Now what is the thing he didn't draw? Well, the constr. of the pentg. – But what's this look like?
  ‘But the sentence simply says that there isn't such a thing & that therefore he can't have drawn it.’
  Is it then analogous to: Sm. can't have
drawn a Centaur? – No. – Or is it like saying: he can't have drawn a woohoo, because there isn't such a thing? Let's see! This is the one answer of which you can say that I want you to give.