If the blind lead the blind, both shall fall into the ditch

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.’