W.B. YEATS in his early days made strenuous efforts to prove that William Blake was Irish by blood and an O'Neill by name, or at least by ancestry. Few people believe this any more - Blake, though he appears to have had some pro-Irish sympathies, was surely a uniquely English phenomenon. England has always produced, and sometimes even understood or accepted, men and women of strange and eccentric genius, which he certainly was. Even in that category, however, he remains a special case and it is unlikely that critical opinion will ever quite reach an agreement about his ultimate importance, either as a poet or as a visual artist.

In his lifetime, many people considered him mad, and some still do, though the testimony of those who knew him really well is that the man was sane. It was common knowledge that he talked with spirits, and even drew or depicted them - mostly great figures from the past, since Blake had a keen sense of history and was widely read. He did not, however, claim to have mediumistic or clairvoyant powers and these visions appear to have been projections of his powerful imagination, rather than mere hallucinations. In his own opinion, most people had similar imaginative powers if only they knew how to use them properly. He might even have agreed with his contemporary Coleridge's description of such phenomena as "ideas with the power of a sensation".

Politically Blake was a radical, a supporter of the French and American Revolutions and an admirer of France, a believer in sexual liberation, and hostile to the reactionary monarchies of his time. His radicalism inclined towards anti-nomianism and even anarchism: "Prisons are built with stones of law, brothels with bricks of religion," he wrote. For a time he was one of a circle around the London bookseller Joseph Johnson, men and women who were mostly radicals or deists and included Mary Wollstonecraft, the apostle of women's rights. Tom Paine was another, and it is a tradition that Paine went into exile in America directly after Blake warned him in person that he faced arrest, and probably hanging, if he did not leave England at once. He drifted away from most of these people, however, and in his later years became increasingly isolated, living in two shabby rooms near the Strand with only the company of his devoted wife Catherine (the Blakes were childless). He also appears to have abandoned his belief in political action, which probably was safer for his own well-being - England in the aftermath of the Napoleonic wars was a reactionary country, as is shown by the sometimes venomous attacks on Shelley and Keats and Leigh Hunt, all of whom leaned politically towards the liberal Left. Blake, however, was too obscure to invite attack and was left to himself and a small circle of initiates and admirers. Who cared about an eccentric, shabbily dressed Cockney mystic who saw visions and produced incomprehensible books which scarcely anyone bought or read?

Yet in spite of his radicalism, Blake was bitterly hostile to most of the nationalistic ideas of the Enlightenment and he particularly detested Voltaire and Rousseau. Brought up in the tradition of the Swedish mystic, Swedenborg, by his father, a London draper, he inherited a tradition of mystical Dissent and he combined an intensely religious nature and vision with a deep distrust of all organised religions. He was soaked from childhood in the Bible, as so many thinking (and unthinking) Englishmen of his time were, yet he rejected the belief in Christ's divinity while exalting him as a man and a prophet. Blake's intellectual and religious system - he evolved his own personal pantheon - is complex and odd, perhaps deliberately so in the sense that he inherited the old Gnostic belief that it is safer and wiser not to spell things out for the vulgar.

Certain successful artists such as Flaxman, Stothard and Fuseli, who had known him as a young man, kept up a certain degree of contact with him over the years and from time to time even put some small commission or patronage in his way. His private admirers included Sir Thomas Lawrence, the hugely fashionable Regency portraitist who owned drawings by Blake, and the poets Coleridge, Landor and Wordsworth were all aware of him to an extent and praised some of his poetry. Yet Blake, trained in boyhood as a commercial engraver, barely scraped a living in his later years and was reduced to hackwork such as supplying illustrations for Wedgewood catalogues.

