Philosophy: Getting It Right
In the complacent 1950s, it was received wisdom that we know a given proposition to be true if, and only if, it is true, we believe it to be true, and we are justified in so believing. This consensus was exploded in a brief 1963 note by Edmund Gettier in the journal Analysis.
Here is an example of the sort used by Gettier to refute that theory. Suppose you have every reason to believe that you own a Bentley, since you have had it in your possession for many years, and you parked it that morning at its usual spot. However, it has just been destroyed by a bomb, so that you own no Bentley, despite your well justified belief that you do. As you sit in a cafe having your morning latte, you muse that someone in that cafe owns a Bentley (since after all you do). And it turns out you are right, but only because the other person in the cafe, the barista, owns a Bentley, which you have no reason to suspect. So you here have a well justified true belief that is not knowledge.