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In this article, St. Thomas designates the mode of God's goodness, i.e., God does not have goodness accidentally, but essentially is goodness.
St. Thomas demonstrates this by the three ways in which a think is perfected.
First, it is perfect in the constitution of its being. In this, it contains what it is. Thus, a fire is "perfect" in that it has the form of fire. Now, God is a simple form. Thus, he is supremely perfect in the constitution of His being. For, all that is in God is God.
Second, it is perfect in that it has all the requisite accidents that are necessary for its perfection. In this too God is supremely perfect. For, he is not perfected by any accidents, but by his essence.
Third, it is perfect in that it attains its end. Yet, God does not only attain his end, but, is his end.
Thus, we may say that God is more perfect than all perfections and contains his goodness supremely and simply.
SC: Boethius says (De Hebdom.) that all things but God are good by participation. Therefore they are not good essentially.
Respondeo: God alone is good essentially. For everything is called good according to its perfection. Now perfection of a thing is threefold: first, according to the constitution of its own being; second, in respect of any accidents being added as necessary for its perfect operation; third, perfection consists in the attaining to something else as the end. Thus, for instance, the first perfection of fire consists in its existence, which it has through its own substantial form; its secondary perfection consists in heat, lightness and dryness, and the like; its third perfection is to rest in its own place.
This triple perfection belongs to no creature by its own essence; it belongs to God only, in Whom alone essence is existence; in Whom there are no accidents; since whatever belongs to others accidentally belongs to Him essentially; as, to be powerful, wise and the like, as appears from what is stated above (Q. 3, A. 6); and He is not directed to anything else as to an end, but is Himself the last end of all things. Hence it is manifest that God alone has every kind of perfection by His own essence; therefore He Himself alone is good essentially.