Biography has become too popular for historians. Ben Pimlott defends his own art form; Writing a biography is like entering a deep cavern. The cavern is a human life, the walls are the evidence. Until you crawl around you don't know what you will find.
Christmas is coming, and biographies are getting fatter. They are also getting on to the syllabus. One enterprising university has just started an MA in Biography. Why? And why is the course unique?

More important, what is biography? This is a poser that hasn't caused me too much bother. Like a bowler who doesn't worry whether the way he sends the ball down the pitch is quite cricket, I have simply got on with the job.

There comes a point, however, when you have to take stock: and I have become aware of a hole in the literature. For despite the growing quantity (and size) of biographical works, historians seldom talk about them as a distinct form.

Silence could denote contentment with it. In this case it often denotes contempt. The truth is that many scholars look down on biographers. Some adopt the attitude of Edward Gibbon's patron who, encountering his protege at work, remarked: "Another damned thick square book! Always scribble, scribble, scribble, eh Mr Gibbon."

Occasionally, they do address the issue head on. In his 1961 classic What is History?, the historian E H Carr linked biography to what he called "the Bad King John theory of history" - namely "the view that what matters in history is the character and behaviour of individuals". This he considered archaic. "I think," he concluded, "we are entitled by convention to reserve the word 'history' for the process of inquiry into the past of man in society. Yet if Carr was entitled to exclude biography from his own definition, we are equally entitled to say that the problem is not whether narrowly self-defining historians accept it into the club, but how to establish independent credentials for biography as a way of inspecting the human condition.

First, why write it at all? One answer is that modern biographies (as Professor Patrick O'Brien of the Institute of Historical Research put in a recent article) are "media and market driven". But there is also a more elevated reply. A good reason for writing biography is the scope its tight conventions provide for springing intellectual surprises. Writing a biography is like entering a deep cavern. The cavern is a human life, the walls are the evidence. Until you crawl around, you don't know what you will find. There will always be crevices out of reach. But when your exploration is finished, you will not only have a unique appreciation of the particular cave, but a better feeling for geology in general.

Biography deals with a cosmically arbitrary, but biologically simple, unit of time and psychic space. The idea of taking a single human span, from cradle to grave, is not only easy to comprehend; it is basic to the way everybody leads their own life.

The most exciting aspect of biography is that it links together human events in the way human beings actually experience them. Thus, the intellectual case is that, so far from lying on the margin of history or any other related discipline, it is more ambitious than any of them.

Every sentient being is an instinctive biographer. Only human amoeba fail to construct chronologies of their own lives. Our self-description consists, to a large degree, of how we construct our own past. Job applicants present curricula vitae. Few authors, even historians who disapprove of biography, decline to write little biographies of themselves for the dust jackets of their books.

Indeed, the notion of the model life is central to several of the great religions. It is significant that the key part of Christian scripture consists of four short biographical accounts. If biography is so ingrained, this is a prima facie reason for regarding it as a part of a continuing literary tradition. Ah, says Carr, with a lip curl: literary, but not historical. The charge isn't hard to answer. Professor John Tosh, in his The Pursuit of History, stands Carr on his head. Of course, he says, economics and social forces are very important. But plainly, "the motives of individuals have some part to play in explaining historical events." Alas, sometimes it is necessary to state the blindingly obvious. Yet do we need to defend biography in such terms? My own claim is for the exploratory and evocative possibilities of biography. I should not at all object to a description that placed it on a cross-roads between academic precision and creative writing. Because this is delicate ground, I need to reinforce one point. Precision is as much a feature of good biography as of good history. What Virginia Woolf once called "the fertile fact" in biography, has to be an accurate fact.

The real difference between the biographer and the narrative historian lies, not in method, but in the conception of the work as a whole. Biography at its best is more concerned with impression than with presenting a case.

In the past, the intended impression has been a flattering portrait. To some extent there has been a move from hagiography to what the writer Tony Gould calls "de-bagiography" - a delight in showing subjects at their most ridiculous. Yet individuals are still frequently given an exaggerated importance. Indeed in my own view of biography, there is a core paradox. While any such work has to focus on the individual, it is not really about that individual at all. The character is a device by which the writer gains the attention of the reader. Furthermore biography is not (as Carr alleged) ever about character abstracted from an environment. On the contrary, it is character-in-an-environment that makes it such a powerful form.

To be as bold, inventive, creative, iconoclastic and unconventional as possible, within the cave walls - that should be the aim of the biographer. And here I would point to a difference between biography and history. I do not see them as truth-seekers in identical senses.

The historian builds an argument, a thesis. Biography is, by contrast, to a large extent non-logical. The historian finishes his article or book with a chapter labelled "Conclusion". The biographer, like the novelist or painter, has no comparable ending. The conclusion is the whole - take it or leave it.

Indeed, to this extent I agree with Carr: a modern biography is a deliberate deceit. It purports to show the rounded individual, but such a thing is impossible. I doubt if anybody ever recognises themselves in a biography. Dostoevsky implicitly made this point, when he dismissed the idea of auto- biography, on the grounds that there are things "which a man is afraid to tell even to himself".

Will there be a biographical Lenin, or perhaps Bill Gates - to revolutionise a genre that has kept its essential structure since Plutarch? I don't know the answer. But I am sure the oldest form of human writing will be around until the end of homo sapiens as an articulate species.

This article is based on the St Cross College Visiting Fellow Lecture delivered in Oxford last week.