The beginnings of the answer can be found in a just-published history of the popular press by the Guardian journalist Matthew Engel. In his preface, he makes an important confession.
"I have come to realise," he writes, "that it is a great deal easier working for a paper like the Guardian, where one is, in essence, writing for one's chums, than for a newspaper with whose readers one might not instinctively identify."
Engel identifies two quite distinct relationships between a newspaper and its readership. In the first, the paper is at one with its audience - "one's chums" - but, in the second, there is a discontinuity. If, he suggests, he were to write for a tabloid, he would have to perform an imaginative leap, an act of identification with readers with whom he has little in common. Crudely, Guardian readers carry an earnest package of social concerns; tabloid readers are into, as Derek Jameson famously said at the launch of the Daily Star, "tits, bums, QPR and roll-your-own fags".
At one level, the gulf between reader and writer implied in the second relationship may be seen as inevitable. The spread of literacy in the late 19th century, when the modern press was formed, meant journalists and proprietors who aspired to popularity felt they had to package their material for the barely educated masses. Some of this is with us still, much of the basic technique of modern journalism consists of simplifying copy for readers who are assumed to be ignorant, inattentive or merely confused.
But, at a deeper level, something has changed. Now there is a polarisation between papers whose style is based on a sense of being at one with their readers and those which carry a more generalised, plural concept of their audience. At the extremes, the division is clear. The broadsheets as a whole aim at a diffuse audience with a wide range of brow levels and obsessions; at the other end of the scale, the Daily Mail, with an awesome, indeed legendary, thoroughness and professionalism, speaks with one voice to a readership who may not be "chums" but are certainly assumed to share a consistent world view with its journalists.
In between there is a spectrum of paper-reader relations. The Guardian is close to the Daily Mail in its assumption of shared preoccupations, but this is combined with a broadsheet inclusiveness. The Daily Telegraph retains its middle- to upper-class constituency with a new, slightly broader appeal to a metropolitan elite. The Sun speaks a stylistically consistent, generalised working-class argot to a readership that is, in practice, highly diffuse. And so on.
But it is the Mail that most thoroughly identifies with and defines its readership. It is the Mail which makes the most radical assumption that a news organisation can speak with a single voice to a socially, morally and politically homogenous constituency. This month, the paper is 100 years old, an anniversary celebrated by another book - SJ Taylor's The Great Outsiders: Northcliffe, Rothermere and the Daily Mail - and, more significantly, by a continuing phase of extraordinary success. Circulation is rising and the distinctive tone of the Mail is more confident than ever.
Yet, on the face of it, this success is anomalous. It is an axiom of most contemporary cultural analysis that society is becoming more diverse. Everything, from lifestyle to opinions, is becoming more variable, less predictable. Politics have escaped from old categories. Identifying oneself as either right wing or left wing has become almost meaningless in the face of issues such as Europe or when confronted with a fissiparous Tory party and a radically reformed Labour Party. Logically, the Mail readership should be fragmenting, fleeing from the fierce certainties of its editorial posture.
But social fragmentation may be precisely what drives people into the arms of the Mail. Anybody picking up the newspaper, however much they may disagree with its hard, often brutal conservatism, knows what they are going to get - a strong, normative, bourgeois ideology that subjects every issue to its demands for order, stability, patriotism and hard work. To even the most hardened Guardian reader, such certainties can be consoling.
There is, however, a more significant stylistic appeal to the Mail. Most journalists would agree that it is the most technically proficient newspaper in Britain and, since British newspapers are, by a wide margin, the best in the world, the Mail has a reasonable claim to be the most technically adept paper on the planet. This quality is often discussed but seldom fully understood, and it arises directly from the paper's own conviction that it is at one with its readership.
SJ Taylor's book provides some, though not enough, clues to the roots of this quality. Lord Northcliffe would lie in his bed comparing every line in the Mail with its rivals and pouncing on any evidence that his staff had failed. "Every day," he said, "there is an event which ought to be the outstanding feature of the news column." Note the certainty. Where most of us see the world as an indecipherable flux in which news decisions are simply arbitrary, convention-driven choices, Northcliffe saw it as a readable text in which the good journalist would instantly see the story. A splash was not a choice as such, it was an objective reality. There was one right splash and the rest were wrong. Equally, there was a right way to do a story and a wrong way.
His legacy is clear on every page of the Mail today. The paper is more ferociously edited than any other precisely because its staff are in perpetual pursuit of the objectively right way of doing things. They work according to unwritten rules of reportage and presentation whose rigour would glaze over the eyes of almost any other newspaper employee. Above all, they know what the Mail wants and the Mail, in turn, knows precisely what its readership wants.
This may sound frightening, it is, but it is also admirable and it may also be the key to the future of newspapers. What is incredible about the British press is its continued success. Almost daily, people point to declining circulations, but, in reality, this decline is astonishingly small in view of the proliferating competition they face. But that competition has meant that newspapers have been obliged to become more distinctive. Readers will see no point in buying a newspaper that flatly reports and analyses issues hours after they have been similarly reported and analysed on radio, TV or the Internet. Nor will they see any point in a newspaper that presents the world as chaotically undifferentiated as those other media. They will want either a carefully expanded area of interest or a voice.
The Mail, more than any other paper, provides a voice. This may be infuriating and its voice may be as utterly wrong as any other. But other voices are available. The British press still offers an energetic clamour unlike any other in the world.
And the final point is that, in some general sense, we all see the world the way the Mail sees it - partially, with a strict hierarchy of significance to ourselves and fiercely edited to our own requirements. It is a paper which, just like us, takes everything personally and that, in the impersonal future, may well be the most difficult and important thing to do. So happy birthday, Daily Mail, not that, in these hesitant but at least curious columns, it will mean a thing to you.