“When I imagine a triangle, for example, I don’t merely understand that it is a three-sided figure, but I also see the three lines with my mind’s eye as if they were present to me; that is what imagining is. But if I think of a chiliagon, although I understand quite well that it is a figure with a thousand sides, I don’t imagine the thousand sides or see them as if they were present to me …This imagining, I find, takes more mental effort than understanding does; and that is enough to show that imagination is different from pure understanding."

Rene Descartes, Meditations on First Philosophy; Sixth Meditation, 1639