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cf., ST.I.Q58.A5; ST.I.Q85.A6; Sent.I.D19.Q5.A1.Rep7; SCG1.C59; SCG3.C108; QDeVer.Q1.A12; Peri.Bk1.L3; DeAn.Bk3.L11; Metaph.Bk6.L4; Metaph.Bk9.L11
In this question, as was said, St. Thomas first denies that falsehood formally exists in things. Thus, by consequence, it must exist in us. Further, in the second article, he has denied that it formally exists in our senses (with the appropriate qualifications given above), THUS the only place left to inquire about is in the intellect.
Now, one can distinguish between three acts of the intellect, 1. Simple Apprehension, 2. Judgment, 3. Argument. One apprehends the what-ness of a thing, then he makes judgments about it, then he draws conclusions from this by combining with other judgments.