Full Idea
When we say that a man assents to what is false and does not doubt it, we do not say that he is certain, but merely that he does not doubt, that is, that he assents to what is false, because there are no causes sufficient to make his imagination waver.
Gist of Idea
A man who assents without doubt to a falsehood is not certain, but lacks a cause to make him waver
Source
Baruch de Spinoza (The Ethics [1675], II Pr 49)
Book Reference
Spinoza,Benedict de: 'Ethics', ed/tr. White,WH/Stirling,AH [Wordsworth 2001], p.89
A Reaction
This is a seventeenth century rationalist desperate to say that the reason can deliver certainty, in the face of idiots who are totally certain about astrology, fairies and what not. Vain hope, I'm afraid. Fallibilist rationalism is required.