Hell is other people. So said Jean-Paul Sartre and it’s hard to argue with him when you’ve been stuck in a jam on the M5 to the West Country for three hours or had your nose rammed into someone’s smelly armpit on the London Underground for the length of your journey. Just as well that we’re all having fewer babies and the population is falling. Except that it isn’t. It’s rising.

The last census two years ago counted 58,789,194 of us and they missed about a million for one reason and another. That is an increase of 17% compared with 50 years ago. We are the fastest growing of the large countries in Europe. According to official projections there will be 65m of us by 2036 and then it may fall a little. But some independent experts say we shall hit 70m around 2050. Worrying, eh? Well, maybe.

Governments are notorious for never looking beyond the next election. When Harold Wilson was accused of being brilliant at tactics but hopeless at strategy he denied it. “It’s quite untrue,” he is reported to have said. “I think at least six weeks ahead.” But some departments today are thinking way beyond that. “Horizon scanning” is the jargon.

The Department for Environment, Food and Rural Affairs is worrying about vital services such as water and waste disposal — where it’s all going to come from and where it’s all going to go. The result of its worrying is that, for the first time, we may have a real population policy. We will have to take population growth a lot more seriously than we have in the past.

I’ve always been a small-is-beautiful man myself. Normally I’m with people like Sir David Attenborough and Jonathon Porritt who support an outfit called the Optimum Population Trust. They not only want to halt the increase in our population. They want to cut it. We’re not talking about a little trimming here and there. Chop it in half over the next 30 years, they say. Cut it to 30m — the same as it was in 1870.

Theirs is essentially a green argument. We must consume less. And anyway our island nation is already too crowded. The roads are jammed, the hospitals struggle to cope and class sizes are too big. You can’t get away from people however hard you try. And the countryside is covered with concrete. My heart is with them. My head says it is nonsense.

You can’t get away from people if you enjoy Devon in August but you can if you like walking in the Cambrian hills. It depends where you want to go and what you want to do. As for the myth of the endless concrete jungle, try driving in a straight line from where you happen to be now. Unless you’re smack in the middle of London and the rush hour is under way you will pretty soon be in some pleasant countryside. And then, if you dare to leave the car and set out on foot, you will soon be alone. That’s if you want to be. Most of us don’t.

For a long list of reasons most gravitate to where there are already lots of other people — whether to holiday or to live. As the population increases they end up in the big conurbations, especially the southeast. And what’s wrong with that? London is sparsely populated compared with, say, Hong Kong. Lord Rogers, the architect, has been banging on for years about how cities become more vital and stimulating if we agree to live in greater densities. London has the best theatre and the greatest diversity because it has the most people.

The National Health Service is struggling, not because there are too many people but because we have yet to find the best way to run it. Education was not twice as good when we had half the population; class sizes were actually bigger. Roads are congested during peak hours but the solution is not to reduce the population so there are fewer cars. It is to make us use our cars less at the times of greatest congestion.

Then there is the economic argument. Again the head wins over the heart. In the columns of this newspaper’s Business section last week Christopher Smallwood, economics adviser to Barclays Bank, produced one of the most powerful economic statistics I have ever read. The American economy is about 20% bigger than that of the European Union. By the middle of the century it will be 2Å times the size.

So much for the EU “challenging” the supremacy of the United States. And the main reason is population growth. Today Europe has 100m more people than America. By 2050 there will be 40m fewer of us than them. More specifically, the working population of the EU will decline after 2010 whereas America’s will keep growing. After 2025 it will accelerate. The economic argument is clear: we cannot afford not to increase our population.

There is a lot to argue about here. Economic growth is all very well, but what about the pollution it produces and the resources it consumes? Have we learnt enough to deliver growth that is environmentally sustainable? On this one, I’m a bit of a Trotskyite. You can’t be a good green for one country. Reducing the population of the world is an admirable ambition. Doing it for us is a bit beside the point. The trick is to get people, whatever the number, to use the world’s resources sensibly.

All these things can be debated in a civilised way. There is an issue at the heart of the population debate, though, that has defied rational discussion. Politicians go green around the gills. Normally sane and sensible people begin to foam at the mouth. It is this: if we agree that there should be more people, where will they come from? The answer is: where they are already coming from.

In part, they come from the ranks of the old. In only a few years there will be more people of pensionable age than there are children. That has never happened before. There are two reasons why it is happening now. Life expectancy continues to increase and the birth rate to fall. You need a birth rate of 2.1 children per woman for a stable population. It is now 1.65.

But the big reason for our increased population is immigration. In the next 50 years the experts reckon that about two-thirds of the extra people on these islands will be immigrants. Some believe that is an underestimate and the real proportion will be more like three-quarters. The trend is well under way. The figures have been going up rapidly since 1997. And it is happening because the government wants it to happen.

It is intended to allow about 150,000 immigrants a year into Britain. The reason is contained in Smallwood’s statistics: the economy. Even these extra newcomers won’t do the trick. The numbers still won’t quite add up. Immigrants get older, too. Which means we either persuade people to work longer and have more children or we accept an increasing number of immigrants. Or we constrain immigration, allow the population to fall and settle for a lower rate of economic growth.

This tends to put the immigration issue in a different light. For most of the past 30-odd years each side in the debate has simply anathematised the other. If you oppose immigration you are a racist. If you support it you are a smug liberal with a nice house in a nice neighbourhood whose only experience of immigration (legal or otherwise) is that helpful Asian newsagent on the corner. Margaret Thatcher used the emotive word “swamped” when the Tories were still in opposition. When David Blunkett used it more recently there was blood on the carpet. Real argument did not happen. Now it is beginning to.

Already it is revealing some unlikely alliances. Labour’s position (or at least its rhetoric) was always pro-immigration. The Tories sounded broadly hostile. Yet Oliver Letwin, the shadow home secretary, sounds anything but. He is careful to observe that we live in a “small and crowded island” but says that where the economic case can be made, immigration should be welcomed. On the other side is Bob Rowthorn, the left-wing Cambridge economics professor. He dismayed the bien pensants by writing a thoughtful essay in Prospect magazine warning of the threat to our national cohesion if the prospective increase in immigration is allowed to happen.

So let’s have the argument. And as we do, I’m with Sartre. I shall ponder these issues while I walk the Cambrian hills.