For whatever it's worth, I found this explanation useful, at least when rendered visually, when first encountering this stuff.

But clearly the geometric viewpoint isn't important to everybody, because the author of the post would have mentioned it if so!

See also, maybe: http://psychclassics.yorku.ca/Galton/imagery.htm http://www.nytimes.com/2015/06/23/science/aphantasia-minds-e... https://www.facebook.com/notes/blake-ross/aphantasia-how-it-...