Full Idea
If a mental state provides no guarantee against error, then it cannot serve as a foundation for knowledge.
Gist of Idea
Mental states cannot be foundational if they are not immune to error
Source
Ernest Sosa (The Raft and the Pyramid [1980], §4)
Book Reference
'Epistemology - An Anthology', ed/tr. Sosa,E. /Kim,J. [Blackwell 2000], p.136
A Reaction
That assumes that knowledge entails certainty, which I am sure it should not. On a fallibilist account, a foundation could be incredibly secure, despite a barely imaginable scenario in which it turned out to be false.