HARMONY
Harmony is the requisite of all art. Harmony of composition, of structure. Whether art, sculpture, dance, drama and (certainly) writing, we look for a harmonising of the elements of content and form. If there is no harmony of composition, the truth will not be clear and the narrative will come over as false, posturing, self indulgent emoting; and the beauty is lost if the composition is skewed. How has it all been brought together? Or not? Where is the poetry?
And that is where the writing gets tough. Most poetry, no matter how well intentioned, will not survive, or will consist of interesting failures. (I remember Walcott agonising as he worked on his Collected Poems, wondering what of his poetry would survive. He hoped a few poems would.)  Eliot again: “The mystic of verse is not a line by line matter, but a question of the whole poem.†After the attempt to capture the Truth of the emotion, experience or insight; after the effort to speak in Beauty by image and rhythm, has the poet brought all into a Harmony of Content and Form, with a fine balance?
Because it is the final Harmony of Content and Form, of Truth and Beauty, that will lead the Reader into the higher regions of Meaning. What does the poem all mean? Truth, Beauty and Harmony are the guides that lead us from the Inferno through the Purgatorio to the Paradiso of Revelation. To the “Aha†moment, to the “I see!†The achievement of Harmony, the finish of a well-composed work, will lead to more correct interpretations of the work, and avoid, hopefully, spurious speculation.  A well-harmonised work will lend itself to close, accurate, critical reading, with all the needed markers in place, well fitted together.
As to this matter of interpretation and finding of meaning, hear how Eliot opens this up even further, and takes us beyond the basics I have dealt with to the ultimate achievement and purpose, of the poem: “A poem may appear to mean very different things to different readers, and all of these meanings may be different from what the author thought he meant….the reader’s interpretation may differ from the author’s and be equally valid – it may even be better. There may be much more in a poem than the author was aware of. The different interpretations may all be partial formulations of one thing; the ambiguities may be due to the fact that the poem means more, not less, than ordinary speech can communicate.†(Welcome to Pandora’s box of the thriving industry of post-modern literary criticism!)
As an expert master, after we have looked at fundamentals, at basics of poem composition, which I have summarised as Truth, Beauty and Harmony, after we have done that and hope we have succeeded to some extent, Eliot then points us to the further regions of Meaning and Interpretation, with all their complexities, and to the role of the Reader to whom we were always heading. Which Reader will then make application to themselves, and raise all the questions of the ultimate purpose of poetry. What is it for?
Poetry finds its completion, and further new life, in the Reader and the Reading of it.
Conclusion
Eliot, in his time, was an innovator and led the modernist literatures of the 20th century which turned from the 19th century and earlier forms which had become worn. On this matter of experimentation, he said the following: “the true experimenter is not impelled by restless curiosity, or by desire for novelty, or the wish to surprise and astonish, but by the compulsion to find, in every new poem as in his earliest, the right form for feelings over the development of which he has, as a poet, no control.â€
And for me, whether traditional forms or experimental forms are used, whether the language be the so-called established or national language, whether book-read or platform-spoken, the fundamentals of Truth, Beauty and Harmony remain a necessary framework within which the poem finds life. And, if successful, long life.
I do not know whether I have answered the question, “what is poetry?†It seems to me that I have been speaking more about the firming and shaping of poetry. The well-springs of poetry remain — in the words of master Eliot, and of Wordsworth — in the emotions of the poet. Which proves the words of Seamus Heaney, who spoke of “this honoured but hard to define category of human achievement called poetry.â€