A passage of Rebecca Solnit's Men Explain Things to Me that I like a lot:
Despair is a form of certainty, certainty that the future will be a lot like the present or will decline from it; despair is a confident memory of the future, in Gonzalez's resonant phrase. Optimism is similarly confident about what will happen. Both are grounds for not acting. Hope can be the knowledge that we don't have that memory and that reality doesn't necessarily match our plans; hope like creative ability can come from what the Romantic poet John Keats called Negative Capability.
I had not a dispute but a disquisition with Dilke, upon various subjects; several things dove-tailed in my mind, and at once it struck me what quality went to form a Man of Achievement, especially in Literature, and which Shakespeare possessed so enormously — I mean Negative Capability, that is, when a man is capable of being in uncertainties, mysteries, doubts, without any irritable reaching after fact and 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.
[rereads: 1, edits: added archive link to the Wikipedia article]