Towards the end of his life, the painter John Linnell became a discriminating patron and it is thanks largely to him that some of Blake's late masterpieces were created. And a generation of young Romantic artists - the so-called Ancients - discovered him, including Samuel Palmer, George Richmond, Edward Calvert etc. To them he was a prophet and a genius, even a saint, to be treated with reverence, and through them his influence was fed into English painting after his death. But the early Victorian age quickly forgot Blake as an artist, and it was only with the Pre-Raphaelite generation that he was rediscovered. Even then, however, admiration was highly selective, and in any case the "prophetic" books he had written, illustrated and printed mostly survived only in a few rare copies. It was not until the end of the century that he could be seen at last in the round, or something approximating to that. (Yeats in his younger days co-edited a volume of his poetry, and with his contemporary "AE" brought Blake early into the bloodstream of the Irish Literary Revival.)

There is currently a major Blake exhibition at the Tate Gallery in London and it is interesting to read some of the critiques and to see how the idea that Blake was mad has resurfaced. Linnell, Palmer and his other followers all insisted that he was perfectly sane, but it is plain that he lived increasingly in a world of his own, preoccupied with his message and his own strange, composite, barely penetrable mythological system. Blake saw himself as a moral and religious prophet, in the Biblical line of Isaiah and Ezekiel (with whom he claimed to have conversed "in imagination") and the prophetic books were largely intended as his testament to his fellow-countrymen.

Blake produced, in all, about 20 illuminated books, in small (sometimes very small) editions, since he found it hard to find patrons and subscribers. He wrote the texts in longhand and evolved a system of "colour-printed drawing" to illustrate them. "Illustrate" is a weak, inadequate word since they are an indissoluble union of text and picture; his technique for these was terribly involved and time-consuming and drew on all his skills as a professional engraver, but with various refinements and complications added.

A new book published by Thames and Hudson collates them all into a single volume of replicas in full colour, which is quite a literary and artistic event. Songs of Innocence, an early book though not the first of them, contains most of Blake's best-loved lyrics, which announced the dawn of the Romantic Movement. Their idyllic, songlike freshness is continued in "The Book of Thel", a short narrative poem whose pastoral charm conceals a good deal of Blakean symbolism and allegory. "America: a Prophecy" and "Europe: a Prophecy" relate to the French and American Revolutions, while "The Marriage of Heaven and Hell" shows Blake as a powerful epigrammatist. It is with the later epic books (which should really be called "symbolic" rather than "prophetic") that many or even most readers are likely to find themselves bogged down in a marshland of the creative imagination, or astray in a dense forest. "Milton" contains some magnificent, visionary passages along with many arid and declamatory ones, but "Jerusalem", by far the longest of them, is almost all rhetoric, convoluted symbolism, and even more convoluted allegory. The ageing artist saw it as his greatest work and laboured over it for years, supplying it with many inspired (and some rather uninspired) drawings and designs. For many years these late works were regarded as hopelessly obscure and even crazy or nonsensical, but since then the labours of great scholars such as Sir Geoffrey Keynes have decoded many or most of Blake's meanings and made sense of his private pantheon. Blake studies, in fact, grew into a wholesale industry for several decades and he himself became a cult, almost a religion - especially for quasi-Jungian interpreters of whom the poet Kathleen Raine has been the high-priestess (her book, William Blake, though cultist and rather apocalyptic, is recommended reading).

Art scholars such as Martin Butlin and Raymond Lister have added much to our understanding of the paintings and graphic work, while of the various biographies Mona Wilson's is probably still the best (written back in 1927, it has since been thoroughly updated).

Yet even if Blake the poet was virtually dead by then, Blake the visual artist produced probably his greatest works in old age - the woodcuts for Thornton's translations of Virgil, the illustrations to the Book of Job, the visionary series of watercolours inspired by Dante's Divine Comedy (which he did not live long enough to complete). Since none of these masterworks were produced for his own texts, they make you wonder: was Blake's intricate private mythology ultimately a drag on his creative imagination? Certainly his final soarings were achieved without its cumbrous apparatus weighing him down and clipping his wings.

William Blake: the Complete Illuminated Books, with an introduction by David Bindman, is published by Thames and Hudson

William Blake, an exhibition of Blake's work is at the Tate, London, until February 11th, 2001