Jeremy Corbyn resembled a socialist Father Christmas at his speech this morning, promising a £500bn investment fund, a million new homes (including half a million council houses), universal childcare, the renationalisation of the railways and more publicly-controlled bus services. But when asked how he would fund this largesse, the Labour leader was strikingly vague. "You pay for it through an expanding economy and driving down tax evasion," he said. He added that "we are not proposing cuts in top rate of taxation or corporate rates of taxation". But short of running up the largest deficit in history, Corbyn's plan would necessitate significant tax rises.

Yet while vowing to "reconfigure our tax system to create a more equal society", Corbyn had remarkably little to say on the subject. He did not match Owen Smith's pledge of a wealth tax on the top 1 per cent or his promise to restore the 50p tax rate. Nor did he repeat his past promise of "people's quantitative easing" (even as economists rush to endorse the idea). The only revenue-raising measures he announced were tightening corporation tax reliefs (saving £10bn) and restoring headline rates to the levels in George Osborne's first term. Corbyn rightly argues that increased investment would raise growth but that alone would not fund his cornucopia of promises. Smith, who published costings for his 20 pledges, will aim to capitalise on Corbyn's vagueness in the first leadership hustings tonight.

Corbyn's lack of detail is unlikely to do him much harm in the contest. Party members wield banners, not calculators. But should Corbyn lead Labour into the next general election, as he pledged to do today, he will need to offer a far better answer to pass the IFS test.