xiv.). Q.E.D.

PROP. XLII. Knowledge of the second and third kinds, not
knowledge of the first kind, teaches us to distinguish the true
from the false.

Proof.--This proposition is self--evident. He, who knows how
to distinguish between true and false, must have an adequate idea
of true and false. That is (II. xl., note ii.), he must know the
true and the false by the second or third kind of knowledge.

PROP. 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. Coroll.). 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.
Coroll.), 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 ans