This is from Matthew, xv, 14. It means that those without knowledge themselves
should not try to teach the ignorant.

> ‘He claimed to know a good deal about television sets, which was more than I
did. He asked me for a screwdriver, tinkered about for half an hour, than
said, “Right. Now you can switch on.” I did. There was a blue flash and the
set caught alight. Talk about the blind leading the blind! The fellow hadn’t a
clue.’