Full Idea
A true belief is not knowledge when it is deduced from a false belief (as when deducing that the late Prime Minister's name began with B, believing it was Balfour, when actually it was Bannerman).
Gist of Idea
True belief is not knowledge when it is deduced from false belief
Source
Bertrand Russell (Problems of Philosophy [1912], Ch.13)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.76
A Reaction
Correct me if I am wrong, but isn't this the 'Gettier Problem'? It raises the central question of modern epistemology, which is what will be counted as adequate justification to make a true belief qualify as knowledge. How high do we set the bar?