The green belt, celebrating its 50th birthday this week, is one of those subjects that many of us free-marketeers would rather not think about. Our heads tell us that there is clearly a shortage of housing in many parts of the country - and particularly in the South-East - and that the answer must surely be to build more. There are places where the shortage is so acute that as many as two thirds of would-be first-time buyers cannot afford even the grottiest one-bedroom flat. That causes a great deal of unhappiness, which could be lifted almost at a stroke by handing over huge swathes of the green belt to property developers.

Our heads tell us, too, that a sound principle of government is that the less we have of it, the better. Why should politicians dictate where we can build houses, and where we can't? Why don't they leave the free market alone, to work its ancient magic of matching supply to demand?

But then our hearts kick in, with a mighty thump, and we realise that we love the green belt. It happens to me every time I drive the family west from London along the A40, to visit my mother-in-law in Oxfordshire or my sister-in-law in Gloucestershire. The first part of the drive is profoundly depressing - mile after mile of furniture warehouses, car showrooms, speed cameras and urban sprawl. The road through the suburbs is lined with identical 1930s semis, screaming of boredom and wife-swapping at everyone who drives past.

And then, suddenly, we hit the green belt and we are in the England that we all adore: shadows of clouds, scudding across fields of golden corn; church spires in the distance, with time-weathered cottages clustering beneath them. There is even the occasional cow to be seen - although there are surprisingly few of them around, these days (something to do with the Common Agricultural Policy, I imagine). Oh, how tragic it would be if all this beauty were to be violated by hideous new housing estates, supermarkets and a thousand more branches of McDonald's.

I know that I am not alone in these feelings, because a survey commissioned this week by the Campaign to Protect Rural England found that 84 per cent of us want to save the green belt, and to ensure that it remains open, undeveloped and free of new buildings. Yes, of course we should be wary of surveys commissioned by pressure groups, since they always seem to find whatever it may be that those who commission them want to find. But this one rings true to me. In all my 51 years on this earth, I have yet to meet anyone who has come across a honey-coloured village in the Cotswolds and said: "Wouldn't this be a lovely place to build a Barratt estate?"