"When the facts change, I change my mind. What do you do, sir?" So ran John Maynard Keynes's celebrated riposte. It is essentially this defence that David Cameron will employ when he breaks his pledge to hold a referendum on the Lisbon Treaty.
He will argue that his promise was made at a time when the treaty was unratified. He will point out that he never promised a retrospective referendum. But it was still foolish of him to use the formulation "cast-iron guarantee" when promising a referendum to Sun readers  back in 2007.
The arch-Eurosceptic Bill Cash articulated the thoughts of many on the right of the party when he said:
We need a full referendum on Lisbon as we were promised and as we voted in the House of Commons. No ifs or buts. This is about the government of the United Kingdom operating in line with the democratic wishes of the electorate.
Cameron should be able to ride out grass-roots discontent over Lisbon provided it doesn't spread to the shadow cabinet. In a fascinating column on Sunday , Peter Oborne identified William Hague as the man most likely to take on the Tory leader over Europe. He wrote:
Intellectually formidable, he is a very live alternative prime minister. More dangerously still, he no longer yearns for power and was only persuaded to return to front-line politics with difficulty. There is very little to stop Hague from resigning and, were he to do so, he could scarcely avoid becoming a very powerful focus of resistance to a Cameron premiership.
I would be surprised to see anything but resolute discipline before the next election; the Conservatives can't wait to get their hands back on those red boxes. But should Cameron fail to repatriate economic and social powers once in office, we could see another civil war over Europe.
ConservativeHome's Tim Montgomerie writes : "If Britain's relationship with the EU is fundamentally the same after five years of Conservative government, the internal divisions that ended the last Tory period in government will look like a tea party in comparison."
This may look like a piece of dispassionate analysis but it is also a clear threat. Those such as Montgomerie who favour EU withdrawal will maintain their pragmatic support for Cameron's stance only for so long.