Imagination and Possibility

'One other issue deserves mention. According to Hume "'Tis an established maxim in metaphysics, That . . . nothing we imagine is absolutely impossible. We can form the idea of a golden mountain, and from thence conclude that such a mountain may actually exist. We can form no idea of a mountain without a valley, and therefore regard it as impossible." (Treatise, I, ii, 2). (Note that, for Hume, "idea" explicitly meant mental image.) He seems to have meant that whatever is imaginable is possible, and, conversely, if something is impossible it is also unimaginable. Many philosophers have found this view significant and attractive. It would certainly be very convenient for metaphysicians if there were a mental faculty capable of providing a reliable test for possibility!

Hume's maxim is very questionable, however. Although examples that seem to favor it can be multiplied, it is also not hard to come up with apparent counter-examples. It seems to me that I am incapable of imagining curved space-time, but I am reliably informed that it is not only possible but actual. Conversely, countless science fiction buffs have imagined traveling faster than light, which is supposedly impossible. Perhaps some version of the maxim can be saved by sufficiently ingenious maneuvers, probably including the restriction of its scope to some or other subspecies of possibility (perhaps it applies to logical, conceptual, or metaphysical, but not to physical possibility), but such issues lie well beyond the scope of this discussion. It is, however, worth mentioning that the maxim has very little purchase if imagination is interpreted after the fashion of those who would deny its essential connection with imagery. Clearly we can pretend or mistakenly believe that impossible things are possible, and we suppose an impossibility every time we set up a sound reductio ad absurdum proof. On the other hand, if "imagination" is to be understood (as some recent philosophers claim) as something like "the faculty that envisages possibilities" (Rorty, 1988; c.f. White, 1990), then Hume's maxim would seem to be true but trivial. Who but a philosopher, however, would dream of denying that imagination has to do with imagery?'

http://philosophy.uwaterloo.ca/MindDict/imagination.html