When I finally saw the Mona Lisa, after my 50 years on this Earth, I found it a terrible disappointment. I don't know quite what I was expecting. I suppose that I was hoping at least for some glimmer of understanding of why this was the most talked-about painting in the world - at best, for a rush of joy at the sheer beauty of this, the real thing.

These philistine thoughts have been prompted by my first ever visit to the Louvre, where I went with my family last week. Like so many other visitors, we made a bee-line for the Mona Lisa, because everyone knows that this is the painting that you just have to see when you are in Paris.

But, if I am right, most of us don't get a great deal of pleasure from paintings - and we wouldn't appreciate them very much no matter how long we stood gazing at them.

. . . I dare say that millions will go on paying their entrance fees to stare at paintings that they don't really appreciate - half pretending, half longing to enjoy them. I shall certainly be among them.

A mere difference of attribution can make a hundredfold difference in the amount that people are prepared to pay for a painting. "After" Vermeer? Let's call that £140,000. "By" Vermeer? Stick a couple of noughts on the end. Never mind that the picture is exactly the same - no more or less beautiful - whether it was painted by Vermeer or the girl next door.