There are so many questions that flow from this, but possibly the most important are: is it true? And what is the political impact of this story?

First, the truth. Is it true? It doesn't matter. As Lyndon Johnson knew, the real point of accusing your enemy of sex with pigs is to force him to deny it. True or not, the image is so vivid it sticks in the mind, indelibly colouring someone's reputation. And LBJ was operating in the pre-internet age. Today online debate is emotion first, facts a long-distant second.

Barely 12 hours after publication, the internet has decided that David Cameron did something unspeakable with a pig. Nothing can change that. Even a the most convincing and comprehensive refutation of the story, a sworn confession from everyone involved that they made it up, won't change the story: it would just become a footnote on a Wikipedia page, a thing clever people say in years to come when the story is retold in pubs ("actually it wasn't true"). Some stories are so compelling the truth can't change them. Look up who really deleted Milly Dowler's voicemails to see what I mean.