The novel Flatland was written in part as a way of explaining what is meant by transcendence in religion. It posits a group of people living in a two dimensional world, and thus many of the phenomena that can only be understood from a three-dimensional perspective are mysterious to Flatlanders, as the Flatlanders cannot really understand three-dimensional bodies.
First, since Flatlanders have no depth perception, they can only see each other as lines. Even the concept of two-dimensional shapes has to be deduced through a long chain of reasoning; only the well-educated can actually figure out another Flatlander's shape accurately.
The narrator Square actually imagines a "Lineland" with only one dimensional beings -- line segments of varying lengths -- and then imagines that Linelanders would not understand the shapes of Flatlanders, but would see Flatlanders simply as lines.
Finally, when Square meets Sphere, he slowly comes to understand that Flatland isn't really flat, but that because Flatlanders perceive only two dimensions, they cannot understand that they live on the surface of a three-dimensional body.