Scientific inquiry requires a clear distinction between the two:
On the Origin of Species by Means of Natural Selection Or, The Preservation of Favoured Races in the Struggle for Life By Charles Darwin
This important passage occurs not too long after a slightly better known (and very, very often misunderstood) passage on the same subject:
On the Origin of Species by Means of Natural Selection Or, The Preservation of Favoured Races in the Struggle for Life By Charles Darwin
One can think of any number of scientific discoveries and ideas that are quite literally unimaginable in themselves but that reason can discover.