The story so far on Evangelical Realism, is that Deacon Duncan decided to criticize C.S.Lewis famous book, Mere Christianity.
"..in Chapter 1, he writes,
The idea was that, just as all bodies are governed by the laws of gravitation, and organisms by biological laws, so the creature called man also had his law — with this great difference, that a body could not choose whether it obeyed the law of gravitation or not, but a man could choose either to obey the Law of Nature or to disobey it."
The big problem with this is that Lewis is making an analogy, that morality is a law, which, like gravity is natural, but unlike gravity, we can choose to ignore, which makes this law, named 'The Law of Nature", by Lewis, to avoid naming it morality, a prescriptive law, in need of a Law-giver, who, much to no-one's surprise is going to turn out to be none other than God.
Nick's objection is that Lewis, as a Thomist, is just using this analogy for convenience and doesn't really believe it.
I think that this just makes Lewis doubly disingenuous since he is hiding his prescriptive law of morality in with descriptive natural laws and naming it, "The Law of Nature" specifically to come to the conclusion that it IS prescriptive and that God is the prescriber.
The simpler analogy would have been that morality is a law, but of course everyone would see through this right away and Lewis is trying to sound fair.
Nick's objection that Lewis was a Thomist is a simple diversion basically saying that Deacon Duncan cannot object to Lewis' use of this analogy because Lewis doesn't actually believe the analogy himself, is somewhat tricky as it puts Lewis' reasoning above criticism.
All's fair in Christian apology, I suppose.