to understand the idea of Peter, and still less the idea of the idea of
Peter. This is the same as saying that in order to know, there is no
need to know that we know, much less to know that we know that we know.
This is no more necessary than to know the nature of a circle before
knowing the nature of a triangle. But with these ideas the contrary is
the case; for in order to know that I know, I must first know. Hence it
is clear that certainty is nothing else than the subjective essence of a
thing: in other words, the mode in which we perceive an actual reality
is certainty. Further, it is also evident that for the certitude of
truth no further sign is necessary beyond the possession of a true idea;
for, as I have shown, it is not necessary to know that we know that we
know....

He who has a true idea knows at the same time that he has a true idea,
nor can he doubt the truth of the thing. For no one who has a true idea
is ignorant that a true idea involves the highest certitude; to have a
true idea signifying just this, to know a thing perfectly or as well as
possible. No one, in fact, can doubt this, unless he supposes an idea to
be something dumb, like a picture on a tablet, instead of being a mode
of thought, that is to say, intelligence itself. Moreover, I ask who can
know that he understands a thing unless he first of all understands that
thing? that is to say, who can know that he is certain of anything
unless he is first of all certain of that thing? Then, again, what can
be clearer or more certain than a true idea as the standard of truth?
Just as light reveals both itself and the darkness, so truth is the
standard of itself and of the false.

_Of Falsity_

There is nothing positive in ideas which can constitute a form of
falsity. But falsity cannot consist in absolute privation (for we say
that minds and not bodies err and are mistaken); nor can it consist in
absolute ignorance, for to be ignorant and to be in error are different.
Falsehood, therefore, consists in the privation of knowledge which is
involved by inadequate knowledge of things or by inadequate and confused
ideas. For instance, men are deceived because they think themselves
free, and the sole reason for thinking so is that they are conscious of
their own actions, and ignorant of the causes by which those actions are
determined. Their idea of liberty therefore is this--that they know no
cause for their own actions; for as to saying that their