(2) If an architect conceives a building properly constructed,
though such a building may never have existed, and amy never exist,
nevertheless the idea is true; and the idea remains the same,
whether it be put into execution or not.  (69:3) On the other hand,
if anyone asserts, for instance, that Peter exists, without
knowing whether Peter really exists or not, the assertion, as far
as its asserter is concerned, is false, or not true, even though
Peter actually does exist.  (4) The assertion that Peter exists is
true only with regard to him who knows for certain that Peter does
exist. 

[70]  (1) Whence it follows that there is in ideas something real,
whereby the true are distinguished from the false.  (2) This reality
must be inquired into, if we are to find the best standard of truth
(we have said that we ought to determine our thoughts by the given
standard of a true idea, and that method is reflective knowledge),
and to know the properties of our understanding.  (70:3) Neither must
we say that the difference between true and false arises from the
fact, that true knowledge consists in knowing things through their
primary causes, wherein it is totally different from false knowledge,
as I have just explained it: for thought is said to be true, if
it involves subjectively the essence of any principle which has no
cause, and is known through itself and in itself. 

[71]  (1) Wherefore the reality (forma) of true thought must exist
in the thought itself, without reference to other thoughts; it does
not acknowledge the object as its cause, but must depend on the
actual power and nature of the understanding.  (2) For, if we
suppose that the understanding has perceived some new entity which
has never existed, as some conceive the understanding of God before
He created thing (a perception which certainly could not arise 
any object), and has legitimately deduced other thoughts from said
perception, all such thoughts would be true, without being
determined by any external object; they would depend solely on the
power and nature of the understanding.  (71:3) Thus, that which
constitutes the reality of a true thought must be sought in the
thought itself, and deduced from the nature of the understanding.

[72]  (1) In order to pursue our investigation, let us confront
ourselves with some true idea, whose object we know for
certain to be dependent on our power of thinking, and to have
nothing corresponding to it in nature.  (2) With an idea of this
kind before us, we shall, as appears from what has just been said,
be more easily able to carry on the research we have in view.
(72:3) For instance, in order to form the conception of a sphere,
I invent a cause at my pleasure - namely, a semicircle revolving
round its center, and thus producing a sphere.  (4) This is
indisputably a true idea; and, although we know that no sphere in
nature has ever actually been so formed, the perception remains
true, and is the easiest manner of conceiving a sphere.
(72:5) We must observe that this perception asserts the rotation
of a semicircle - which assertion would be false, if it were not
associated with the conception of a sphere, or of a cause
determining a motion of the kind, or absolutely, if the assertion
were isolated.  (6) The mind would then only tend to the
affirmation of the sole motion of a semicircle, which is not
contained in the conception of a semicircle, and does not arise
from the conception of any cause capable of producing such motion.
    (72:7) Thus falsity consists only in this, that something is
affirmed of a thing, which is not contained in the conception
we have formed of that thing, as motion or rest of a semicircle.
(8) Whence it follows that simple ideas cannot be other than
true - e.g., the simple idea of a semicircle, of motion, of rest,
of quantity, &c.
    (72:9) Whatsoever affirmation such ideas contain is equal to the
concept formed, and does not extend further.  (10) Wherefore we
form as many simple ideas as we please, without any fear of error.

[73]  (1) It only remains for us to inquire by what power our mind
can form true ideas, and how far such power extends.  (2) It is
certain that such power cannot extend itself infinitely.  (3) For
when we affirm somewhat of a thing, which is not contained in the
concept we have formed of that thing, such an affirmation shows a
defect of our perception, or that we have formed fragmentary or
mutilated ideas.  (4) Thus we have seen that the notion of a
semicircle is false when it is isolated in the mind, but true when
it is associated with the concept of a sphere, or of some cause
determining such a motion.  (73:5) But if it be the nature of a
thinking being, as seems, prima facie, to be the case, to form true
or adequate thoughts, it is plain that inadequate ideas arise in us
only because we are parts of a thinking being, whose thoughts - some
in their entirety, others in fragments only - constitute our mind.

[74]  (1) But there is another point to be considered, which was not
worth raising in the case of fiction, but which give rise to complete
deception - namely, that certain things presented to the imagination
also exist in the understanding - in other words, are conceived
clearly and distinctly.  (2) Hence, so long as we do not separate that
which is distinct from that which is confused, certainty, or the true
idea, becomes mixed with indistinct ideas.  (3) For instance, certain
Stoics heard, perhaps, the term "soul," and also that the soul is
immortal, yet imagined it only confusedly; they imaged, also, and
understood that very subtle bodies penetrate all others, and are
penetrated by none.  (74:4) By combining these ideas, and being at the
same time certain of the truth of the axiom, they forthwith became
convinced that the mind consists of very subtle bodies; that these
very subtle bodies cannot be divided &c.

[75]  (1) But we are freed from mistakes of this kind, so long as we
endeavor to examine all our perceptions by the standard of the given