No matter how many times Theresa May reminds us, it is easy to forget that Labour’s manifesto committed it to delivering Brexit.
Equally it is hard to remember that the notorious motion passed by the last Labour conference that opened the door to the party’s possible support for a Brexit referendum - as a last resort - was also a restatement of the party’s pledge to deliver its own vision of how to leave the EU.
So it was rational for the prime minister to respond in good faith to Jeremy Corbyn’s written offer to negotiate Brexit terms that he and his party could support.
And quite apart from the convention that manifesto commitments should be honoured, she will presumably know - since almost everyone else in the UK does - that Corbyn is less attracted to a referendum than he would be to a job offer from Goldman Sachs.
And by way of further evidence, if such were needed, I am told that the original draft of Corbyn’s letter to May, which was written by Labour’s shadow Brexit secretary Starmer, contained reference to the party’s fallback position of a referendum. And this was struck out by Corbyn’s office before the letter was sent.
But none of that means there is a deal to be done between Corbyn and May - because the scale of compromise for both may well be beyond what their parties can wear and bear.
To get Corbyn and Labour on board, May would have to sacrifice some of the putative freedoms - such as the ability to ever diverge from the EU on Labour or environmental rules, or to negotiate free trade deals with non-EU countries - that for many Tory Brexiter MPs represent the whole point of Brexit.
And to get his party on board, Corbyn would have to explain why he would be doing a deal whose effect could be to sustain the Tory government in office till 2022.
But as I have said before, there is a deal to be done between May and Corbyn that would command parliamentary support - it would be a version of what some Remainy MPs have styled Common Market 2.0 - so long as neither mind that their respective parties would fracture as a price of that deal.
The point is that May’s and Corbyn’s visions of a tolerable Brexit are much more aligned than the views of the Brexiter and Remain wings of their own respective parties.
So it is May’s and Corbyn’s resolve to deliver Brexit and damn the consequences for the institutions that have sustained and nurtured them throughout their entire adult lives that will determine how and even whether the UK leaves the EU.
For both, it is all about whether their perception of the national interest trumps party interest.
PS I am told Starmer is not the happiest member of the frontbench, to put it mildly - according to multiple sources.
He had agreed that the final part of Corbyn’s letter to May would say “if you do not accept this [Brexit offer] there will be a People’s Vote”.
A source tells me “LOTO [the leader of the opposition] agreed to this. But then Keir discovered after the letter had been sent and published that the People’s Vote para had gone”.
Starmer “called LOTO and was told ‘oh we must have forgotten that paragraph’”.
Apparently Starmer’s reaction has not been one of unbridled joy.
And even erstwhile Corbyn loyalists are becoming grumpy at what they see as his refusal to follow the revealed will of Labour members and supporters that their should be a referendum.
One said: “The only interest” of Corbyn and his aides is “seeing a Tory Brexit through so they can wash their hands of it”.