Keats' letters were first published in 1848 and 1878. During the 19th century, critics deemed them unworthy of attention, distractions from his poetic works. During the 20th century they became almost as admired and studied as his poetry, and are highly regarded within the canon of English literary correspondence. T. S. Eliot described them as "certainly the most notable and most important ever written by any English poet." Keats spent a great deal of time considering poetry itself, its constructs and impacts, displaying a deep interest unusual amongst his milieu who were more easily distracted by metaphysics or politics, fashions or science. Eliot wrote of Keats' conclusions; "There is hardly one statement of Keats' about poetry which ... will not be found to be true, and what is more, true for greater and more mature poetry than anything Keats ever wrote."
Few of Keats' letters are extant from the period before he joined his literary circle. From spring 1817, however, there is a rich record of his prolific and impressive skills as letter writer. Keats and his friends, poets, critics, novelists, and editors wrote to each other daily, and Keats' ideas are bound up in the ordinary, his day-to-day missives sharing news, parody and social commentary. They glitter with humour and critical intelligence. Born of an "unself-conscious stream of consciousness," they are impulsive, full of awareness of his own nature and his weak spots. When his brother George went to America, Keats wrote to him in great detail, the body of letters becoming "the real diary" and self-revelation of Keats' life, as well as containing an exposition of his philosophy, and the first drafts of poems containing some of Keats' finest writing and thought. Gittings describes them as akin to a "spiritual journal" not written for a specific other, so much as for synthesis.
Keats also reflected on the background and composition of his poetry, and specific letters often coincide with or anticipate the poems they describe. In February to May 1819 he produced many of his finest letters". Writing to his brother George, Keats explored the idea of the world as "the vale of Soul-making", anticipating the great odes that he would write some months later. In the letters, Keats coined ideas such as the Mansion of Many Apartments and the Chameleon Poet, concepts that came to gain common currency and capture the public imagination, despite only making single appearances as phrases in his correspondence. The poetical mind, Keats argued:
has no self – it is every thing and nothing – It has no character – it enjoys light and shade;... What shocks the virtuous philosopher, delights the camelion [chameleon] Poet. It does no harm from its relish of the dark side of things any more than from its taste for the bright one; because they both end in speculation. A Poet is the most unpoetical of any thing in existence; because he has no Identity – he is continually in for – and filling some other Body – The Sun, the Moon, the Sea and Men and Women who are creatures of impulse are poetical and have about them an unchangeable attribute – the poet has none; no identity – he is certainly the most unpoetical of all God's Creatures.
He used the term negative capability to discuss the state in which we are "capable of being in uncertainties, Mysteries, doubts without any irritable reaching after fact & reason ...[Being] content with half knowledge" where one trusts in the heart's perceptions. He wrote later: "I am certain of nothing but the holiness of the Heart's affections and the truth of Imagination – What the imagination seizes as Beauty must be truth – whether it existed before or not – for I have the same Idea of all our Passions as of Love they are all in their sublime, creative of essential Beauty" again and again turning to the question of what it means to be a poet. "My Imagination is a Monastery and I am its Monk", Keats notes to Shelley. In September 1819, Keats wrote to Reynolds "How beautiful the season is now – How fine the air. A temperate sharpness about it ... I never lik'd the stubbled fields as much as now – Aye, better than the chilly green of spring. Somehow the stubble plain looks warm – in the same way as some pictures look warm – this struck me so much in my Sunday's walk that I composed upon it". The final stanza of his last great ode: "To Autumn" runs:
Where are the songs of Spring? Ay, where are they? Think not of them, thou hast thy music too,- While barred clouds bloom the soft-dying day, And touch the stubble-plains with rosy hue;
Later, To Autumn became one of the most highly regarded poems in the English language.[nb 6][nb 7]
There are areas of his life and daily routine that Keats does not describe. He mentions little about his childhood or his financial straits and is seemingly embarrassed to discuss them. There is a total absence of any reference to his parents. In his last year, as his health deteriorated, his concerns often gave way to despair and morbid obsessions. The publications of letters to Fanny Brawne in 1870 focused on this period and emphasised this tragic aspect, giving rise to widespread criticism at the time.