How do political parties realign themselves. That is a peculiar mystery in the UK, where everything seems desperately constipated.
It also depends on context. The various explanations about how rival factions came together in 1859 to form the Liberal Party are usually too inward-looking to explain much. The threat of a looming continental conflagration never gets mentioned for example, but it is also clear that groups of MPs had been moving together for some years.
To listen to UK politicians, especially those who dream of a realignment of the left, you would think these things had to be all cut and dried and agreed by the gods before anyone does anything. As if politicians were all centralising lawyers, steeped in regulation.
The reverse is actually the case. Parties realign when MPs and politicians find themselves working together to achieve joint objectives. Not the other way around.
There in a nutshell is why the Progressive Alliance, for example, has been relatively slow on the launchpad.
But it also explains how that realignment is already emerging. The story yesterday, for example, suggests that Theresa May ought to make common cause with some disaffected Labour MPs to get her Brexit plans through without interference from the hard brexiteers.
Or the idea, put about by the Daily Express and others, that she should sack her 12 rebels, which would then by default become a centrist group alongside the Lib Dems.
Do I think either of these will happen? Well no, I don’t actually. But the fact that they are even being discussed suggests how fluid the political situation may be.
But, as I say, what turns fluidity into actual realignment is not ideas or negotiation or directives. It is joint action, doing stuff together.