Knowing that the existence of such objects is possible, Descartes then turns to the prevalence of mental images as proof. To do this, he draws a distinction between imagination and understanding; imagination being a non-linguistic “faculty of knowledge to the body which is immediately present to it […] without intellection or conception”, which therefore exists like a mental photograph; and understanding (or apprehending) being something that is not necessarily pictured. He uses an example of this to clarify:
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 (ad intelligendum); and this special exertion of mind clearly shows the difference between imagination and pure intellection (imaginatio et intellectio pura). (Descartes, Meditation VI: On the Existence of Material Objects and the Real Distinction of Mind from Body).
Descartes has still not given proof that such external objects exist. At this point, he has only shown that their existence could conveniently explain this mental process.