Two undefeated chess grandmasters meet at a chess tournament. No ties are accepted: this is a mental fight where only one can win. Each believes that they will win the match, and in fact has good reasons to believe that they will win: they each have defeated all previous games. For one of them their belief is true, for the other their belief is false, yet the winner cannot be said to know. True belief (mere opinion, doxa) does not make knowledge (episteme). We want a way to sort out true from false beliefs, and not just accidentally stumble upon truths like one of the grandmasters. To cut a long story short, we need some reliable way of sorting out true from false beliefs.