I am not a cognitive neuro-scientist but I recently met one at a party. He told me rather a lot about memory. I've forgotten most of it, of course, but I do recall him telling me that memory comes in long-term and short-term varieties and that neither is much cop. The short- term memory's feeble; the long- term memory's fictional.
Every second of every day we are besieged by trillions of bits of sensory data. Most of it simply bypasses the memory banks and heads straight for the great sea of oblivion. If it didn't our heads would soon bulge out to one side and then burst.
Apparently the short-term memory, even when in mid- season form, can only hang on to about half a dozen separate bits of stuff.
Permanently excluded from that half dozen are the name of the person you've just been introduced to and where you put your glasses. But at least the short- term memory is honest. What little it hangs on to is by and large true.
The long-term memory is a lot more capacious but a lot less honest. It works like a Marxist historian. It revises the past.
Imagine you lived in Alaska and were attacked, as happens frequently in those parts, by a grizzly bear.
Your response would be threefold. First you would fight to survive. The correct way to do so, apparently, is not to run, scream, sing a hymn or kick the bear in the crotch. That sort of carry-on only keeps the bear interested, especially the hymn-singing.
The thing to do is to lie very still and wait for the bear to get bored and go away.
I'm not sure I could manage that but it comes naturally to Alaskans thereby allowing them to move on to the second stage of the process which is to go to hospital for repairs. The third and final stage is to go to a party to tell people about your battle with a bear.
Because the event was traumatic, your memory will have retained a vivid and reasonably accurate record of it. So you corner your victim between the fridge and the cooker and tell him about it.
Halfway through your riveting life-and-death narrative, he glances over your shoulder, pretends to catch sight of someone he knows, promises to be back in seconds and you never see him again.
You now have two choices. Either you accept that your grizzly encounter was nothing startling on the Alaskan cocktail circuit.
Or you resolve to tell it better. In order to tell it better you heighten the good bits and expunge the dull bits. You turn it, in other words, into a story. And at the same time you turn it into a new memory.
Because according to Mr Neuro, and who am I to argue with science, though I did for a bit out of habit, the long-term memory resembles a computer program that keeps receiving updates from Microsoft Central.
Every time you tell a story, the memory erases what actually happened and replaces it with your latest version of what happened. Eventually, there's none of the original program left and the truth and your story have had a divorce.
Mr Neuro also told me that the memory, in common with most of us, doesn't like nasty things. So it tends to discard them or soften them. We can remember, for example the fact of feeling pain, but we can't remember the actual sensation of pain.
All of which seemed to me to explain rather a lot.
It explains why mothers are prepared to have a second child. It explained why Messrs Glenn and Peters can tell conflicting versions of the same event and both believe they're telling the truth (though I'll wager a pig to a peanut that one's far closer to the truth than the other).
It explains why people keep buying new pieces of exercise equipment. It explains why the sun always shone when you were a kid. It explains the enormous importance we attach to stories. It even explains why people continue to travel. "Travel," said Paul Theroux, and he's done a fair bit of it, "is only glamorous in retrospect."
But retrospect is all we've got, and it tells fibs. So we sit on our suitcases to close them once more and head off to the airport like innocents.
And it explains nostalgia. Today is just a welter of sensory data that is neither satisfactory nor coherent. But 20 years from now, September 2008 will be the wonderland of yesterday when the weather was perfect, kids knew discipline, people knew their neighbours, everyone pulled together, the All Blacks were invincible and petrol was so cheap they effectively gave it away.
"The past is a foreign country," wrote L P Hartley, "they do things differently there." It isn't. They don't. We've merely rewritten it in our heads.
