A stopped clock shows the right time. Do you know the time?

A clock reads 2:00, so you believe it's 2:00, and it is. But the clock stopped 12 hours ago.

Many philosophers long defined knowledge as a true belief you have good reason for. Your belief fits: it's true, and a clock is a good reason.

But it feels like luck, not knowing. In 1963, philosopher Edmund Gettier used cases like this to challenge that definition, in a 3-page paper.