When was the last time Mr Cameron walked into a shop and bought something?

Cameron, Clegg and Osborne are what used to be called ‘men of independent means’.

Thanks to the way Sarah Montague reported it on Friday morning, and the fact that I hadn’t had my first cup of coffee, I thought our Prime Minister didn’t know the price of petrol: “David Cameron told an American student yesterday that he would probably faint if he knew how much we pay for petrol here.”

But it makes you think, doesn’t it? I’m sure Dave does know how much a gallon of petrol costs, but only because he has people around him whose job it is to make sure he knows. Because the alternative — a PM being caught out not knowing the price of petrol, or milk, or butter — would be political kryptonite. It would be the equivalent of an American presidential candidate being asked who the leader of Pakistan is and saying: “General. I can’t name the general. General.”

It would be an especially sensitive subject for Cameron, Clegg and Osborne, because they are what used to be called “men of independent means” — so independent they could probably afford their own personal fleet of Waitrose delivery vans, all disguised with a Lidl logo in case the press saw them.

Nowadays prime ministers are obliged to watch state-of-the-nation briefing films prepared by their spin doctors: these give a snapshot of the things they might need to know in any given week: anything from who is number one on the bestseller list, to what the latest plot twist is in Coronation Street. And of course the price of butter, milk and bread.

Margaret Thatcher, the grocer’s daughter, always prided herself on knowing the price of things. She based her economic policy on the principle that managing a budget for a country was no different from managing one for a household. But there is a difference between knowing the price of petrol, milk or butter and “feeling” it in a visceral way — realising how much lighter your wallet feels when you buy things.

It’s not Cameron’s fault, but when was the last time he actually walked into a shop and bought something, when it wasn’t part of a publicity stunt? Does he even carry money? Multi-millionaire businessmen are divorced from everyday experience, too. When I met Gerald Ratner last year he told me that one of the hardest things he found about losing his jewellery empire (after that unfortunate remark) was trying to remember how to do everyday tasks, such as filling a car with petrol. It was something that was always done for him by his chauffeur. He got it wrong and spilled petrol down himself.

We all suffer from this a bit, even those of us who aren’t multi-millionaires or politicians. After hearing that item on the news, I asked a straw poll of parents at my children’s school if they knew the price of petrol, butter and milk: estimates varied, partly because of three-for-two offers, partly because so many of us still think in pounds and pints, not kilos and litres, partly because credit cards distance us from the cost of things.

On a macro scale, this is where the economy went wrong in 2008. People thought of money as an abstraction. A derivative is a financial instrument that allows bankers to trade in “paper” that has become detached from the assets that give it value. But what is a derivative of a derivative of a derivative? It’s through the looking glass, is what it is. By the way, in my straw poll most people thought a gallon of petrol was about £1 (they were probably thinking of litres). It’s actually £6. (Faints.)