When I have a mental image of a triangle, for example, I don't just understand that it is a figure bounded by three lines; I also "look at" the lines as though they were present to my mind's eye. And this is what I call having a mental image. When I want to think of a chiliagon, I understand that it is a figure with a thousand sides as well as I understand that a triangle is a figure with three, but I can't imagine its sides or "look" at them as though they were present.… Thus I observe that a special effort of mind is necessary to the act of imagination, which is not required to conceiving or understanding (); and this special exertion of mind clearly shows the difference between imagination and pure intellection ("").