Who's been thinking in my mind?

TODAY'S 'Thought for the Day' is provided by the well-known psychologist Dr Ernst Dreher. All yours, doctor.

Hello. Do you know that moment when someone says to you: 'A penny for your thoughts'? And when you try to work out what you are thinking about, you realise that you are thinking about absolutely nothing at all? In other words, that your mind is unemployed? Yes, your mind, that supposedly subtle and expansive machine which, we are informed by people who are paid to know this sort of thing, is finer and faster than any computer ever built, and yet seems to have ground to a halt? You know the feeling?

If you don't know what I'm talking about, here are some more examples. You go into the kitchen to pass on some precious thought to your loved one, or perhaps just to the man who has come to look at the washing- up machine, and you open your mouth - and nothing comes out. You can't remember what you were going to say. Your mind, which in its capacity has been compared by some experts to the biggest reference library in the world, is suddenly incapable of remembering an idea you had framed five seconds previously, ready for instant use.

Or you switch on the radio and hear someone say: 'And now it's Start The Week with Melvyn Bragg . . .' and the next thing you are aware of is Melvyn Bragg saying that he will be back again next week at the same time, and you realise that you have let an hour of radio slip past without a word of it impinging on your mind. You tell yourself that it may have been an unmemorable conversation and just went easily through both ears, but in your heart of hearts you know that it's something to do with your inability to take in the sparkling talk.

Do you want to know what I think? I think the brain is not like a computer or a reference library at all. I think those are self-deluding ideas that make us feel grand. The brain reminds me much more of a supermarket trolley that we wheel round with us, somewhat crookedly, putting on a fact here, a half-baked notion there. And, of course, dropping things off as well, or forgetting to put them on.

And you know that moment when you leave the trolley by itself for a moment in aisle 18 to go back and get the paprika in aisle 17? And you have a sneaking fear that when you get back someone will have taken it? We can leave our minds behind for a moment, too. When you're driving down a motorway, for instance, and you suddenly realise you haven't taken in any of your surroundings for 10 miles and you haven't even thought about the way you're driving - well, that's because you left your mind in aisle 18 all that time.

Or maybe our minds are not so much like supermarket trolleys as space rockets - maybe our minds travel at the speed of light and visit other places while we are not using them. Sorry, we say, I was miles away, and then we laugh, but what if we were miles away? What if the expression 'absent-minded' is literally true?

Of course, I don't really think the mind is like a trolley or a space rocket. What I really think is that our minds are time-share residences. I think that we use our minds only for a certain percentage of the time and that somebody else is using them at the other times. This is why we sometimes greet total strangers, or think we recognise places we know for certain we have never been to before. It's why we suddenly think about things for no apparent reason, or why our minds suddenly go blank. Somebody else is using our minds. And they are filling them with their own memories, their own thoughts and their own fancies, and they are not always clearing everything away when it is their turn to vacate the premises.

'For oft, when on my couch I lie / In vacant or in pensive mood . . .', wrote Wordsworth. He wrote truer than he knew. Sometimes the mind is vacant. The other time-sharer has moved out and we have not yet moved back in. People are sometimes accused of being untidy-minded. Now you know why. The outgoing mind-sharer has not done his share of the tidying-up. This also explains why some people seem to have split or dual personalities. The mind is being occupied by two different people . . .

I'm sorry. I seem to have lost my train of thought . . .

Today's 'Thought for the Day' was provided by Dr Ernest Dreher, or perhaps by someone else quite different. One of them will be back again soon, we hope.