Returning from France after a fortnight of snoozing by the pool and reading books about the Second World War, it is cheering to see on my return that Labour leader Jeremy Corbyn has sorted out the party’s anti-semitism problem and put all that nastiness to bed.

Or maybe not. Somehow, the meltdown is getting worse, with the Corbynite hordes assaulting deputy leader Tom Watson because he dared to say at the weekend that the party should root out racism and discrimination.

I hold no candle whatsoever for Watson. There was his involvement in removing Tony Blair. Okay, that was quite funny. But his anti-free press activities were chilling. And there was that ‎strange and creepy business airing allegations of an establishment criminal web that turned out in the end to be bogus.

But so crackpot are the Corbynista social media loons that in their attacks they have cast Watson as a Blairite stooge. To do that you have to ignore simple historical reality. Watson was a Brownite. That’s why he organised the plot to get rid of Tony Blair and replace him with Gordon Brown. This is ‎elementary fact-based stuff. Gordon Brown was not a Blairite. Really, he wasn’t. The clue is in the name. Tom Watson supported Gordon Brown.

Now some other Corbynista chap on the party’s policy-making forum has said Watson has Jewish donors and has betrayed the sainted Jeremy. Jewish money? Judas? What are they playing? Corbynite antisemitic bingo?

Meanwhile, the bearded-one issued an unconvincing video shot in a shifty manner in which he says that anti-semitism ‎must be booted out of Labour.

The uselessness of his performance in that short film – and the chippy demeanour which screams “why am I even having to do this? ” – reminded me that a core part of the problem is that, even putting to one side his Marxist views, Corbyn is a bear of little brain. He cannot take criticism or challenge and sticks only to what he knows, that is his own supposed brilliance and purity. Convinced of his own moral superiority, cheered on by fans who regard him as being of unquestionable integrity, he thinks about the world now exactly what he thought three or four decades ago.‎ That’s just weird, certainly in a leader. Most people worth trusting develop, adapting their thinking when it collides with reality, such as the fall of the Berlin Wall, holding on to fundamental principles hopefully but developing some understanding of change and complexity.

Corbyn shows no sign of being intellectually capable of ‎such development. His capabilities are all about campaigning, not thinking.

This is not to knock his achievement (or the achievement of his smarter supporters) in capturing Labour.‎ Corbyn proved the commentators wrong on the centre-right (raises hand) and on the left including those such as Owen Jones (Squealer from Animal Farm) who said he was heading for disaster.

Still, Corbyn is stubborn to the point that he cannot process that anyone could doubt him or even think differently ‎and be a decent person.

This is a weird situation. The leader of the opposition’s popularity rests on his “principled” consistency, that is sticking wedded to bad ideas because his brain can’t process anything else. What are the implications of Corbyn being doggedly determined but not the sharpest tool in the box? It seems to me possibly as follows:

1) He will not give up, which is impressive in its way. Yet, there is no emotional inflection point at which he switches position on anti-semitism or on anything else. That requires, as well as an emotional reaction, the processing power to justify or explain and communicate coherently a shift.

2) It is possible – unlikely, but possible – that sufficient key voters will decide that this is the kind of person (not adept at thinking) suited to national leadership. British Prime Ministers have been a mixed bunch since 1945, but all in their way sharp thinkers. Thinking and responding is a central requirement for the post, although no guarantor of success. Corbyn isn’t fit to share a category – Labour Prime Minister – with, say, Jim Callaghan.

3) That means Corbyn is very much beatable at an election by someone not called Theresa May, by someone‎ capable of exhibiting emotional range and mental capacity in televised debates and on the trail. Do the Tories have such people in the Commons? Yes, and they had better hurry up and get them ready.

4) It is said that Corbyn has such a hold that he could never be replaced or removed. That may be true, right now. Around him are some very clever, and dangerous, people who hate the West, markets and freedom, however. Corbyn is their useful idiot, and if he can’t win he’ll find himself spending even more time on his allotment.

In this way the antisemitism crisis is exceptionally revealing. It illustrates Corbyn’s major operational and electoral weakness, as well as showing  how over the years he shared platforms and promoted the views of appalling people.