Full Idea
Nothing should be taken as certain without foundations.
Gist of Idea
Nothing should be taken as certain without foundations
Source
Gottfried Leibniz (Letters to Antoine Arnauld [1686], 1687.04.30)
Book Reference
Leibniz,Gottfried: 'Philosophical Writings', ed/tr. Parkinson,G.H.R. [Dent 1973], p.71
A Reaction
This might leave open the option, if you were a modern 'Fallibilist', that something might lack foundations, and so not be certain, and yet still qualify as 'knowledge'. That is my view. Knowledge resides somewhere between opinion and certainty.