For most of philosophical history, "knowledge" was defined as *justified true belief*. To know that the cat is on the mat, three things had to be true: the cat IS on the mat (truth), you BELIEVE the cat is on the mat (belief), and you have GOOD REASONS for the belief (justification). Simple enough — until 1963.
Edmund Gettier published a three-page paper that broke the field. He constructed cases where someone has a justified true belief that doesn't intuitively count as knowledge. A standard example: you look at a clock that, unbeknownst to you, stopped exactly twelve hours ago; it currently reads 3:00, and the actual time IS 3:00. You believe it's 3:00 (true), you're justified (clocks usually work), and the belief is true — but you don't know what time it is. You got lucky.
Sixty years of "Gettier responses" tried to add a fourth condition. None has held. Some philosophers (Williamson) gave up on analyzing knowledge into parts and treated it as basic. Others (virtue epistemologists like Linda Zagzebski) argued that knowledge is a kind of cognitive achievement — knowing requires success that's CREDITABLE to the knower's competence, not luck. Whichever direction the field eventually settles, Gettier permanently changed what philosophers think they're doing when they say "I know."