Goethe remarked, in a phrase Yeats liked to quote, that the poet must know all philosophy, but keep it out of his work. Yeatss notion was not to exclude philosophy, but to admit it on his own terms. He came to study it through its dingy back entrances, those cults which W. H. Auden has appropriately termed Southern Californian [sic; Permanence of Yeats, p.345.], and at first he saluted more reputable thinkers with something of the reluctance of an officer of another service. But in later life he investigated them carefully to find parallels between his thought and theirs. In the nineteen-twenties and thirties, especially, he read fairly widely in Plato, Plotinus, Croce, Whitehead, Russell, Hegel, G. E. Moore, and others, search out whatever he could find on the relation of the antinomies, on the connection between the sphere of reality and the gyres of illusory appearance, on subjectivism and kindred subjects. (p.216.)
[Quotes Yeatss letter to Sturge Moore on the theories of subjective/idealism and objective/realism versions of the world:] He chooses [..] that interpretation of the external world which gives the mind autonomy, while at the same time  insisting that no ones mind is isolated. His choice is largely pragmatic, the proof of the philosophy being its adaptability to poetry and its power to free the mind from abstractions. (p.318; view extract from Yeatss letter to Sturge Moore in Ricorso, W. B. Yeats - Quotations [infra].)
'He conceived of a corporate imagination which he called Anima Mundi or Spiritus Mundi. It is this which creates and stores archetypes, and the man who is able to let his imagination fuse with this corporate imagination has all the images ever wrought by men available to him as well as the power to create new ones. Because anima mundi is corporate all men participate in it a little and can respond to its images. (p.219.)
[Discussing Byzantium:] Now used Plato and Plotinus as his whipping boys, and attacked them as all transcendence. Later he admitted that they did not deserve this designation, but he evidently considered it accurate enough to keep them as symbolic targets: And I declare my faith: / I mock Plotinus thought / And cry in Platos teeth, / Death and life were not /  Till man made up the whole, / Made lock, stock and barrel / Out of his bitter soul, / Aye, sun and moon and star, all, / And further add to that / That, being dead, we rise, / Dream and so create Translunar Paradise. [Ben Bulben.] These remarkable declarations are not the blend of night-time and day-time images, of moon and sunlight into one inextricable beam, that Yeats desired earlier in the poem, but rather a forced merger of all things into night-time images created by the human imagination. In the poem Death, written two years later, he would say again, Man has created death. His subjectivism is a dramatic cry of defiance against those who would denigrate man or subject him to abstractions like death, life, heaven or hell. God, Plotinus One. Platos Good or eternal ideas. Humanism rushes to the point of solipsism; rather than concede anything to the opposition, it erects man as not only the measure but also the creator of all things. Yeats utters his assertions in part because they are not acceptable, out of an obstinacy which he wants to be as mulish, and as heroic, as he can make it. / (pp.224-25; see further under Berkeley.)
'At the end of the twenties Yeats thought he had found an ally for his most extreme positions in Berkeley. Before Berkeley became Bishop of Cloyne he had proved, Yeats decided, that the world was a vision. Only in pious, debilitated old age, when ecclesiastical preferment had sapped his thought, did he pretend that the vision was Gods. To the young Berkeley, Yeats announced with more vehemence than accuracy, the vision was mans, as if man had built the world out of his imagination and then contentedly started to live in it. Yet this position of Yeatss prose is not one to which he rigidly adheres in his verse. [p.225; .; &c.]
