Keats believed that great people (especially poets) have the ability to accept that not everything can be resolved. Keats was a Romantic and 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.
Keats expressed this idea in several of his poems
Negative capability is a state of intentional openmindedness that has many parallels in other writers' literary and philosophical stances. Much has been written about this. Walter Jackson Bate, Keats's authoritative biographer, wrote an entire book on the topic. The footnote to the negative capability letter in the 1958 Harvard UP edition of the Letters of John Keats references the work of Woodhouse, Bate, C. L. Finney, Barbara Hardy, G. B. Harrison, and George Watson, all prior to the edition’s printing in 1958. In the 1930s, the American philosopher John Dewey cited Keatsian negative capability as having influenced his own philosophical pragmatism, and said of Keats' letter that it "contains more of the psychology of productive thought than many treatises." Additionally, Nathan Scott (author of a book titled Negative Capability), notes that negative capability has been compared to philosopher Martin Heidegger’s concept of Gelassenheit, “the spirit of disponibilité before What-Is which permits us simply to let things be in whatever may be their uncertainty and their mystery." Even Philip Pullman excerpts from Keats's letter and prominently incorporates the concept in his fantasy novel The Subtle Knife.