A man reads a clock that stopped exactly twelve hours ago. It says three, and it is three. That’s Bertrand Russell’s example (1948). In 1963 Edmund Gettier’s three-page paper, with cases of his own, made the point famous: a belief can be true and justified and still not be knowledge.
How to play it
Ask him the time, as often as you like. Try to catch him being right.
That’s the whole collection examined, every room. Now imagine doing this with your own life, for a year, with two people who come to know it well. That’s membership.