I’m not sure what the quote is that you are looking for (I have some books on Aquinas but I have yet to read them). The answer that you mentioned seems to just be an inverse of the Eurypthro dilemma restated in terms of evil, which doesn’t seem to solve the problem. I didn’t think this was St. Thomas’ view although I may be wrong.
The reason the dilemma does not apply to God is because God and Goodness and one and the same. So it’s not the case that God sat down one day and decided on the Ten Commandments or something like that because then all divine law is arbitrary. God would basically be like some sort of tyrant forcing His will on others, only He’s the biggest kid on the block, which doesn’t correspond to the classical understanding of God. On the other hand, if God is merely commanding certain behavior because there’s an external standard above Him that He adheres to, then He’s not really God because the standard of goodness is above Him and His being is dependent on this higher standard.
The solution is that something is good if it conforms to the nature of God, who is Goodness Itself. Evil is seen as a privation of good meaning that what is evil excludes God’s will or final ends to various degrees. I forget the terminology but this whole understanding of God involves something like the “convertibility of the transcendentals.” In other words, God’s being is His Power, is His Knowledge, is His Goodness, is His Will, is His Action, etc. Due to our limited natures we perceive separations between these transcendentals when they are all really one and the same in God. Does this make sense?
Alright. The fact is, I do remember a quote from St. Thomas Aquinas addressing this specific problem. It is simply that I do not remember what the exact quote was.
But yes, I know that God is Goodness itself, and the measure of all Good. Which means that, to seek goodness and righteousness, is to seek God Himself and that Good and Evil do not exist apart from the Divinity.
But my understanding from the quote of St. Thomas Aquinas’ quote was twofold.
First, because God is the Lawgiver, He commands that which is Evil, and that which is Good, and man is bound to give his assent, and live by God’s commands. But to be more specific, God’s commands can be related to the Divine Law. The Divine Law, in my understanding, essentially refers to the Ten Commandments, which God
revealed through Moses to Israel, and then to the nations. Through the Divine Law, God reveals that which is Good and Evil, and binds man to live by His commands. Hence, the first point to be made is,
God is Goodness, and for that fact, is the measure of Good and Evil. As such, man can know Good and Evil, because God reveals them.
The second consideration, is to consider Good and Evil under the aspect of the Natural Law. The central notion is to understand that
God forbids an act because it is Evil. It is true, as stated above, that God is Goodness, and the measure of Goodness Itself, since He is the Lawgiver, and the dictates of His Reason determine the rightness and the wrongness of human acts. At the same time, however, the sentence
God forbids an act because it is Evil can be taken to mean the following. It means that, although God is Goodness and the Author of the Divine Law, it is also possible to discover the rightness or the wrongness of an action apart from the Divine Law. To be more explicit, it means that,
human reason apart from the Divine Law, can discover the central truths of Good and Evil. This understanding can more or less be equated with the notion of the Natural Law.
In retrospect, let us summarize the two statements.
“Something is Evil because God commands it”: This means that man comes to know Good and Evil through the Revelation of God as Lawgiver, primarily in the Divine Law. In other words, man comes to the knowledge of Good and Evil, because God commands it.
Or, something is Good or Evil, because Divine Reason judges it to be Good or Evil.
“God commands something because it is Evil”: This means that although God is Goodness Itself, is the measure of Good and Evil, is the Lawgiver, and is the One who reveals Good and Evil, it is possible for human reason to discover the truths of rightness and wrongness apart from Divine Revelation, and primarily in the Natural Law. In other words, the knowledge of Good and Evil, can be discovered by human reason apart from Revelation.
Or, something is Good or Evil, because (human) right reason discovers it to be Good or Evil (apart from God’s Revelation).
This, I think is how I would explain the matter.