by Brian CathcartLet us try for a moment to see the actions of the Metropolitan Police in relation to apparent leaks of information to the Guardian from the point of view of senior police officers.Operation Weeting, led by deputy assistant commissioner Sue Akers, came into being because of a failure by the Met to pursue evidence of wrongdoing. The original investigation into phone hacking in 2005-7 was closed down after two convictions even though there were abundant grounds to delve further into the activities of News of the World journalists associated with Glenn Mulcaire. Detectives had good leads, in other words, and they chose not to follow them, and for that they have since been rightly condemned. Now Akers, alongside her investigations into hacking and also into improper payments to police officers, is apparently dealing with a leak or leaks of information to the Guardian, notably in relation to the Milly Dowler story which broke in July. The Met clearly takes the view that this leaking may have been illegal. It may be (and I am not saying I agree with this view) that senior officers feel that, in the circumstances, they are obliged to pursue the matter to the utmost. They have the lead, so they have no choice but to follow it. A detective constable has been arrested and the Guardian’s Amelia Hill has been questioned under caution. Now the Met is seeking to use the Official Secrets Act to force the Guardian to disclose its sources. If it is heavy-handed, they might say, so be it. How could they justify showing the Guardian a consideration they would not (now) extend to News International?I don't want to appear naive about the police, or complacent about the Guardian's present position, because I am not, but let us say for a moment that what I have described is indeed the explanation. In those circumstances, are the police wrong?To a journalist the answer is obviously yes. We do not reveal our confidential sources. (This is a matter of principle but also of practicality, since we could hardly expect people to confide in us otherwise.) If Guardian reporters have sources in the police or elsewhere telling them about hacking then most journalists would be appalled if the paper identified those sources to the police. Even if, by publishing the leaked information, the paper risked impeding the prosecution of a suspected offender? (That, according the the paper, is the relevant passage of the Official Secrets Act.) Yes, even in those circumstances I’m sure most journalists would say the paper was right to remain silent. You don’t reveal sources, and reporters have gone to prison for refusing to do so.Moreover, the Guardian can in this case point to the dramatic consequences of the Milly Dowler story as evidence of an overwhelming public interest in publication. Without it, the dramatic events of the summer (public debate, resignations, further revelations, establishment of an inquiry) might never have occurred. But to the police, and possibly to the public, these things are not so clear. The Met to believe that the Guardian has in its possession evidence of illegal leaking by a police officer and they want that evidence to build a case. They assert that they do not believe the Guardian has a public interest defence. It is not a simple matter. There is a risk that journalism will appear to be placing itself above the law -- giving itself the right to choose when to obey the law and when not to. Who else is allowed to do that?The public interest is obviously the key here, but the public interest itself is not a clear concept to the public (partly, it has to be said, because unscrupulous people mainly in the tabloid press have been grossly abusing the concept for many years. And it is obviously unclear to the police, who have chosen to force the issue in a very heavy-handed fashion.It is time for clarification, perhaps by the courts but much better by the Leveson inquiry, which can look at matters in the round. Let us have a definition of the public interest, a test which can be generally understood and which leaves no doubt, for example, about the gulf of ethical and moral difference between what News of the World journalists were doing and what Guardian journalists are supposed to have done. It may be difficult; such matters often are. There is no absolutely satisfactory definition of manslaughter, or of privacy, or even of human life, but in those spheres we have not been afraid to try. With public interest journalism there will always be tough cases, but we can certainly produce a practical, general definition for our times. Brian Cathcart teaches journalism at Kingston University and is a founder of Hacked Off. He tweets at @BrianCathcart