Various attempts have been made in recent years to state necessary and sufficient conditions for someone's knowing a given proposition. The attempts have often been such that they can be stated in a form similar to the following:
(a) S knows that P IFF (i) P is true, (ii) S believes that P, and (iii) S is justified in believing that P.
... These ... examples show that definition (a) does not state a sufficient condition for someone's knowing a given proposition.
Edmund L. Gettier, "Is Justified True Belief Knowledge?", Analysis, Vol. 23, No. 6 (Jun., 1963)