We shall continue considering the essence of the one God by turning now from His simplicity (Q3) to His perfection.
And because everything in so far as it is perfect is called good, we shall have to address this difficulty by speaking of His goodness (Q6) -- and goodness in general (Q5) -- as well as His perfection (Q4).
Note that Aquinas also treats this difficult topic in his Exposition of the "On the Hebdomads" of Boethius.
We shall begin considering the pefection of God with three articles: