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cf., Sent.I.D41.Q1.A3; SCG3.C163; QDeVer.Q6.A2; Ioan.C15.L3; Rom.C1.L3, C8.L6, C9.L3; Eph.C1.L1, L4
Now, he inquires into whether there is a cause on the part of the object, i.e., whether there is something (merit) in such and such a man that would cause God to predestine Him.
At this point, one might, from a simple consideration of what was said in the previous article and in the articles on the Divine Will and Providence answer in a sharp "no!," but, as often is the case, it is no so simple.
Rather than flatly denying that foreknowledge of merits is the cause of predestination, St. Thomas answers with a distinguo.
This is most helpfully laid out in QDeVer.Q6.A2, which I will use for my explanation (he only spends the last two paragraphs treating this in his Summa article).