Tricky one this. It is hard to get a clear definition, even though everybody from Bill Bryson to In-flight magazine journalists behave as if they know exactly how and when to use it. The phrase is Keats`s by the way, and he kept using it in letters to friends in slightly different, and annoyingly vague ways. Somebody must have taken him to task about it (not that it did much good), because in 1817, Keats tries to define “negative capability” in yet another letter.
“….that is, when a man is capable of being in uncertainties, Mysteries, doubts, without any irritable reaching after fact & reason”.
Some interpreted this to “a way of dealing with life” to trust the subconscious and not to try too hard in coming at a truth.
Other people appear to think that negative capability is related to empathy. Or integrity. Or subjectivity. Hardly surprisingly some academics have built a whole career out of this.