We cannot get away from the fact that although we know that everything is rounded and three-dimensional, we have to separate one thing from another by delineating them. Logically, everything is separated into ‘things’ with edges, and with spaces in between them. Otherwise we would be bumping into things all the time.
“We live in a three-dimensional world, or four of you include Time; or up to eleven if you follow modern physics. This means we live in a mysterious world – we cannot know the other side of things. We cannot see it all at once.” Andrew Marr
“For Nature is made up all of roundnesses....Boughs are rounded, leaves are rounded, stones are rounded, clouds are rounded, cheeks are rounded; there is no more flatness in the natural world than there is vacancy” John Ruskin
(both quotes from the excellent ‘A Short Book About Drawing’ by Andrew Marr)

We cannot capture this three-dimensionality exactly, as our paper, after all, is flat! We can give a good representation of it though, as long as we draw with our eyes and what we really see, and not what our brains tell us. I find it easiest to explain this to students by asking them to pretend that, when drawing, the world is completely flat. Everything in this flat world fits together like pieces of a jigsaw.
Holding up your pencil again, with one eye closed, something close to you will ‘touch’ the edge of the pencil in space, and so will something in the far distance. Looking up and down the edge of the pencil, everything will touch it in a flat sense. The world is FLAT!
Besides anything else, holding up your pencil like this and squinting at the world beyond it shows you the most important function of your pencil; it is NOT for drawing with – you can draw with anything, from sticks to boot polish. No, the most important function of your pencil is to make you LOOK like an Artist. If you sit in front of a piece of paper and perform this exercise with conviction, it really impresses onlookers..........