Observe that Descartes starts his system by using “error” and its synonyms
or derivatives as “stolen concepts.”
Men have been wrong, and therefore, he implies, they can never know what is
right. But if they cannot, how did they ever discover that they were wrong? How
can one form such concepts as “mistake” or “error” while wholly ignorant of
what is correct? “Error” signifies a departure from truth; the concept of
“error” logically presupposes that one has already grasped some truth. If truth
were unknowable, as Descartes implies, the idea of a departure from it would be
meaningless.
The same point applies to concepts denoting specific forms of error. If we
cannot ever be certain that an argument is logically valid, if validity is
unknowable, then the concept of “invalid” reasoning is impossible to reach or
apply. If we cannot ever know that a man is sane, then the concept of
“insanity” is impossible to form or define. If we cannot recognize the state of
being awake, then we cannot recognize or conceptualize a state of not
being awake (such as dreaming). If man cannot grasp X, then “non-X”
stands for nothing.