Keats’ theory of “negative capability” was expressed in his letter to his brothers dated Sunday, 21 December 1817. He says 
I had not a dispute but a disquisition with Dilke, on various subjects; several things dovetailed in my mind, & at once it struck me, what quality went to form a Man of Achievement especially in literature & which Shakespeare possessed so enormously – I mean Negative Capability, that is when man is capable of being in uncertainties, Mysteries, doubts without any irritable reaching after fact & reason – Coleridge, for instance, would let go by a fine isolated verisimilitude caught from the Penetralium of mystery, from being incapable of remaining content with half knowledge. This pursued through Volumes would perhaps take us no further than this, that with a great poet the sense of Beauty overcomes every other consideration, or rather obliterates all consideration.
Keats believed that great people (especially poets) have the ability to accept that not everything can be resolved. Keats, as a Romantic, believed that the truths found in the imagination access holy authority. Such authority cannot otherwise be understood, and thus he writes of “uncertainties.” This “being in uncertaint[y]” is a place between the mundane, ready reality and the multiple potentials of a more fully understood existence.