I think you must be referring to perspective... which gives indeed an idea of 3D. Medieval paintings lacked perspective. But technically it's still 2D: both eye

I think you must be referring to perspective... which gives indeed an idea of 3D. Medieval paintings lacked perspective. But technically it's still 2D: both eyes see the same thing. We are used to 2D: if you look around with one eye, or at very long distances: the moon, the sun and the stars are in 2D. And we're used to 2D in drawings, paintings and photographs. 3D tries to trick you into seeing 3D by showing a different picture for each eye, but the illusion is far from perfect: according to this article we receive clues that point in different directions. And that, it claims, isn't healthy in the long run.