern Ireland are entitled to leave the Union, shouldn't England have the same right?
And the DUP has insisted that the referendum on Brexit was an all-UK referendum and that the majority here against it is irrelevant. By the same argument, a majority here for retaining the Union might also be undermined.
I am speculating here on what a truly reckless prime minister might come up with. For now, something else holds Johnson back. He has pledged himself to denying a referendum on independence to Scotland. That is another shallow and senseless position to take, since it will only enrage Scots further and increase the demand for independence. What's he going to do? Send in the Army?
But while he holds to that insistence on keeping Scotland, he can not trifle with the idea of losing Northern Ireland - no matter how attractive that idea is made by harder and yet harder Brexiteers whispering in one ear and the DUP boring him through the other.
Making the idea potentially more attractive still comes the claim from Professor Vernon Bogdanor, a constitutional expert, that Britain would be in breach of the Act of Union if it made a trade deal that excluded Northern Ireland, which it would have to do if it agreed to standards or regulations which diverged from those of the European Union which are binding here.
That means that we could see the Supreme Court striking down a trade deal with the United States. We'd then be back to an all-UK backstop - if the EU would agree to it - and the only alternative would be somehow getting Northern Ireland out of the Union, or going back to the discarded idea of a land border. Johnson may soon have reason to be as exasperated with Northern Ireland as Wilson and Douglas-Home were.
It would be tactically wise of the DUP not to add to that exasperation.