Full Idea
The claim that someone knows a proposition if it is true, it is believed, and the person is justified in their belief is false, in that the conditions do not state a sufficient condition for the claim.
Gist of Idea
Being a true justified belief is not a sufficient condition for knowledge
Source
Edmund L. Gettier (Is Justified True Belief Knowledge? [1963], p.145)
Book Reference
'Knowledge and Belief', ed/tr. Griffiths,A.Phillips [OUP 1968], p.144
A Reaction
This is the beginning of the famous Gettier Problem, which has motivated most epistemology for the last forty years. Gettier implies that justification is necessary, even if it is not sufficient. He gives two counterexamples.