"Fernando Pessoa, strictly speaking, doesn't exist." These words were written by Alvaro de Campos, naval engineer, opium-eater, absinthe-drinker, dandy and futurist - and one of at least 72 "heteronyms", fictive identities through the medium of which Pessoa produced some of the most remarkable poetry and prose of the 20th century. C R Anon, an English philosopher who wrote with a sharply anti-religious edge on questions of free will and determinism; Ricardo Reis, a Portuguese monarchist, neo-Horatian poet and sometime Latin teacher; Alberto Caeiro, the pagan author of The Keeper of Sheep, a poetic critique of language and metaphysics as subtle and forceful as anything to be found in Wittgenstein - these are a few of the personae adopted by Pessoa in the course of a life that anticipated and embodied the post- modern condition, in which no one has a single, stable identity. Of Pessoa's alter egos, the closest to the man himself - if such a person existed - was Bernardo Soares, the fictional author of The Book of Disquiet.
Soares's "factless autobiography" must rank as the supreme assault on authorship in modern European literature. Even in its publishing history, it displays the beguiling elusiveness that surrounds everything to do with Pessoa. Born in Lisbon in 1888, he passed his days writing, drinking and chain-smoking, scraping a meagre living from translations and devising English crosswords, a reclusive figure with few friends and almost no love life. To him, obscurity became a kind of vocation. When he died in 1935, most likely from liver failure, he was admired in Portugal as an accomplished essayist and experimental poet, but was otherwise largely unrecognised in Europe. The greater part of his work consists of more than 25,000 pieces, some apparently unfinished, others mere scraps, which were found in a trunk after his death. Pessoa left no guidance as to how these jottings were to be assembled. Not long before his death, he placed some of them in an envelope marked with the book's title, but not all of his selections seem to belong there, and it remains unclear which of his literary remains were meant to be included.
As a result, there can be no definitive edition of The Book of Disquiet. Written on and off over a period of more than 20 years, seemingly beginning as a book by another of Pessoa's heteronyms, Vicente Guedes, and slowly evolving into the imaginary testament of Soares, it is a dishevelled album of thoughts, sensations and imagined memories that can never be fully deciphered. Any version is bound to be a construction. In his notes on the text, Richard Zenith recognises this and suggests that readers "invent their own order or, better yet, read the work's many parts in absolutely random order". Despite this disclaimer, readers of Zenith's edition will find it supersedes all others in its delicacy of style, rigorous scholarship and sympathy for Pessoa's fractured sensibility.
"Pessoa" can mean "person" and - by a well-known classical extension - "mask", but Pessoa's fictive identities were not disguises. It was widely known that he was the creator of many heteronyms. On the contrary, Pessoa's jostling aliases expressed his absolute belief that the individual subject, the core of European philosophy, religion and morality, is an illusion. As one of his heteronyms put it: "Each one of us is an assembly of subsidiary psyches, a badly made synthesis of cellular souls." That the self is multiple, not singular, was Pessoa's most intimate experience. But in writing as he did - mediumistically, as the channel for a number of personalities with quite disparate histories and values - Pessoa did more than record his own experiences of dissociation. He diagnosed a condition that was to become widespread in 20th-century Europe - a highly developed self-consciousness combined with a lack of any fixed view of the world. With neither religion nor the banal humanist faith of the Enlightenment to sustain him, Pessoa retreated into himself, only to find doubt and turmoil. This is the condition that Bernardo Soares, right at the start of Zenith's version of The Book of Disquiet, calls decadence: "I reasoned that God, while improbable, might exist, in which case he should be worshipped; whereas Humanity, being a mere biological idea and signifying nothing more than the animal species we belong to, was no more deserving of worship than any other animal species . . . And so, not knowing how to believe in God and unable to believe in an aggregate of animals, I, along with other people on the fringe, kept a distance from things, a distance commonly called Decadence."
Pessoa is not the only early 20th-century writer to have made the predicament of the person without any settled identity or beliefs his central subject matter. So did Robert Musil, in his clairvoyantly prescient and inexplicably neglected masterpiece, The Man Without Qualities. But no one has lived an ineffectual life as intrepidly as Pessoa did, or written about it with such insight and charm. Reading at times like Rainer Maria Rilke's The Notebooks of Malte Laurids Brigge, at others like the early, pre-prophetic Nietzsche, and at still others like a faintly fraudulent self-help book for losers and incorrigible dreamers, The Book of Disquiet is an inexhaustible jumble of epigrams, images, dreams and fantasies, the self-revelation of a disoriented and half-disintegrated soul that is all the more compelling because the author is himself an invention. Not a complete invention, since all of Pessoa's characters were born of a painful personal necessity: he confessed that Soares was not even a genuine heteronym, but a semi-heteronym, a distorted version of himself, edited and abridged to create a personality in some ways more substantial than his own. In this way, Pessoa was able to do in thought what he could not do in practice - escape the confines of a timid, introspective life. By allowing his alter ego a reality that he refused for himself, he turned himself into a fiction - a triumph over brute actuality that is recognised in Jose Saramago's beautiful novel, The Year of the Death of Ricardo Reis, in which the recently deceased Pessoa appears as a ghost haunting one of his own creations.
It is curious, and at the same time somehow fitting, that Pessoa should continue to be so little known. Long before postmodernism became an academic industry, Pessoa lived deconstruction. Yet few of those who write so laboriously about postmodern irony have heard of its supreme practitioner. True, Pessoa has not entirely escaped academic notice. The doyen of American criticism, Harold Bloom, has gone so far as to include him among the 26 writers he thinks are essential to the "Western Canon" - an honour Pessoa would surely have received with a smile. But, for the most part, Pessoa remains as he was during his lifetime: an obscure, almost inexistent figure, among whose many aliases are to be found some of the most authentic voices in European literature.
John Gray is professor of European thought at the London School of Economics