But Descarte says, 'well i think i can represent this visually the same way Euclid was studying these triangles and these circles' he said 'why don't I ?' if we view a piece of paper. if we think about a two-dimensional plane. you could view a piece of paper as kind of a section of a two-dimensional plane. we call it two-dimensions because there's two directions that you can go in. there's the up down direction, that's one direction. so let me draw that, i'll do it in blue. because we're trying to visualize things so i'll do it the geometry colour. so you have the up down direction and you have the left right direction. that's why it's called a two-dimensional plane. if we're dealing with three-dimensions. you have an in out dimension. and it's very easy to do two-dimensions on the screen because the screen is two-dimensional. and he says 'Well, you know there are two variables here and they have this relationship.