And who, I ask, can know that he understands anything, unless he do first understand it?
In other words, who can know that he is sure of a thing, unless he be first sure of that thing?
Further, what can there be more clear, and more certain, than a true idea as a standard of truth?
Even as light displays both itself and darkness, so is truth a standard both of itself and of falsity.
I think I have thus sufficiently answered these questions--namely, if a true idea is distinguished from a false idea, only in so far as it is said to agree with its object, a true idea has no more reality or perfection than a false idea (since the two are only distinguished by an extrinsic mark); consequently, neither will a man who has a true idea have any advantage over him who has only false ideas.

Further, how comes it that men have false ideas?
Lastly, how can anyone be sure, that he has ideas which agree with their objects?
These questions, I repeat, I have, in my opinion, sufficiently answered.

The difference between a true idea and a false idea is plain: from what was said in II.xxxv., the former is related to the latter as being is to not--being.

The causes of falsity I have set forth very clearly in II.xix.and II.xxxv.

with the note.