Yeats, it has been said, wrote books of poetry rather than individual poems, crafting and reshaping his work to create integrated collections, each with a distinct identity, and ruthlessly dropping or amending poems which did not suit the developing canon as he conceived it. There is enough truth in this to make the question of his Collected Poems a contested arena. Should it include the poems, some of them brilliant, which are studded into his plays - or, indeed, those plays that are themselves in verse? Should it embrace poems which never appeared in, or disappeared from, his books? Should it separate out lyrical poems from those that are narrative and dramatic? Above all, should it try to reproduce the last poems in the order which he drew up for a projected individual volume, or in the order chosen by his widow and editor, and apparently canonised by the posthumous Last Poem and Two Plays (1940), the limited two-volume Poems of W. B. Yeats (1949) and Collected Poems of 1950? This last volume brought his astonishing oeuvre together, after years when much of it had been out of print due to wartime conditions and vagaries of fashion. It more or less coincided with the first major critical studies by Richard Ellmann and A. N. Jeffares, and remained a familiar vade mecum for generations of readers as Yeats’s reputation became more and more firmly established as one of the very greatest poets writing in English in the 19th and 20th centuries.

Yeats begins as a late-Victorian, and the early work carries languorous echoes of Tennyson and Swinburne, as well as Spenser, Shelley and the Young Irelanders Davis and Mangan - not to mention the translations from Irish mythological cycles which shaped his first major publication, The Wanderings of Oisin in 1889. But at the same time that work announced something decisively new, in its visionary conjunctions, its unexpected but firmly controlled metre, its distinctive use of off-rhymes and unexpected assonances. Above all, Yeats implicitly announced an enterprise that he never really abandoned: a determination to exoticise Irishness, to proclaim the essential difference and originality of his country’s culture. This may reflect his own uncertain status - Protestant, slightly déclassé, living between London, Dublin and Sligo at the whim of his splendidly bohemian father. At the beginning of his career he stamped his work by the use of magnificently sonorous Irish names, whether of the Sligo lakes and mountains of his youth or of heroes from the sagas - as well as by a deliberate invocation of the Fenian tradition of sacrificial nationalism. His nationalist fervour cooled notably from the turn of the century, but he would continue to identify Irish cultural individuality by claiming Dublin as the home of a distinctively modern drama, focused on the astonishing plays of J. M. Synge, a constant presence in his elegiac poetry. Later, Yeats’s evocation of the salty wisdoms spoken through the mouths of Irish bawds and beggars differentiates them from English bourgeois niceties; and throughout his life he claimed a place for Ireland in the mainstream of European culture.

The other affirmation of “difference”, and indeed exoticisation, which pulses through his poetry from start to finish is a sense of the supernatural. In Irish folklore, which influenced Yeats immensely at the most formative stage of his life, the veil between the living and the dead is very thin indeed; fairies are evidence of this, in many ways, and so are the rituals and invocations of traditional country life. Yeats would combine this with a belief in a shared universal mind, accessed through psychic communication, ghosts and dreams: collecting folklore with his greatest friend, Augusta Gregory, he felt “again and again that we had got down as it were to some fibrous darkness, into some matrix out of which everything had come”. He was equally immersed in the arcane mysteries of the society he joined in 1893, the Order of the Golden Dawn, where initiation involved a rite of death and resurrection. Psychic research, séances, and above all his wife’s automatic-writing experiments from 1917 took up an immense amount of his attention for much of his life.

This has exasperated several critics and admirers, notably W. H. Auden, but the preoccupation brought him - as he himself said - “metaphors for poetry”. Some of his most intriguing short poems, as well as major works such as “All Souls’ Night”, testify to the results. But the point must be also made that Yeats’s powerful mind was by no means indiscriminatingly credulous; and that his fascination with the occult and supernatural was accompanied by a lifelong immersion in philosophical reading and speculation, particularly in eastern religion and philosophy but also in the Christian mystical tradition, in Berkeley’s thought and in Italian philosophy of the early 20th century. The argumentative edge and bite of long poems such as “Ego Dominus Tuus”, “A Dialogue of Self and Soul” and above all perhaps “The Tower” is evidence of this; but so is the compressed perfection of a meditation like “Meru”. “Genius”, he wrote in his auto-biography, “is a crisis that joins the buried self for certain moments to our trivial daily mind.”

That current of creative argument with oneself continues throughout: Yeats, after all, defined it as one of the sources of poetry. The dialogue form is very pronounced in his poetry, and creates the structure which enables a poem such as “Man and the Echo” or - very differently - “Adam’s Curse” to present profound ideas in an accessible and even idiomatic way. The audacity of Yeats’s technique developed alongside increasingly challenging ideas about history, politics, authority, tradition and sex. It might be noted how many poems close on a question (and closing a poem is one of Yeats’s consummate abilities). Even the love-lyrics of the early pages, which include the eternally popular “Cloths of Heaven” as well as “The White Birds” and other poems addressed to his eternal goddess, Maud Gonne, are preoccupied with death, burial and essential solitude. This is not concealed by the “embroidery” (his own word) which adorns them. From the turn of the century, coinciding with a number of disillusionments in his own life (including Gonne’s disastrous marriage), that embroidery would be consciously unpicked. The distinctive new voice that came through from around 1912 owes something to collaboration with Ezra Pound, but there were clear precedents in his early conditioning.

