If there is one mistake you should avoid at all costs, it is to be interested in a particular subject and then listen to politicians debate it. If you thought you were ignorant at the beginning, you certainly will be by the end. So it was with David Cameron’s unveiling of defence “investment”, he called it. I got the general gist of it. We’re spending a lot of money on weapons of various sorts.

Cameron’s opening gambit was like listening to a party political broadcast, which it actually was, really. “Economic security”, “long term economic plan”, “strong economy” and that sort of thing. Oh and our “long term economic plan”. On no account forget that one. He took a while to hit his stride but by the time he did I was drowning in a sea of statistics. £198 billion here, two “rapid strike brigades” there. There was much more in between. More frigates, more planes, more pens for all those extra civil servants to push. Dave was on one.

It all sounded pretty impressive, except that if you read the small print, the “rapid strike brigades” would not actually be in place for 10 years. It was hard to be impressed with this news since, so far as I am aware, the small matter of terrorism is here already. I very much doubt whether a bunch of psychopathic islamic fascists could be persuaded to just hold fire with any potential beheadings and suicide murders at least until we’ve got the boys in place.

But, love him or hate him, Dave is good at this sort of thing. He was rather rushing through his speech, in the manner of someone looking forward to a good claret, but he was clearly familiar with his brief, which was more than you could say of Jeremy Corbyn.

The Labour leader’s “spontaneous” reply had plainly been scripted long in advance of Cameron’s announcement and it sounded like it too. He paused occasionally to scowl at the baying Tories opposite as well as losing his place and running out of breath during some of the longer points and questions. Worst of all, you could see his heart wasn’t in it. Praising the military, calling on the government to stop cuts to the police – these things all stuck in his throat – and it sounded like he didn’t mean a word of any of it.

Once Corbyn’s ramble had ended, Cameron climbed smoothly to his feet and dealt with, not necessarily answering, Corbyn’s points, punctuating a good number of them with quotes from Corbyn and his shadow chancellor John McDonnell to devastating and toe-curling effect. The worst thing watching it was how easy it all was for Cameron. The PM is no Churchill, no Thatcher and not even a Tony Blair but he’s a decent parliamentary performer. And when he is up against someone who is, incredibly given his experience in the House of Commons, such a lumpen and dull performer, Cameron turns into some worldly wise statesman.

And Cameron stayed on his feet off and on for two more hours, easily dealing with questions from both sides of the House with increasing confidence, much of it surely gained from seeing off Labour’s latest leader. I am just glad it was on the Parliament channel and not on BBC 1.

The chamber gradually emptied as questions continued, especially the Labour front bench which, after an hour consisted solely of Corbyn, Hillary Benn and Dianne Abbott, the latter of whom was texting throughout the debate, having presumably finished writing all her Christmas cards during last week’s meeting of the Parliamentary Labour Party (PLP). All in all, it was a sad, not to say pathetic sight.

Later this week, David Cameron will return to the House to argue that Britain should join in the bombing of ISIS, along with our friends from…er…Saudi Arabia amongst others. I might normally say something along the lines of “Good luck with that one, Dave” but I am not so sure now. To date, he has not put a case to the public let alone parliament as to why we should start bombing. I left Labour when Blair took us into Iraq. I am not saying I was lied to but it felt like it once the dust had settled. Iraq was a disaster, still is; Afganistan was a disaster, still is. What will be different about Syria?

We know that British planes would fly only for political reasons because there is already sufficient firepower over there, so what happens if one of our planes crashes, the pilot is captured and then burned to death in a cage? In an air campaign which was political, symbolic. And when we go in, then what? What are we going in for, what will we achieve and how will we get out again? Questions, questions, Dave.

Corbyn needs a few answers too. It’s becoming clear that he is a pacifist, so he will vote against military action whatever parliament decides. What policy will he instead advocate?

As Andrew Rawnsley pointed out yesterday in the “Observer”: “The next time a murderous jihadi is located in Raqqa, perhaps the Labour leader will volunteer to parachute there and perform a citizen’s arrest.” It’s funny because it’s too near the truth. Of course it would have been better of Mohammed Emwazi had been captured, but how to do it is slightly more problematical.

After hearing today’s debate in parliament, I feel I know less than I did before, but then I was watching politicians, not experts in their fields, so that’s not a surprise. I’m just hoping Cameron and Corbyn can come back on Thursday and persuade me one way or the other. I’m not holding my breath.