For those unfamiliar with the reference (from http://www.ams.org/notices/200410/fea-grothendieck-part2.pdf):

  One striking characteristic of Grothendieck’s
  mode of thinking is that it seemed to rely so little
  on examples. This can be seen in the legend of the
  so-called “Grothendieck prime”. In a mathematical
  conversation, someone suggested to Grothendieck
  that they should consider a particular prime number.
  “You mean an actual number?” Grothendieck asked. 
  The other person replied, yes, an actual prime number. 
  Grothendieck suggested, “All right, take 57."

  But Grothendieck must have known that 57 is not
  prime, right? Absolutely not, said David Mumford
  of Brown University. “He doesn’t think concretely.”