Many years ago I had a friend who was a foreign exchange dealer. He told me about a dinner he was at with other foreign exchange dealers. In the centre of the table was a pineapple. Being foreign exchange dealers, and having passed the port more than once, they started making books on how many leaves there were on the pineapple. For perhaps 20 minutes or so, they would buy and sell from each other. The dealers who made money - they were City foreign exchange dealers so they played for real bucks, of course - were the ones who bought and sold purely according to 'market forces'. The ones who lost money were the ones who obsessed about how many leaves actually were on that damn pineapple.

All well and good and clearly a parable for how market prices can sometimes bear little resemblance to the underlying fundamentals. Indeed, the real world can actually impinge on the ability of market makers to make a market.

But not a lot - and not for long. One can safely presume that over-long bulls who tried to bid the market up above, say, 500 leaves on the pineapple would soon have been eaten by the bears. Likewise, short-seller who tried to force the market below, perhaps, four leaves would soon find their lunch was in the stomachs of the bulls.

If the game had carried on for longer, eventually the pineapple would have gone rotten. Eventually the leaves would have started dropping off the leaves and the market would have headed south faster than you can say 'pineapple-backed securities'. The reality as to how many leaves were actually on the pineapple - and, indeed, going to remain on the pineapple by the time the accounts had to be squared - would have kicked in big-time.

So why am I talking about bloody pineapples.

The Conservative party over here and the US legislative bail-out bill over there both call in to question fair value accounting - the mark-to-market mechanism that some are saying is to blame for the incipient spin of mortgage-backed securities values. Tory shadow Chancellor George Osborne called for a three-month suspension of mark-to-market accounting while the bill before Congress requires the SEC to look again at the concept.

So if I get this right, we wouldn't be in this mess if only the accountants had let the bankers pretend that it wasn't happening. Better to ignore the truth of the matter, to ignore the fact that Main Street bank lenders have allowed Joe Public to borrow money with little regard to Joe's salary, Joe's ability to repay and Joe's ability to find someone prepared to pay even more for his home than Joe himself did. Better to ignore the fact that Joe's mortgage and millions like it have been swept up into big bundles, and that some of those bundles contain real stinkers (but we don't know which). So much better to believe that it will be all right in the end, and please don't make anyone pay any heed to the fact that the leaves are dropping off the pineapple faster than you can say 'pineapple-backed securities'.

You can suspend or even scrap mark-to-market accounting. But there ain't a rug in America big enough to sweep all the fallen leaves under.

Andrew Sawers