In all her time as prime minister, there will be no policy as important for Theresa May as her Chequers plan for the UK's future commercial relationship with the EU.
Which is why, as I perhaps needlessly pointed out yesterday, for the country and for May's future in Number 10, it matters that the EU's main Brexit negotiator Michel Barnier has said that her central proposals are unacceptable (pour memoire: these are collecting tariffs for the EU in a so-called Facilitated Customs Arrangement and being part of the EU's single market for goods but not services).
His opposition, plus that of at least 80 Tory MPs, means - in the words of one of her ministerial colleagues - that Chequers is deader than the deadest of dead things.
Now normally that would mean she too would be most of the way to the prime-ministerial knackers yard: what leader could survive the rejection of a policy designed to implement the most important decision this country has taken for half a century, and especially when that policy has already led to the estrangement and resignations of two of her three most senior ministers, Johnson and Davis?
But these are the strangest of times, and according to her fiercest critics on her backbenches, the True Brexiters of the European Research Group, convention is wrong on this occasion.
They tell me that the ONLY way she can survive in office is to ditch Chequers and revert to a version of the Canada-plus free trade agreement that Davis was designing till he quit as Brexit secretary in July.
And if that sounds like a threat, that is is because IT IS a threat.
Led by Rees-Mogg and Steve Baker - and supported by Davis - they say that what they want is an approach to Brexit for which they can vote. And that matters more to them than who occupies 10 Downing Street.
They are explicit: do the right Brexit thing, and Boris Johnson's schmoozathon (he has embarked on a remorseless campaign of wooing potential MP supporters and buying off putative rivals) will be as fatuous as Jeremy Corbyn looking for votes in Tel Aviv.
And to make it as easy as possible for her, in the coming few days they will publish their own long and detailed plans for that free trade deal and how the border between Northern Ireland and the Republic can be kept open without imperilling the integrity of the UK - along with a blueprint for trading under WTO rules if there is no Brexit deal.
So at the moment that Barnier and they are ripping to shreds the intricate Chequers artifice she designed with Olly Robbins and the Whitehall machine, they will deliver to her a model whose engine - the free trade agreement - is one that Barnier and many Tory MPs have already said they favour.
So will they capture May and turn her into their cipher?
There is no sign of that: her new Brexit minister Dominic Raab put on his bravest face when trying to persuade MPs on the European Scrutiny Committee that the Chequers parrot is only sleeping; and her chief whip Julian Smith yesterday made the challenging claim to her ministers that she "has the numbers" to win a parliamentary vote on Chequers.
"No she does not!" say the ERG, "because unlike those wimps among the posho Remainers in our party, we will vote against a Brexit deal we don't like - whether or not it brings her down, or the party down. For us this matters more than whether or not a few of our colleagues have official chauffeurs and grace-and-favour houses".
Oh dear, it all feels too much like the end of Hamlet, or perhaps Reservoir Dogs.