Quintessentially English in form, the essay allows the writer to say a little while suggesting a lot, writes Terry Eagleton.

Cynically speaking, an essayist is a writer who has no more than 20 pages' worth of knowledge of any particular topic. An essay, on this view, is a kind of verbal equivalent of a nudge and a wink: it intimates that you could say a good deal more than you actually do. While others drone on discursively, you confine yourself with winning modesty and exquisite tact to a well-wrought aper?u .

The essayist is like the experienced drunk who knows that if he allows himself more than the odd pregnant comment the slurring in his speech will be instantly obvious. You can toss off an essay, but you cannot toss off an epic.

This, to be sure, hardly accounts for a Bacon, Montaigne or Chesterton.

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Some of the most innovative philosophical interventions, by scholars such as Quine, Putnam and Davidson, have taken the form of essays. Milton's magnificent defence of free thought, Areopagitica , is hardly a full-blown book, neither are Shelley's Defence of Poetry or John Stuart Mill's On Liberty . The essay is a minor form that has often served major purposes.

It is also a form for which the English have a special affection, given their nervousness of systematic thought. No subject is too lowly for it.

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You can write a whimsical few pages on dormice or pepper pots, as you could not easily write a play or a novel on such topics. This penchant for whimsy is another reason why the English have held the essay in high regard. In the hands of a Charles Lamb or a Virginia Woolf it has the quirkily idiosyncratic quality that the natives of this island particularly admire.

It conveys the taste and texture of a uniquely individual mind, and the English love a "character" as much as they love a lord. If Byron had a fan club to outdo Mick Jagger's, it was largely because he was both.

The anti-systematic nature of the form also made it popular with maverick Marxists, such as Walter Benjamin and Theodor Adorno, who used it to challenge the traditional theoretical treatise. This kind of essay is a conscious assault on the book, which is regarded as outmoded. Some modernist thinkers did not just produce books about deconstruction; they also sought to deconstruct the book. Benjamin dreamed of writing a book that consisted of nothing but quotations, just as Ludwig Wittgenstein wanted to write one consisting of nothing but jokes.

What is often most striking about the essay is not what is said, but the quality of the mind that says it. This is why people will buy volumes of collected essays on a diverse range of subjects, not all of which any individual reader is likely to find engrossing. We buy them because we want a grandstand view of the mind of an Orwell or a Sontag at work, even if some of the topics they deal with interest us hardly at all.

The essay is a supremely individualist mode - another reason why it is so popular among the spiritually privatised English. In its looseness of structure, impressionism and open-endedness, it is a quintessentially liberal form. As the word "essay" suggests, it is a tentative, trying-it-on sort of venture, the reverse of doctrinal or didactic. It is a Protestant rather than a Catholic phenomenon, one that flies a kite rather than lays down the law. This is why the sceptical, non-committal 16th-century French aristocrat Montaigne is its rightful progenitor, a man who chose the essay form not because he lacked more than several pages' worth of knowledge, but because he lacked more than a few pages' worth of certainty.

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There are two main branches of the form, which contrast sharply. On the one hand, there is the rambling, conversational, impressionistic piece that the English find most seductive, from Sir Thomas Browne to J. B. Priestly. It is a style of writing that sacrifices structure to detail, the analytical to the elegant. It is prose saturated in the atmosphere of an individual mind, which takes its time in getting to the point with all the genial unbuttonedness of an after-dinner speech.

The other kind of essay is very different in tone. It is pithy, pungent and polemical, as in the writing of Swift or Hazlitt. In such hands, the essay becomes an activist mode, a partisan intervention on some pressing issue that cannot be postponed until such time as a book has rolled from the press. In fact, a lot of what we now regard as literature, and thus see as bathed in an aura of timelessness, first emerged in the form of pamphlets, essays or sermons in specific religious or political debates. The essay is a timely form, well adapted to local wrangles and instant ripostes.

What really ensured its future, however, was the growth of periodical literature. In 18th-century journals such as Tatler and The Spectator , essays played a key role in the formation of middle-class morals and manners. Later they became the stomping grounds of the Victorian sage, where Carlyle, Arnold, Mill and George Eliot would debate moral and social issues. The periodicals redefined authors as journalists, able to throw off a hasty piece on whatever subject readers desired. The essayist had become the hack.

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From there, the essay graduated to the specialist journal, in the shape of critical articles and scientific papers. And this, for the most part, is how we encounter it today. The brief, well-focused piece seems peculiarly well adapted to an age in which the traditional gravitas of literature has given way to the instant consumability of newsprint. The aphorisms of the classical essayist live on as the soundbites of the columnist. The essay was always an evanescent form, with no illusions about its own immortality; and this is no doubt one reason why it lends itself to modern audiences.

Another reason for its popularity is that the fragmentary, subjective and provisional - all qualities of the classical essay - have perhaps never been as obvious as in these relativist, tentative times. In this sense, the essay is a rather longer version of the recurrent "like" of the American teenager's speech, a word expressing an uncertainty that captures the postmodern sense that nothing can be said for sure. A rough metaphorical approximation is now the closest we can come to truth. (I have heard of a US professor who makes his students put a quarter in a glass for every "like" uttered in class, and who is growing extraordinarily well heeled on the proceeds.) The idiosyncrasy of the essay, a form that usually has "as I see it" as an invisible subtitle, chimes well with the postmodern sense that there are no longer any normative forms of knowledge, just partial, partisan ones. The essay is no longer a leisurely form of letters but a kind of cognitive capsule, to be hastily swallowed by those who take their knowledge, like everything else, on the hoof. It is a literary survivor, as epic and pastoral are not, but only because it is so adept at changing its functions to suit fresh needs. The death of the novel has been announced with farcical regularity, but never the demise of the essay. There will always be room for swiftly digestible think-pieces, not least in a society that finds discursive prose increasingly hard to handle. Complex plots and extended narratives are still with us, but nowadays they are known as movies rather than literary fictions.

As for myself, writing essays and producing full-length books have never felt like very different kinds of activity. But this, I suspect, is because I enjoy writing as such, regardless of the genre involved. In fact, I enjoy it so much that I am embarrassingly overproductive, a problem about which it is naturally impossible to expect the least breath of sympathy from my colleagues. It is like complaining to the Savoy management that your suite has so many rooms it's impossible to find your way around.

Terry Eagleton is professor of cultural theory and John Rylands Fellow at Manchester University. The latest symptom of his overproductivity is The English Novel: An Introduction , published by Basil Blackwell. The Times Higher and Palgrave Macmillan humanities and social sciences essay-writing prize is launched this week.

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