Most directly, the faculty of reviving or especially creating images in the mind's eye. But more generally, the ability to create and rehearse possible situations, to combine knowledge in unusual ways, or to invent thought experiments. Coleridge was the first aesthetic theorist to distinguish the possibility of disciplined, creative use of the imagination, as opposed to the idle play of fancy. Imagination is involved in any flexible rehearsal of different approaches to a problem and is wrongly thought of as opposed to reason. It also bears an interesting relation to the process of deciding whether a projected scenario is genuinely possible. We seem able to imagine ourselves having been Napoleon, and unable to imagine space being spherical, yet further reflection may lead us to think that the first supposition is impossible and the second entirely possible.