Negative capability is a term that was first coined by Keats, and is used to refer to the ability that humans have to transcend and think beyond the way that humans are conditioned to think. It is a phrase that refers to the power of imagination in relation to our ability as humans to think above and beyond our contexts. Keats used it in one instance, where he was criticising Coleridge, who, in the opinion of Keats, wrote his poetry in order to search for truth and as a result missed out on beauty and its elevating affects.

Negative capability can be seen in many of the poems of Keats and is clearly something that is vital to his poetry, as they all focus on nature or beauty and the ways that they allow humans to transcend their limitations. Note, for example, the following example from "Bright Star!":

No--yet still stedfast, still unchangeable,
Pillow'd upon my fair love's ripening breast,
To feel for ever its soft swell and fall,
Awake for ever in a sweet unrest...

For the speaker, the power of the imagination allows him to look at the moon and imagine possessing a combination of the moon's steadfastness and the human ability to be intimate and close to capture the moment he is enjoying so deeply forever. Negative capability is something therefore that lies at the very heart of the poetry of Keats in the way that it allows humans to escape the restrictions of their lives and imagine and experience different worlds.

I think that Keats would feel that negative capability is such a part of poetic expression because it opens the door to imagination.  For Keats, the role of imagination is vitally important in the development of his poetry.  In his own mind, Keats believed that the role of negative capability played a vital role in the development of his art:  “In uncertainties, Mysteries, doubts, without any irritable reaching after fact & reason... all end in speculation."  For Keats, the role of negative capability is the essence of poetic expression because it underscores what he sees as the fundamental element of the human condition.  Keats' belief in negative capability is where the human imagination enters into the poetic understanding of being in the world.  It is in this where individuals fully understand their own place in the world, one that resides "in speculation."  In poems like "Ode on a Grecian Urn," the closing couplet is one in which negative capability is an essential element of both the poem and being in the world.  The idea of "Beauty is truth and truth beauty" as well as this representing all "one need to know" is an idea in which the construction of truth, an element of certainty, is actually rooted in a lack of certainty.  In this, one sees how negative capability is perceived on Keats' part to be an essential element of both poetry and life.