ABSTRACT
Standing on the world you see a part of it, your neighborhood. Move about and you discover other parts of it; these are glued in your memory so as to obtain your own map of the world. Did you ever detect for yourself (or even find a proof) that the world is round? How? From childhood on we have learned to observe complex objects by looking at specific points, edges, faces, shaded areas, bumps,... and then compose a global picture. Most of the geometrical objects we are studying now are too complex to be visualized, even with the use of sophisticated computers the images or at best approximations and we cannot see more than three dimensional shapes. Again abstract mathematics follows closely the way our thinking and observing have evolved.
