Rupert Murdoch is a great businessman. The Wall Street Journal is a great newspaper. Which of these reputations is likely to survive Mr Murdoch's prospective purchase of the Journal? Despite the high price he offers, "his" is the plausible answer.

Business magazines and business television stand on the shoulders of the few newspapers that do the reporting, and analysis, day in and day out. The Journal is the world's leader, if only by circulation. It is also two papers in one. Its news coverage is independent, questioning and authoritative. Its research is superb and its editing professional. Meanwhile, its editorial pages are the engine of US conservatism.

The WSJ's readers should imagine what their world would be like if it disappeared. The obvious response is that the WSJ is not threatened with disappearance. But it is threatened, instead, with inclusion in the world's most dynamic media empire. There it will reside alongside such beacons of excellence as the UK's News of the World and the New York Post.

Why should that matter? After all, Mr Murdoch owns some better products: the perennially loss making London Times, for one, and the more profitable Sunday Times, for another. Many admired former colleagues of mine work for News International.

Yet I wonder how many even of his admirers would argue that Mr Murdoch, for all his successes, has created even one serious, authoritative and truly independent newspaper. That is not what Mr Murdoch is about. He is a populist, a lover of tabloids and a brilliant businessman. We already know three things about his influence on his publications.

The first is that the editorial line will almost certainly be pro-business and conservative. In the case of the WSJ, this would mark no change from its present line. That was untrue in the case of the New York Post, to take a celebrated earlier example. Indeed, since Mr Murdoch is sensitive to changes in the political mood, he may take the WSJ's editorial line to the left (it can hardly be taken to the right). But wherever it ends up, it is unlikely to be as independent.

The second thing we know is that the paper is likely to become shriller and more populist, across the board. Down-market is the direction Mr Murdoch knows. That has been the direction in all of his publications with which I am familiar. Mr Murdoch can take substantial credit for the tide of vulgarity that now floods the UK. For good or ill, he has helped transform my country.

The last and most important thing we know about Mr Murdoch is that business considerations matter. The controversies are numerous: over Australian coverage of Ansett airline, half-owned by News Corporation; over the dropping of the BBC from Star TV, whose coverage Mr Murdoch wanted to stretch into China; over the publication of the memoirs of Chris Patten, the UK's controversial last governor of Hong Kong; and over coverage in The Times at the time of the Hong Kong handover.

For the readers of the Journal, this last issue must be the most important consideration. Many newspapers exist to entertain. Business papers exist to inform. Any doubt about the accuracy and independence of their coverage destroys their value as tools. Such questions are, in this case, inevitable.

A notionally independent board can be created to protect editors' independence, as at The Times. But in the end, editors must be aware of the interests and prejudices of the people who employ them. In some cases, they can be sure that the owner will not interfere. Where Mr Murdoch and a paper as influential as the WSJ are concerned, how credible could that belief possibly be?

A cynical employee of the FT might argue that any faltering of the Journal can only be to its benefit. I am not that cynical. The world needs at least two respected, editorially independent and authoritative English-language business papers. One is too few. None would be a catastrophe. Great newspapers are more than just businesses. They provide the public good of reliable information on which our knowledge-intensive society depends. Competitive markets do not provide public goods well. We may discover quite soon just how bad at it markets can be.