How do we know what knowledge is? In his remarkable 1963 article, Gettier proves that Plato's 2000-year-old definition of knowledge is flawed—in just 930 words. Plato defined knowledge as "justified true belief" that meets three criteria: (1) it is true, (2) you believe it to be true, and (3) you are justified in believing it is true. But, using two ingenious examples, Gettier demonstrates that a justified belief can be true because of mere luck. This, he argues, means that justified true belief is not necessarily knowledge. Among the most cited philosophical works ever published, Gettier's short article forced a total rethink of a key philosophical theory.