In a wonderfully dry manual of theology on my husband’s bookshelves, written in Latin and printed in Naples in the 1830s, there is a discussion of whether ‘rustics and idiots’ are supported in their belief by ‘motives of credibility’, such as miracles. The same question has been asked about belief in Jeremy Corbyn, except that the city stands in for the country, and the idiots are often useful ones.

‘I am the only candidate who can offer a bold but credible vision,’ Andy Burnham has said. ‘I’ll have the confidence to reject Tory myths and the credibility to demolish them,’ countered Yvette Cooper. John Curtice, the political scientist, noticed that Labour MPs think Mr Corbyn’s ‘economic policy is not credible’. But credibility should be applicable to candidates as well as their policies. This double application of credible, to men and to evidence, is no new discovery of politics; in the 15th century, both meanings were used in their letters by members of the Paston family.

Even so, to be credible is a grudging sort of recognition: a hurdle to be surmounted. More inspiring is to be incredible. Jeff Brazier, the widower of Jade Goody, spoke recently about the present woman in his life: ‘I think she’ll be an incredible mum; she’s already showing signs of being an incredible stepmum.’ Rachel Treweek, the first woman diocesan bishop, as the new Bishop of Gloucester, said it was an ‘incredible feeling’ to be consecrated.

This ‘hoorah’ term has been knocking around for some time. The famous but unknown Monk of Evesham, who wrote a travel book about Purgatory more than 100 years before Dante, mentions at one point an ‘incredibulle swetenes of ioyfull conforte’. He did believe it, but he wouldn’t have believed it before he smelt it.

One more meaning of credible has lurked in the thickets of its history, and that is ‘credulous’. Even though grammarians since the 19th century have been denouncing this meaning as a blunder, it has hung on. Poor old Norman St John Stevas remarked 30 years ago: ‘I’m afraid people are credible and gullible. This is one of the results of original sin.’ Perhaps so. Certainly what seems credible to voters is sometimes simply incredible.