It is clear that the landmark collection The Wild Swans at Coole owes much to the upheavals in Yeats’s life after 1914, and especially after 1916. The Easter Rising and the development of the Irish revolution, culminating in the Anglo-Irish war and the treaty of 1921, altered his public life, and enlisted him (if rather ambivalently) once more as the poet of nationalism. But his personal life was also convulsed, by his final attempt to make Gonne marry him, his subsequent infatuation with her daughter Iseult, and then his sudden marriage - at 52 - to the much younger Georgie Hyde-Lees and the beginning of their supernatural experiments together. These were also the years when he decided to return to live in Ireland - for some of the time in the ancient tower, Thoor Ballylee, which he had bought and renovated. This was only a few miles from Coole Park, home of Augusta Gregory, where he spent his summers from the early 1890s and which he loved above all other houses. All these themes and developments are reflected in his poetry. This is not to say that Yeats’s creative writing is autobiography (even if his autobiography sometimes approaches creative writing). “A poet writes always of his personal life,” he wrote in a “General Introduction for my Work” unpublished in his lifetime, “in his finest work out of its tragedy, whatever it may be, remorse, lost love, or mere loneliness; he never speaks directly as to someone at the breakfast table, there is always a phantasmagoria.” Appreciation of the poems from 1916, which move from disillusionment through catharsis to revelation, is greatly enhanced by an understanding of the development of his emotional life, and his preoccupation with the great political crises of 1916 to 1922 - not only in Ireland, racked by guerrilla warfare and military oppression, but in Russia, Italy and central Europe.

This is the background to poems such as “The Second Coming” as well as the marvellous sequences which would play such a key part in the architecture of The Tower: “Meditations in Time of Civil War” and “Nineteen Hundred and Nineteen”. The Tower also contains “Sailing to Byzantium”, where Yeats’s personal philosophy of the eternal artifice of art received its most memorable expression, and which also inaugurates one of the preoccupations of his late work: the inconsistencies, indignities and desolating insights of old age. Long before, he had referred to a section of his work as “Poems Written in Discouragement”; from the late 20s, his poetry is as often written in rage as discouragement, and the personae he adopts - such as Crazy Jane, with her unabashed memories of sex and her taste for abusing bishops - speak in a new language, very different from the hieratic formality of his earlier creations such as Michael Robartes and Owen Aherne. This is not contradicted by the tragic dignity of the anticipatory elegies for Gregory, “Coole Park, 1929” and “Coole Park and Ballylee, 1931”. But when her death did come, in May 1932, it brought Yeats first of all a severe case of writer’s block, and then a determination to throw himself into what was left of life with a kind of energy and fury reminiscent of one of his heroes, Jonathan Swift.

The range, excitement and sheer nerve of the poems written between 1936 and 1939, and originally published as New Poems and Last Poems and Two Plays, were not always appreciated at the time. Some of them stem from the period when Yeats flirted with an Irish proto-fascist movement, the Blueshirts (though he quickly disassociated himself from it); others reflect his interest (shared by many on the right and the left) in the sinister pseudo-science of eugenics, and his disillusionment with Irish politics. There is in some of this work a kind of jaggedness inseparable from the “late style” of some other great artists, like Beethoven or Ibsen, along with the supreme accomplishment of poems about artistic inspiration, such as “Long-legged Fly” and “The Circus Animals’ Desertion”. But there is also, intriguingly, a turning back to his earliest inspirations. Indian philosophy recurs (notably in the idea that sexual consummation can be a route to spiritual revelation); so do the ballad forms he had adopted in his very first collections, though they are now used to project challenging philosophies and assertive nationalism, rather than nostalgic otherworldly traditions. The very last poems he wrote, on his deathbed in the south of France, return to Irish myth and saga: ghostly armies ride out of hillsides, the homecoming of mythic heroes is forecast, and the Irish hero Cuchullain is chosen as the poet’s alter ego, to travel into the world of the shades.

Many of these themes, often expressed to jarring effect, are summed up in “Under Ben Bulben”, Yeats’s own epitaph-poem which was drafted before many of the literally “last poems”. One of his last conscious acts was to revise and rename it. In the provisional contents page of the volume he was projecting on his deathbed, Yeats placed it first, as a kind of announcement. Though it ends with the famous epitaph he decreed for himself, this was apparently to precede the poems that came afterwards, which would be spoken, as it were, from beyond the grave. In his scheme, the poems in his last planned volume would end with “Politics”, a lovely evocation of passion above abstraction. The inspiration for “Under Ben Bulben” came from his disagreement with Rilke’s image of a poet’s death as annihilation - wood consumed by flame. Yeats, by contrast, had claimed his own eternal place as a deathless golden bird singing to eternity, and at the very end of his life believed that he had - as declared in “The Tower” - “made his soul”, creatively and spiritually. The injunctions of “Under Ben Bulben” reflect this consciousness, as well as looking back over his astonishing life and all that had made him what he was. The Collected Poems , taken all together, finally vindicate the task he had set himself at the outset of his writing life, “to hammer my thoughts into a unity”.