Thesis Natural law is a form of ethical naturalism, and it needs no ontological naturalism
Where it stands
I keep meeting a confusion, in students and in some books: that "naturalism" is one view, and that a Catholic, by definition, can't be a naturalist.
My thesis: natural law theory is a form of ethical naturalism, because it holds that moral facts are grounded in facts about human nature, and it does not require ontological naturalism, the view that only natural entities exist. Most natural lawyers have rejected ontological naturalism entirely.
For Aquinas, the precepts of natural law follow the order of our natural inclinations: to preserve our life, to the goods we share with other animals such as raising offspring, and to goods proper to reason, such as knowing the truth about God and living in society. What's good for us is fixed by what kind of being we are. That is a naturalist ethics. That God created that nature is a further claim about why the nature is as it is, not a rival account of what makes things good for us.
Philippa Foot's Natural Goodness (2001) gives a secular version of the same structure: goodness in living things is relative to their form of life, so a human lacking justice is defective in roughly the way an oak with shallow roots is defective.
So the word "naturalism" hides at least two different questions. I'd like objections from both sides: from naturalists who think God does no work here, and from anyone who thinks ethical naturalism fails anyway.