'Yeatss poetry is bound together by one unchanging conviction, the desirability of intense, unified, imaginative consciousness. But apart from this central pillar it reveals a series of points of view, sometimes parallel and sometimes divergent. What are we to make of his various attitudes towards reality, truth, life, death, and imagination? The question is of special moment because he kept increasingly as his career progressed to the ideal of writing poems of insight and knowledge which he had marked out for himself in youth. His position hovered for a time that of Keats, who held that he was certain of nothing but of holiness of the Hearts affections and the truth of Imagination - What the imagination seizes as beauty must be truth. Keats would undoubtedly have added, if pressed, that beauty does open her doors to the cheap, the temporary, or the false. The quality of the great writer, he maintained on one occasion, was Negative Capability, that is, when a man is capable of being in uncertainties, mysteries, doubts, without any irritable reaching after fact and reason. Yeatss conception of his art moved beyond this theory, because his verse depended, more than Keatss, on presenting a complete picture of the self, and to do so became of reaching after fact and reason.
'To explain and confirm his practice Yeats evolved a hypothesis which is closer to defining the situation in which modern poet finds himself than negative capability. It might be described as affirmative capability, for it begins with the poets difficulties but emphasizes his resolutions of them. Rejecting Keatss cry for a life of Sensations rather than of Thoughts, Yeats considered it the poets duty to invade the province of the intellect as well as of the emotions. Neither the intellect nor the emotions can be satisfied to remain in uncertainties, mysteries, and doubts; they demand the more solid fare of affirmations. (p.238.)
[Quotes WBY:] I agree with Ezra Pound in his dislike of the word belief. Belief implies an unknown object, a covenant attested with a name or signed with blood, and being more emotional than intellectual may pride itself on lack of proof. But if I affirm that such and such is so, the more complete the affirmation, the more complete the proof, and even when incomplete, it remains valid within some limit. I must kill scepticism in myself except in so far as it is mere acknowledgement of a limit, gradually, in so far as politeness permits, rid my style of turns of phrase that employs it. Even the politeness should be ejected when charm takes its place in poetry. [.] I must though [the] world shriek at me, admit no act beyond my power, nor thing beyond my knowledge, yet because my divinity is far off I blanch and tremble. Further: [.] We even more than Eliot, require tradition, and though it may include much that is his, it is not a belief or submission, but exposition of intellectual needs. [.] I feel as neither Eliot nor Ezra do the need of old forms, old situations, that, as when I recreate some early poem of my own, I may escape from scepticism. [.] Nor do I think that I differ from others except in so far as my preoccupation with poetry makes me different. The men sitting opposite me, in the Rapello restaurant where some days ago the sound of a fiddle made me remember the old situation, are to my8 eyes modern but only a perverted art makes them modern to themselves. The modern man is a term invented by modern poetry to dignify our scepticism (unpub. journal; Ellmann, p.239.] Note: the above is a part-copy only; Ellmann italicises certain sentences.
'His attitude towards scepticism is not entirely defined, but he was working towards some such generalisation as this: In so far as scepticism prevents positive statement, it is a danger to art. (Ellmann., op. cit., p.241.)
'What emerges from a consideration of Yeatss whole poetic career is an impression of its seriousness and importance. All his work, in both poetry and prose, was an attempt to embody a way of seeing. That way is close to that of Blake and the romantics, but not identical with theirs. Like Blake, Yeats conceives of the imagination as the shaping power which transforms the world; but coming a century later, he has a tough-minded appreciation of the worlds intransigence. He has little hope of building Jerusalem in Irelands green and pleasant land, and for him as for Keats, one function of art is to freeze lifes inadequacies so as to render them harmless and beautiful. More than Keats or Coleridge, more perhaps even than Blake, he defends the imagination with the defiance of a man who sees himself as preventing the incursions of chaos. Yet his poetry is as much offensive as defensive. It fights its way beyond the frontiers of common apprehension, and brings previously untamed areas of thought and feeling under strong rule. (p.246.)
The principle of growth and of stability keep constant watch on one another in Yeatss poetry. He was a many-sided man who by dint of much questioning and inner turmoil achieved the right to speak with many voices and to know completely the incompleteness of life. And if, as seems likely, his work will resist time, it is because in all his shape-changing he remains at the centre tenacious, solid, a marble triton among the streams. (p.247; end.)