Full Idea
The mind does not err from the fact that it imagines, but only insofar as it is considered to lack an idea which excludes the existence of those things which it imagines to be present to it.
Gist of Idea
Error does not result from imagining, but from lacking the evidence of impossibility
Source
Baruch de Spinoza (The Ethics [1675], II Pr 17 s)
Book Reference
Spinoza,Benedict de: 'Ethics', ed/tr. Curley,Edwin [Penguin 1996], p.46
A Reaction
These may be the wisest words I have yet found on conceivability and possibility. My example is imagining a bonfire on the moon, which seems possible until you fully grasp what fire is.