Kathryn: I don’t know about it being “so Derb,” but it is not true, as the

author seems to think, that four-dimensional configurations cannot be

visualized. The geometer John Flinders Petrie (1907-72) could, in moments

of intense concentration, answer questions about 4-dimensionsl figures in

such a way as to make it clear that he was visualizing them. Petrie is, of

course, best remembered as one of the co-authors of the 3-D geometry classic

The Fifty-Nine Icosahedra.