so ignorant of geometry that Alberti thought it necessary 
     to explain the words diameter and perpendicular. (33)

     It is necessary for the understanding of the visual take-
off that was to occur with Gutenberg technology, to know that 
such a take-off had not been possible in the manuscript ages, 
for such a culture retains the audile-tactile modes of human 
sensibility in a degree incompatible with abstract visuality or 
the translation of all the senses into the language of unified, 
continuous, pictorial space. That is why Ivins is entirely justified 
in maintaining in his Art and Geometry  (p. 41):

          Perspective is something quite different from 
     foreshortening. Technically, it is the central projection of 
     a three-dimensional space upon a plane. Untechnically, it 
     is the way of making a picture on a flat surface in such a