Full Idea
A true belief cannot be called knowledge when it is deduced by a fallacious process of reasoning. If I know all Greeks are men, and Socrates was a man, I cannot know that Socrates was a Greek, even if I falsely infer it.
Gist of Idea
A true belief is not knowledge if it is reached by bad reasoning
Source
Bertrand Russell (Problems of Philosophy [1912], Ch.13)
Book Reference
Russell,Bertrand: 'The Problems of Philosophy' [OUP 1995], p.76
A Reaction
Another very nice 'Gettier' example, fifty years before Gettier. There is a danger of circularity here, between knowledge, fallacy and truth. Giving them three independent definitions does not look promising.