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This first article has a twofold purpose. First, to establish the judgment "God is perfect" by way of theological argumentation. For, we already know that God is perfect by way of revelation (and nature). Yet, St. Thomas reduces the judgment to its quasi-proper causes (i.e., he founds it upon the metaphysical essence of God from which every perfection flows, i.e., ipsum esse subsistens).
Second, we wishes, in proving that God is perfect, to arrive at a more profound definition of "perfection." For, perfection in creatures is very much different than perfection in God. For, creatures are described as "perfect" in their determinacy to some definite nature (DeDivNom.C13.L1.29-32). For them, "infinity" would be an imperfection, flowing from some lack towards definite being.