Full Idea
What reaches to knowledge, I think may be called certainty; and what comes short of certainty, I think cannot be knowledge.
Gist of Idea
If it is knowledge, it is certain; if it isn't certain, it isn't knowledge
Source
John Locke (Letters to Edward Stillingfleet [1695], Letter 2), quoted by Simon Blackburn - Quasi-Realism no Fictionalism
Book Reference
Alexander,Peter: 'Ideas, Qualities and Corpuscles' [CUP 1985], p.282
A Reaction
I much prefer that fallibilist approach offered by the pragmatists. Knowledge is well-supported belief which seems (and is agreed) to be true, but there is a small shadow of doubt hanging over all of it.