No one, who has a true idea, is ignorant that a true idea involves the highest certainty.
 Baruch Spinoza, Ethics (1677). copy citation

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Author Baruch Spinoza
Source Ethics
Topic certainty ideas
Date 1677
Language English
Reference
Note Translated by R. H. M. Elwes
Weblink http://www.gutenberg.org/files/3800/3800-h/3800-h.htm

Context

“that is, obviously, he must be assured. Q.E.D. Note.—I explained in the note to II. xxi. what is meant by the idea of an idea; but we may remark that the foregoing proposition is in itself sufficiently plain. No one, who has a true idea, is ignorant that a true idea involves the highest certainty. For to have a true idea is only another expression for knowing a thing perfectly, or as well as possible. No one, indeed, can doubt of this, unless he thinks that an idea is something lifeless, like a picture on a panel, and not a mode of thinking—namely, the very act of understanding.” source