Full Idea
Who can know that he understands some thing unless he first understands it? That is, who can know that he is certain about some thing unless he is first certain about it?
Gist of Idea
You only know you are certain of something when you actually are certain of it
Source
Baruch de Spinoza (The Ethics [1675], II Pr 43S)
Book Reference
Spinoza,Benedict de: 'Ethics', ed/tr. Curley,Edwin [Penguin 1996], p.58
A Reaction
This seems to beg the question, which concerns how you get to the state of full understanding or certainty in the first place. Spinoza thinks only certainty counts as knowledge, which seems to derive from Descartes. I prefer Peirce.