One criticism aimed at natural law is that it falls foul of the naturalistic fallacy, but as we will see there are good reasons both for doubting the validity of this very term, and that it could be applied to natural law.
John Finnis, in a conversation with Peter Vardy, says that the neo-scholastics were confused by the order in which we come to know nature and the ontological order itself. We understand human nature by understanding human goods, BUT these goods do depend on the way we are. We look to objects, act, capacities and therefore to nature. The failure to make the distinction between these two ways = the naturalistic fallacy.
“Controversially, Finnis starts with the argument that Aquinas, properly interpreted, was an ethical non-naturalist. He argues that the ‘basic goods’ of natural law appeal directly to reason without any need to make particular observations of nature”
But Finnis is simply noting that Aquinas’ first principles are axiomatic. If they were derived from some prior observations then those observations themselves would require further underpinning.
Thomas Storck says in the post I linked to from here, that:
“Moral goodness is in fact a subset of ontological goodness, a part of ontological goodness applicable only to creatures with intellect and will”
“Moral goodness and badness are simply that part of ontological goodness or badness which is more or less subject to our free choice. And because our possession of intellect and will is what specifically distinguishes us from the other animals, who lack those endowments, the goodness or badness which depend upon our intellect and will mark out a human being as good or bad more clearly than any mere ontological deficits, deficits which have absolutely no moral aspect. Thus a bad man is not someone who is blind or lame, but someone who steals or cheats and so on.”
Finally, Phillipa Foot, in her commentary on ‘thick concepts’ argues that some terms such as ‘rude’ ” undermine the is-ought gap: calling something rude is evaluative because it expresses condemnation in the same kind of way as bad and wrong do, but this evaluation can be derived from evaluative description. “
In summary, criticisms of natural law which rely on some version of the naturalistic fallacy should be used with care.