Edmund Gettier proved that a person can have a justified true belief without actually possessing knowledge.

In a brief 1963 paper, Gettier presented scenarios where someone's belief was both justified and true, yet only by sheer luck. For instance, if you look at a broken clock that happens to show the correct time, your belief about the time is true and justified by the clock, but you do not actually 'know' the time. This simple revelation forced philosophers to rethink the very definition of knowing.