“The responsibility lies with the perpetrator.” This was the defence Theresa May brought with her to Prime Minister’s Questions, to use in the face of inevitable attacks over the ongoing knife crime epidemic.

But this is the trouble with knife crime. You carry a weapon nominally in self-defence, but if you think the other guy’s got a knife too, you pull yours first and things end badly for all involved.

And this is exactly what happened. “The responsibility for these attacks lies with the perpetrator.” She’d said it before Jeremy Corbyn had even asked a question.

Fortunately for her, as weapons go, this was more a spoon than a knife. If she’d been stopped and searched on her way to the chamber, she’d have been laughed at.

They’d have laughed at her, because it’s not a defence at all. It’s not an attack. It’s not anything. It’s barely even a noise.

“The responsibility lies with the perpetrator.” Indeed it does, prime minister. And the responsibility of the government – i.e. you – is to prevent the perpetrator from doing it. It is one of the things, perhaps even the single biggest thing, through which the government claims the right to collect taxes.

Which is, why, the fact that violent crime has shot up at the same time as police numbers have been cut by 21,000, all done by Theresa May, first as home secretary and secondly as prime minister, the blame is being laid at her door.

And which is why, saying, “the responsibility lies with the perpetrator,” is laughable. Which is why, the Labour benches laughed. And her own sat there in stony silence.

The families of the victims of knife crime do not want to have to sit there and be told by their prime minister, that the “responsibility” for what has happened to them “lies with the perpetrator”.

There’s a glimmer of a chance they’re aware of that. What they’re interested in is the responsibility of the government to stop it happening at all.

Arguably it is a window into her entire world view. Why cut a mere 21,000 police officers? Why not just get rid of the entire force, and then “solve” the immediate crime tsunami by driving around town with a loudhailer screaming, “The responsibility lies with the perpetrator! The responsibility lies with the perpetrator!”

She said she wanted to get to the “root causes”. Arguably, the “root causes” bear some resemblance to the “burning injustices” she once stood outside Downing Street claiming to want to tackle, all of which have got worse, and none of which she has done anything about. In the meantime, as far as the victims are concerned, the “root causes” are that there is no one around to prevent any of it from happening. The “root cause” is that they can get away with it, because no one is stopping them, because basic public services are not being provided.

For his part, Jeremy Corbyn took a now rare dip into his inbox, and read out an email from “Mike from Gosport”.

Mike from Gosport has a simple thing to say. “The police do not have the resources. Violent crime has doubled. This has become a really unsafe town to live in.”

As he spoke, Jeremy Corbyn that is, not Mike from Gosport, the Conservative benches sat there in stony silence. Most of them hadn’t even bothered to turn up. Jeremy Corbyn is used to this kind of thing from his own side. Theresa May, frankly, is not. There is no more certain evidence that the Conservatives know they are so hopelessly in the wrong than that they can’t even steel themselves to take random swipes at Jeremy Corbyn at Prime Minister’s Questions. It has never happened before. Not quite like this, anyway.

This is an issue that is not going away. A better defence is urgently needed.