Five-Minute Memoir: Alexander McCall Smith on the day he thought he'd purchased a Raphael

Some of us are collectors, and some are not. You usually find out when you are quite young. You have more model cars or dolls than others; something's brewing. As a teenager, I had large piles of The Listener and The Boy's Own Paper. Then, since the age of 18, I have collected pictures, starting, as I suspect many of us did, with posters.

The buying of posters was part of the rite of passage of leaving home: you went off to university, where you had – oh the heady delight of it! – a room. This room had to be decorated, and this was done by buying posters. In my day, a poster of Che Guevera wearing a beret was more or less mandatory; everybody had that poster, except for those who had Peter Fonda on his motorbike from Easy Rider. I had neither Che nor Fonda, but had, instead, a large poster of a pre-Raphaelite picture. That was quite a statement, I thought, but of what exactly I cannot quite recall.

The next step, the purchase of an actual picture, was crucial, and showed the way things were going to go. That came some time in my twenties, the age at which it occurs to most people that posters are, well, posters, and the urge is felt to replace them with pictures. This is the stage at which you realise that some pictures are what people called originals as opposed to reproductions. Originals were owned by the next generation up – aunts, parental friends, and so on. They were usually rather twee – views of the Falls of Clyde, horses in a field and so on – and viewed with condescension; they were hardly much of an improvement on flying china ducks.

So you bought your own picture; I bought a print by a Danish artist who then lived in Scotland. It cost me £8, the equivalent of about £60 today. I was immensely proud of it, especially of the fact that it was signed and numbered. I pointed that out to friends: "Signed and numbered," I said, knowingly. Usually these friends had nothing signed and numbered themselves, and were impressed.

Time marched on, and I graduated from prints to the cautious purchase of works at the very bottom end of exhibitions. That, of course, is another milestone: to have a red dot put on a picture in a gallery and to know that the red dot means the painting is yours. This is a great pleasure.

I now started to buy paintings that, although not expensive, were by recognised artists. That is another stage in the collector's career – moving from artists of profound obscurity (in extreme cases, unknown even to their friends) to artists mentioned in reference books. I took an interest in Scottish art and, then, it was possible to get paintings by recognised 20th-century Scottish artists for very little.

My collection grew, although it was – and remains – a modest one. What I hoped for was the great discovery. One reads about this – Sir Timothy Clifford, the distinguished art historian, is always finding old master drawings under people's sofas or in dusty boxes in museums. Why should the rest of us not be able to do that – even if only once? This goes to the heart of the collecting impulse: the discovery, the sheer thrill of the real find.

I lived in hope… and then one day my mobile went. It was Guy Peploe, who runs the Scottish Gallery in Edinburgh. "There's a van Huysum coming up in a local auction," he said. "Interested?" Of course I was, and the following morning, Guy bid for it on my behalf and obtained it at a very good price. Jan van Huysum is regarded as the greatest flower painter of all time. It was a lovely picture – not very valuable, but very beautiful.

But the pleasure would have been so much greater had I bought it as something else and then discovered its real nature. That is what collectors really want to do.

Then, a few months ago, I saw in an auction catalogue what was described as a fragment by Ridolfo di Ghirlandaio. It was not at all expensive and I was able to get it. I gave it to the restorers and they started to remove the over-painting. Suddenly, a whole new area was revealed underneath. I showed it to a friend who is an expert on Italian art. He said: "My goodness! Very, very close to Raphael!"

Heart in mouth, I arranged for a further opinion. The result: probably Ridolfo Ghirlandaio, as the auctioneers had suggested. Oh well. I don't really want a Raphael. No, I do. All of us would love to have a Raphael. But that's what collecting is all about: the pleasure is in the searching, not the finding. Or so you must tell yourself. And I do.E

'Sunshine on Scotland Street' by Alexander McCall Smith is published in hardback by Polygon this month. To order a copy at the special price of £15.29 (usually £16.99), including p&p, call Independent Books Direct on 0843 0600 030