Benedict de Spinoza, THE ETHICS
Page: 35

knowledge.

XLIII. He, who has a true idea, simultaneously knows that he has
a true idea, and cannot doubt of the truth of the thing
perceived.

>>>>>Proof--A true idea in us is an idea which is adequate in
God, in so far as he is displayed through the nature of the
human mind (II. xi. Cor.). Let us suppose that there is in God,
in so far as he is displayed through the human mind, an adequate
idea, A. The idea of this idea must also necessarily be in God,
and be referred to him in the same way as the idea A (by II.
xx., whereof the proof is of universal application). But the
idea A is supposed to be referred to God, in so far as he is
displayed through the human mind; therefore, the idea of the
idea A must be referred to God in the same manner; that is (by
II. xi. Cor.), the adequate idea of the idea A will be in the
mind, which has the adequate idea A; therefore he, who has an
adequate idea or knows a thing truly (II. xxxiv.), must at the
same time have an adequate idea or true knowledge of his
knowledge; 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. 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