When it comes to leaving the EU, the prime minister has to do separate deals - one with the EU and another with her own truculent MPs.
In the past few days, all attention has been on the True Brexiters in her party, furious with a proposal - agreed at Chequers and enshrined on Thursday in a 98-page White Paper - that would see the UK following EU rules forever on the production and consumption of food and goods, and also collecting tariffs for the EU at our borders.
That is seen by the Brexiters as - in the spiky characterisation of Jacob Rees-Mogg - "the greatest vassalage since King John paid homage to Phillip ll at Le Goulet in 1200".
In case you are in doubt, that's not - in Rees-Mogg's view - a good thing.
But what about the other counterparty, the 27 heads of EU governments and the officials in Brussels who are their willing vassals (to use the word of the moment)?
Well their initial reaction, to me, may be less learned than Rees-Mogg's but is no less hostile.
One EU official told me that Theresa May's customs plan - her so called "Facilitated Customs Arrangement" or FCA, which involves the UK collecting both EU and British tariffs on imports - is "nuts".
"There's too much red tape, susceptibility to fraud, a huge burden for our machine, uncertainty as to how to update as rules and procedures change, uncertainties about the application of EU law, and so on," said a Brussels source.
Apart from anything else, there is anxiety in Brussels that under the FCA, if the UK did free trade deals with the likes of China, multinational companies based in the UK could import parts from China free of tariffs and then reship finished goods to the EU with the benefit of a significant competitive advantage over manufacturers based across the Channel.
So the UK would have a significant advantage in attracting multinational companies to settle in UK, as a bridge (still) to the EU's single market.
"There is zero chance of the EU agreeing the FCA," says a former British official - who added that he found it impossible to believe that the PM was not aware that her customs plan was dead at inception.
Brussels sources also warn that EU governments have been completely clear that the UK cannot have seamless, frictionless access to their giant market for goods and food, unless the UK is prepared to sign up for free movement of people and services (two of the "four freedoms") - and they see the PM as trying to escape that obligation.
So paradoxically, Brexiters hate May's plan to almost stay in the EU's single market for goods and food, because it gives what they see as too much power to the EU to boss the UK's rules and regulations, whereas Brussels dislikes it because they see the EU's sovereignty eroded.
Or to put it another way, a Brexit White Paper which was supposed to be the moment when the PM made her tough choices turns out - you guessed it - to be another delay in the toughest of her decisions, namely whether to permanently alienate her Brexiter colleagues or the UK's erstwhile EU partners.
And indeed as she has conceded, we're still little the wiser whether we are leaving the EU with a deal or no-deal.