OK, so just how do you define the public interest? It’s easy to find clear-cut examples of what is and what isn’t but, unfortunately, that doesn’t really help with the subject of press regulation. On Tuesday, lovers of fun had the option of pitching up to the Culture Select Committee, which was inquiring into the mores of the news media, after the pursuit of Kate Middleton and the sentencing of Clive Goodman for hacking into royal aides’ voicemails.

On one side of the argument is Richard Thomas, the Information Commissioner, whose job bafflingly combines beefing up privacy by enforcing data protection and freedom of information. Mr Thomas has long been outraged by private investigators and the journalists who have paid them to access phone, police, health or vehicle records. He insists, of course, that he is a believer in the public interest, but searching for “tittle-tattle about footballers and football managers” doesn’t count, and surely we’d all agree with that.

Then, there’s the counter argument. Move away from the culture committee for the moment, but when The Guardian, on the same day, printed a story alleging that Ruth Turner had concerns about Lord Levy’s evidence in the cash-for-honours affair, few would dispute that it had some kind of public interest defence.

Admittedly, The Guardian was helped by getting into dialogue with a High Court judge late in the day, to the point where the trucks with the offending newspapers had rolled out of the pressroom door. Yet, listening to a Labour MP criticising The Guardian for prejudicing a potential trial was hardly credible because it suits the interests of the same political party to subdue reporting of this topic.

So, there you have it, that’s the public interest defined for you. There’s important stuff about politics, crime and health that’s ruled in, and all that tittle-tattle ruled out. Mr Thomas points out how private detectives charge £200 to identify the owner of a number plate; £750 for a phone record, which should concern the general public. Yet, as he admits himself, the primary target in clamping down on such activity should be the private investigators, who will have clients not just in the media. After all, no newspaper has the time to go on fishing expeditions; individuals are only of media interest if they are linked to news or somebody famous. And yet, the key issue in determining the answer to the public interest question is found in the sticky middle ground: the reporting of celebrities’ private lives. Anybody who has tried to sell a newspaper knows that the reporting of celebrities interests most of the people most of the time.

This is not to say that the public interest should be defined by what interests the public. Rather, it is worth imagining what public life would be like if all journalists uncritically accepted the celebrity agenda. Every day would be like a page from Hello! or OK! magazine, and while they have their fans, most of us hanker for something different, particularly when fidgeting in the dentist’s waiting room.

Celebrities have considerable influence, like it or not, so if people who are acting as role models are flawed, the public ought to know. Nobody seriously argued that Kate Moss’s privacy was breached by the Daily Mirror when she was exposed for taking cocaine; and the welter of coverage that followed reflected how much genuine public interest there was in the story.

Some might argue that stories about, say, celebrity love lives should not be told if extracted using subterfuge. No crime after all has been committed this time, yet nobody has the right to get the coverage they demand. Anybody who works with the press to build up a career moves into public life, and in democracies everybody should be up for wider scrutiny. Newspapers are fortunate in enjoying the privilege of self-regulation. When it breaks down as it did in the Goodman affair (his problem was that he didn’t find anything of consequence) there are always calls for a clampdown — as if the media can never restrain itself. Mistakes are made, but as the Kate Middleton episode showed, intervention by the Press Complaints Commission helped to dissipate the crowd of photographers. Self-regulation can work.

The problem for those who believe in a free press is that if the only arguments that are made for the use of clandestine methods are high-minded ones, then it is easier for the critics to push back — and win — restrictions that can be used by criminals as well as the famous. The public interest should be more broadly defined, because the reporting of celebrity exists on a continuum with the reporting of something altogether more serious, a point the culture committee ought to consider.

dan.sabbagh@thetimes.co.uk

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