John, this adaption of Aquinas’ teaching causes the Church a great deal of misunderstanding and is one of the interpretations of evil that David Bentley Hart calls to task in the essay I previously recommended.
Here is another that deals more specifically with this off the cuff, simplistic view of evil:
"But as you can tell by Aidan Walker’s interpretation of Aquinas I’ve been presenting here, evil is a profound alteration of good. Evil, Thomas is telling us, is not a nothing, but a voracious ontological parasite that feeds off of the real in order to clothe its empty center with a shadowy, counterfeit substance with no originality of its own.
Further, there are three considerations that flow from Thomas’ account of evil as “privation of the good” that are crucial for our understanding of how joy relates to evil and how God’s theodicy, His response to the question of evil is, in fact, a deed: the Paschal Mystery of the incarnate Son."
The essay continues here:
payingattentiontothesky.com/2009/07/03/evil-and-joy-ii-%e2%80%9cif-you-do-not-have-this-love-do-not-go-near-these-wounds%e2%80%9d/
My advice John, is get it right because the idea is very complex and for every apologist we have who can explain it the better makes us better as a Church. Conversely for everyone who mumbles “evil doesn’t exist” or is just a “lack of goodness”, the hurt and shock this causes those who confront real evil in our world is immeasurable.
In Christ,
dj