Does the concept of “fake news” logically imply the existence of real news? Otherwise, all news is fake. Good news, bad news, indifferent news: none of it is to be believed. This is the world view that appears to be methodically and unscrupulously promoted by Donald Trump. His willingness to label any inconvenient fact as fake news – a deliberate untruth perpetrated by the media – is mirrored by his tendency to tell deliberate untruths himself.

This is frustrating for Mr Trump’s opponents. How can they land a punch on one so slippery? And even if they did – in a world in which all news might be true or might be fake, how would anybody know? Take the allegation that the British intelligence agency GCHQ was commissioned by President Barack Obama to eavesdrop on Mr Trump during last year’s presidential election campaign. Mr Trump did not actually assert it to be true but he gave this wild allegation authority by mentioning it more than once and praising its direct source, a commentator on Fox News, as “a certain very talented legal mind”. So is this fake news? Or is GCHQ’s angry denial – calling the claim “utterly ridiculous” – fake instead?