For most people the word "essay" conjures up memories of school, and it's usually the essays you didn't want to write that stick in the mind. In Our Mutual Friend, Dickens made it the embodiment of pedagogical narrowness in the figure of Miss Peecher, "who could write a little Essay on any subject, exactly a slate long", and whose essays were always written "strictly according to rule".

Yet the essay is one of the most unruly genres around. It defiantly resists categorisation. Even the great lexicographer Dr Johnson must have known that his own dictionary definition of the essay as a "short, undigested piece" fell short - it could hardly encompass the 600 pages of systematic philosophy contained in John Locke's Essay Concerning Human Understanding of 1689.

That the term can embrace such a miscellany - from narrative to polemic, from flippant to serious, from personal to scientific - says much for the essay's status as the ultimate outsider genre. Throughout history, it has stood outside institutional authority, and usually, whenever it seems to have entered the mainstream, it has withered, only to be reborn again on the margins, whether in the Romantic individualism of Thomas De Quincey and William Hazlitt, or through the subversive decadence of Oscar Wilde and MaxBeerbohm, or in the rigorous anti-authoritarianism of George Orwell, whose short weekly essays in Tribune appeared under the banner "As I Please".

Perhaps because the form is so protean, the history of the essay remains relatively uncharted by scholars despite its richness. Yet unusually among literary genres it can trace its origins to a single moment and a single man: Michel de Montaigne, whose Essais appeared in 1580. The word he coined meant an "assay", a "trial", a "test"; and thus, a sense of experiment, provisionality and questioning were written into the idea from the start. Though his matter was various - from cannibals to sorrow, from liars to parental love - his true subject was himself. As Hazlitt later put it, Montaigne "did not set up for a philosopher, wit, orator, or moralist, but he became all these merely by daring to tell us whatever was in his mind".

Montaigne arguably invented not just a genre, but the modern notion of the author as a subject (in both senses). But the proto-Enlightenment philosophers of the 17th century, such as Locke, attempted to transform the essay into an objective positivistic form. Perhaps reflecting the scientific idea of an assay - laboratory analysis of a metal to determine its ore - they used the essay to test their theories. John Wilkins's Essay towards a Real Character, and a Philosophical Language (1668) even attempted to invent a system in which every word corresponded perfectly to one particular thing in the universe, and every thing to one word. Its aim was to eject ambiguity, uncertainty and relativism - the very qualities out of which Montaigne created the essay form, which for him had been the opposite of a transparent language: prose with a voice.

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