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FOURTH ARTICLE [I, Q. 17, Art. 4]

Whether True and False Are Contraries?

Objection 1: It seems that true and false are not contraries. For true
and false are opposed, as that which is to that which is not; for
"truth," as Augustine says (Soliloq. ii, 5), "is that which is." But
that which is and that which is not are not opposed as contraries.
Therefore true and false are not contrary things.

Obj. 2: Further, one of two contraries is not in the other. But
falsity is in truth, because, as Augustine says, (Soliloq. ii, 10),
"A tragedian would not be a false Hector, if he were not a true
tragedian." Therefore true and false are not contraries.

Obj. 3: Further, in God there is no contrariety, for "nothing is
contrary to the Divine Substance," as Augustine says (De Civ. Dei
xii, 2). But falsity is opposed to God, for an idol is called in
Scripture a lie, "They have laid hold on lying" (Jer. 8:5), that is
to say, "an idol," as a gloss says. Therefore false and true are not
contraries.

_On the contrary,_ The Philosopher says (Peri Herm. ii), that a false
opinion is contrary to a true one.

_I answer that,_ True and false are opposed as contraries, and not, as
some have said, as affirmation and negation. In proof of which it must
be considered that negation neither asserts anything nor determines
any subject, and can therefore be said of being as of not-being, for
instance not-seeing or not-sitting. But privation asserts nothing,
whereas it determines its subject, for it is "negation in a subject,"
as stated in _Metaph._ iv, 4: v. 27; for blindness is not said except of
one whose nature it is to see. Contraries, however, both assert
something and determine the subject, for blackness is a species of
color. Falsity asserts something, for a thing is false, as the
Philosopher says (Metaph. iv, 27), inasmuch as something is said or
seems to be something that it is not, or not to be what it really is.
For as truth implies an adequate apprehension of a thing, so falsity
implies the contrary. Hence it is clear that true and false are
contraries.

Reply Obj. 1: What is in things is the truth of the thing; but what
is apprehended, is the truth of the intellect, wherein truth
primarily resides. Hence the false is that which is not as
apprehended. To apprehend being, and not-being, implies contrariety;
for, as the Philosopher proves (Peri Herm. ii), the contrary of this
statement "God is good," is, "God is not good."