Only one thing matters for the future of the Labour party: does Jeremy Corbyn have any ideas about how to woo back the voters Labour lost to the Tories last May?
We know the Left love him; what we don't know is whether he has any idea how to win enough votes to return Labour to power in 2020. Because what his devoted fans have not yet taken on board is that there simply aren't enough Lefties in Britain to install Mr Corbyn in Downing Street.
This isn't just my opinion; it is a verifiable fact. When UK voters are asked where they place themselves on the political spectrum, the average is seven points to the left of centre. (Before the 2015 election, voters thought Labour was 36 points to the left, and Ed Miliband 40 points to the left. Goodness knows where they'd put Mr Corbyn.)
Yes, some Labour supporters who voted Green or SNP in May may return to the Labour fold under Mr Corbyn. So will some who didn't vote at all - although the evidence suggests that when non-voters decide to vote, their choice doesn't necessarily match the expectations of the Left.
One of the first lessons that any political activist learns is that their way of looking at the world is not shared by everyone. No matter how clever, or how decent, they are, some people simply have different views.
It is a lesson that Mr Corbyn and his supporters might do well to learn quickly. Ask them how they intend to win the next election, given the huge electoral hurdle they have to overcome, and they reply that millions of people who have either deserted Labour or who haven't voted in the past -- the young, the poor, the least educated - will now be inspired to vote, because at last they have a champion in whom they can believe.
It's possible that they're right. Possible, but unlikely. Arithmetic can be cruel, and the numbers are not Corbyn-friendly. Yes, a quarter of a million people voted for Labour's new leader last weekend - and yes, it is a hugely impressive figure. But it is not quite so impressive when compared to the 11.3million people who voted Conservative last May.
Let the numbers do the talking. In a report called "The mountain to climb", the left-leaning Fabian Society spelt them out. Labour will need to gain more than 100 seats in 2020 if it is to win a Commons majority, and four-fifths of the extra votes they'll need to win in English and Welsh marginals will have to come direct from Conservative voters.
The report's key conclusion was this: "The litmus test for Labour's strategy is simple: can the party win over large numbers of people who voted Conservative and SNP in 2015?" The reason is that there aren't enough young, poor and disillusioned voters in the key marginals to make the difference - it doesn't matter if the votes pile up higher than ever in Hackney, Tottenham and Islington, because those seats return Labour MPs anyway.
What matters is votes in all those Labour target seats that the party failed to win in May, and so far, Mr Corbyn has said nothing about what he intends to do to win those voters over.
Some of his supporters insist that winning isn't everything - they loathe the Blairites' constant reminders that Mr Blair won three consecutive election victories - and it is true that winning without ideals or principles is an empty victory. But principles that aren't coupled to an electoral strategy are worthless to the people whom Mr Corbyn's supporters say they care about.
It is too easily forgotten that Britain was a far better place after 13 years of New Labour than it would have been under the Conservatives. The Blair-Brown tandem was a pretty neat invention until it turned toxic, because it sent the charming Mr Blair to woo the middle classes while the quietly wealth-redistributing Mr Brown set about reducing child and pensioner poverty and setting up Sure Start centres. Between them, they introduced the minimum wage and the Human Rights Act, and they hired hundreds more doctors, nurses and teachers. What's more, economic growth, measured as GDP per capita, between 1997 and 2010 was higher than in Germany, the US, France, Japan or Italy.
According to the Institute for Fiscal Studies, this was the Blair-Brown record on poverty reduction: "Reforms since 1997-98 resulted in an £18billion annual increase in spending on benefits for families with children and an £11billion annual increase on benefits for pensioners by 2010-11... Child and pensioner poverty would either have stayed the same or risen, rather than fall substantially, had there not been these big spending increases."
But there was also the invasion of Iraq, a policy error of such magnitude that it has overshadowed everything else. No one much under the age of 30 will remember the pre-Iraq Blair, just as no one under the age of 40 will remember the days of Thatcherism. That, perhaps, helps to explain why Mr Corbyn does so well among the young: they have not yet had a chance to see how a centre-left government can deliver real benefits to those people who need most help, and how the right-wing alternative can do real, lasting harm.
The New Yorker wrote woundingly after Mr Corbyn's victory last weekend: "There is a cruel caricature, hard to erase from the popular imagination, that depicts the archetypal resident of the British far left: a bearded, bicycle-riding, teetotal vegetarian from Islington, in north London. The image is lazy and unjust; in Corbyn's case, unfortunately, it also happens to be true."
Mr Corbyn's first faltering mis-steps as party leader were unforced errors that he should never have made. (Prime Minister's Questions was a rare first-week success.) He can't claim to have been taken by surprise when he came top of the leadership poll, yet no one seems to have warned him that he would be expected to turn up at the Battle of Britain commemoration on Tuesday, and that, yes, he'd be expected to sing the National Anthem.
Wearing a jacket and trousers that don't match, and a tie with the top shirt button undone, doesn't matter a jot to his supporters (it doesn't matter to me, either), but it does matter to the voters Labour needs to win back. Giving the impression that you just don't care what they think is simply unprofessional.
It may be that Mr Corbyn is a quick learner. The word is that from now on, he will mouth the words of the National Anthem when required, and his earlier prevarications on the EU have given way to a much less ambivalent statement that he wants the UK to remain a member. (In the world of old politics, it would be called a U-turn.) The shadow chancellor John McDonnell's apology for having praised the "bravery" of the IRA is another sign that the practice of political pragmatism has not been entirely abandoned.
At least it's a start. But I still believe that Mr Corbyn is so far from the British political mainstream that he will never lead his party to victory. In which case the only remaining question is how long it will take him - and his party - to realise it.