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cf., Sent.I.D19.Q5.A1; QDeVer.Q1.A10; Metaph.Bk5.L22; Metaph.Bk6.L4
Now, in the seventeenth question, St. Thomas begins to discuss falsehood, which is the opposite of truth. First, he treats it in relation to things (A. 1). Second, in relation to the senses (A. 2). Third, in relation to the intellect (A. 3). Fourth, in relation to truth itself (A. 4).
In this article, St. Thomas asks the question of whether things are false. First, we know, from the nominal definition of "false," that it is something that is contrary to the truth. Further, it is a principle of philosophy that "contraries exist in the same subject." Thus, we can know that, just as truth exists formally in the intellect and only fundamentally in things, so also will the true exist formally in the intellect and only fundamentally in things.
Thus, to resolve this question, we must take note of the different intellects that the "false" can exist in. Further, we can distinguish, on the part of the object, the different relations of "dependence" of the object on the knowing subject, as we did above, i.e., whether accidental (disparity of a thing to the intellect upon which it does not depend) or essential (disparity of a thing to the intellect upon which it depends).