e
intellect may be deceived, by attributing to the thing of which it
understands the essence, something which is not consequent upon it,
or is opposed to it. For the intellect is in the same position as
regards judging of such things, as sense is as to judging of common,
or accidental, sensible objects. There is, however, this difference,
as before mentioned regarding truth (Q. 16, A. 2), that falsity can
exist in the intellect not only because the knowledge of the
intellect is false, but because the intellect is conscious of that
knowledge, as it is conscious of truth; whereas in sense falsity does
not exist as known, as stated above (A. 2).

But because falsity of the intellect is concerned essentially only
with the composition of the intellect, falsity occurs also
accidentally in that operation of the intellect whereby it knows the
essence of a thing, in so far as composition of the intellect is
mixed up in it. This can take place in two ways. In one way, by the
intellect applying to one thing the definition proper to another; as
that of a circle to a man. Wherefore the definition of one thing is
false of another. In another way, by composing a definition of parts
which are mutually exclusive. For thus the definition is not only
false of the thing, but false in itself. A definition such as "a
reasonable four-footed animal" would be of this kind, and the
intellect false in making it; for such a statement as "some
reasonable animals are four-footed" is false in itself. For this
reason the intellect cannot be false in its knowledge of simple
essences; but it is either true, or it understands nothing at all.

Reply Obj. 1: Because the essence of a thing is the proper object of
the intellect, we are properly said to understand a thing when we
reduce it to its essence, and judge of it thereby; as takes place in
demonstrations, in which there is no falsity. In this sense
Augustine's words must be understood, "that he who is deceived,
understands not that wherein he is deceived;" and not in the sense
that no one is ever deceived in any operation of the intellect.

Reply Obj. 2: The intellect is always right as regards first
principles; since it is not deceived about them for the same reason
that it is not deceived about what a thing is. For self-known
principles are such as are known as soon as the terms are understood,
from the fact that the predicate is contained in the definition of
the subject.
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