Philip Hensher: A portrait of us as we see ourselves– that is the true value of the census

Concerns about state surveillance have been growing at such a rate in the past 10 years that the census was never going to have an easy ride, this time round.

As it plopped on to our mats this week, you would be forgiven for reflecting that government agencies already gather more information about us than is sensible, useful or even decent. As Heather Brooke's polemic, The Silent State, shows us, in recent years government agencies have sought to collect detailed information on the mentality, health and prospects of citizens from a very young age, while consistently denying the right of citizens to discover anything about the authorities themselves.

In this country, something like four million CCTV cameras supervise our ordinary movements every day. Many millions of citizens have their DNA stored by the law enforcement agencies, whether or not they have been found guilty of any crime. Hundreds of official and pseudo-official agencies have been handed the right to carry out surveillance operations into the intimate lives of individuals, for no better reason, sometimes, than to see if we conform to the rules on the putting out of rubbish.

All this has got so very much worse in recent years that the 2011 census has been greeted by a level of outrage very much worse than last time, in 2001. What business has the Government to intrude into our behaviour in this way?

On the surface, some of the questions are indeed startlingly specific. In 2001, the census asked for the first time what the religion of the respondent was – a classic case of begging the question, since it assumes, for no very good reason, that the respondent had a religion in the first place. This time, we are asked to tell the man in the ministry how we heat our houses, and not only the general field in which we work, but the address of our employer.

In other areas, however, the census is surprisingly unspecific. It has no interest, for instance, in distinguishing between people who have acquired a plain BA and those who have gone on to further academic qualifications, including MA and PhD. All those are covered by the same box, though you would have thought that the drift towards higher and higher qualifications was exactly the sort of thing that the census ought to cover. In other questions, however, the census does seem to probe into quite private areas. This inquisitiveness is bound to lead to rebellion. Though it is compulsory to fill in the census form, last time only a handful of the three million who failed to return one were prosecuted. Many of those who did made a point of adding to the gaiety of the nation by describing their religion as "Jedi Knight" – making it, apparently, the nation's fourth most popular belief system.

Is the census needed any longer? Does its intrusion, which has always been a bone of contention in the 200 years of its history, make sense any longer, now that any number of official agencies collect almost all this data without our consent or participation? Couldn't most of this be collected, and a great deal more besides, from such intrusive pieces of private surveillance as supermarket loyalty cards? We could find out, much more reliably, what people actually earn with reference to bank records, suitably anonymised. What do we need the census for?

Plenty of people have been asking this, and the 2011 exercise may well turn out to be the very last one. If you regard the census as, ideally, an accurate account of the state of the nation, then the main justification for it is that it is much more secure than other repositories of private information. (That's what they say: since, in recent years, millions of private financial details have gone missing when some civil servant left a computer disk on a train, so I don't suppose the census is really as secure as all that.)

But there is another justification for the census which goes alongside its objective information. The census is about the only official exercise which seeks not just to gather factual information about the population, but information about how the population regards itself. It asks questions about racial self-definition, as well as chosen religious affiliations; about how linguistically capable in English the respondent thinks himself to be; about how physically able the respondent is, and other questions where opinion and self-regard play as large a role as objective fact.

These questions are of so much interest that they could usefully be expanded. Following the introduction of the religious question in the 2001 census, some discussion started of whether, in 2011, a question could be posed about sexual orientation. In the event, it hasn't happened, but it would have been a first-rate question, not about the realities of the case, but about perception and willingness. If such a question had been put in every census since the legalisation of homosexuality in 1967, the tendency of gay people to self-identify in increasing numbers would, surely, be very apparent. I understand that a movement is under way to persuade people to answer a question that has not, in the event, been asked. You are, of course, to respond to the question "What religion are you?" by responding "Lesbian".

Similarly, we would enormously value a question, if one had been put over the past two centuries of censuses, which simply asked "What social class are you?". Of course, the answer would be meaningless in objective terms. But what people regard themselves as, and how that has changed over the decades, is an interesting question, too. In fact, the sociological observation which extrapolates solely from people's professional occupations, which is what the census currently works from, is a much broader and less useful tool than a question about people's self-perceptions.

There are serious issues about the willingness of government, over the past years, to collect specific and private information about individuals, and to share that in recklessly offensive ways. In most cases, we are absolutely right to think that it is none of the Government's business to hold information about our private behaviour and to investigate our private communications.

It's a mistake, however, to conflate the creeping surveillance of government into individual lives with the project of the census. The census is bound by any amount of protection of privacy. Independent investigators have concluded that the Office for National Statistics works to a principle that no single individual should be able to identify himself from the statistics published – so, for instance, if it should turn out that there is exactly one Arab man living in Auchtermuchty, the figures may be adjusted slightly to allow for ongoing anonymity.

What the census does permit is a relatively truthful, generalised portrait of the make-up of Britain at the time, and a version of the picture Britain holds of itself, too. After two centuries of the census, its matchless historical value is thoroughly established. Its venturing into private lives is trivial compared with what any number of companies and ill-controlled public agencies undertake without any of the restraints over the ONS. It may need some refinement with the passage of time, but its value as a tool of policy and historical analysis is the same as it ever was.