Why on earth do you bother with Twitter? It’s a good question and you deserve an answer.

It’s a good place to publicise an article or a book is the first thing. It certainly came in handy recently when I published Everything in Moderation earlier this year. I can also answer questions about my views or express an opinion that is too brief or fleeting or even flip for The Times.

And what goes with that is that it is also a good place to hear about other people’s books and articles. The main reason I’m on Twitter really is that I learn about things that would otherwise pass me by.

But a third reason is that the dialogue is endlessly fascinating, despite, or maybe even because, it is sometimes irritating or stupid. It can be riveting to find out how other people think. Sometimes it can make you reflect. But it can also give you an insight into what you continue to think of as epically wrong arguments.

That is what happened to me earlier this week. Angela Rayner, Labour’s deputy leader, had attended a meeting of the Jewish Labour Movement and announced she was willing to expel many people, even thousands, from the Labour Party, in order to rid it of antisemitism. And she received a reply from the Labour activist Mark Seddon.

Now, Mark Seddon is a serious figure on the Labour left. He edited Tribune for more than 10 years, served on Labour’s national Executive Committee for several years and held various moderately senior roles in the United Nations which among other things involved him in speechwriting for the Secretary General Ban Ki-Moon. He is not, in another words, an anonymous avatar with three followers.

Here is how he replied to Ms Rayner’s promise to tackle antisemitism: “Today is the International Day of Solidarity with the Palestinian People. Has Angela Rayner recorded her support and solidarity for those being oppressed?”

Startled by this I replied: “Is it your view that in order to condemn antisemitism it is necessary first to express solidarity with Palestinians? I actually live in Pinner rather than Haifa. In order to express solidarity with me you do not need to make a broad statement of foreign policy first.”

“Yes. And very clearly,” he responded to my dismay. “It would be the right thing to do. The Labour Party supports a ‘two state solution’”. For good measure he pointed out that I was a Conservative. (How do you think that happened, Mark, when it was so tempting to belong to the same party as you?)

The brazen nature of this reply was fascinating. I couldn’t have learned this anywhere else. I think his view is representative of a strand of thought on the left that is both distressing and hard to tackle.

He did not appear to appreciate (indeed when I pointed this out he said I was being “plain silly”) that he was treating me and other British Jews as foreigners.

The left are so bound up with the interchangeable use of Zionist and Jews, and so committed to their attempt to describe the antisemitism problem as merely being about defending Israel from criticism, that they have become tangled up in their own argument.

Mr Seddon appeared seriously to be advocating that before you can support a British ethnic group and protect it from discrimination, you must first adopt the “correct” position on whatever foreign policy questions attach to members of that ethnic group living elsewhere.

How bad is that? So bad that I couldn’t even make an analogy involving any other ethnic group without it rightly being considered too offensive to write in the Jewish Chronicle. That’s how bad.

Sir Keir Starmer is trying. And I’m actually very impressed by that. I think emotionally, intellectually and politically he is committed to tackling antisemitism in the Labour Party. And that means a lot and deserves proper acknowledgment.

But as the argument has gone on, the left has dug in. It goes very deep, I’m afraid. When Ms Rayner suggests that she would be willing to expel thousands of people, I’m beginning to think it might come to that.

Daniel Finkelstein is associate editor of The Times