The Grinnis school, as I understand it, formed in reaction to the rejection of natural law among Western thinkers. The rejection was typically justified by invoking Hume's alleged naturalistic fallacy, that one can't derive a moral imperative, an "ought," from a description of fact, an "is." Grinnis accepted the validity of Hume's criticism, but has tried to rescue natural law theory from Humean criticisms, even going so far as to claim that Thomas Aquinas himself accepted Hume's distinction.
Oderberg argues convincingly that this school severely neglects metaphysical concerns. His main points I summarize thusly:
-Normativity, "oughtness," is based in essence, "is-ness;" there is no fact-value distinction.
-Unmoored in ontology, NLT can acquire an excessively subjective character
-NLT is traditionally grounded in ontology, rather than in epistemology or methodology, and thus requires an extrinsic ordination of law coupled with the promulgation thereof in the hearts of all men. These are metaphysical questions, rooted in knowledge of human nature.
-Yet for the Grinnis school, knowledge of human goods is methodologically prior to knowledge of human nature. Nature doesn't inform methodology. Indeed, the Grinnis school disagrees among itself about whether there really is such a thing as human nature.
-The Grinnis school focuses on inclinations, rather than metaphysics, yet it is that prior order of the universe and the order of human nature itself which makes inclinational knowledge rational in the first place. We have knowledge of nature through inclination, not, contra Grinnis, knowledge of inclination simpliciter.
-Further, the neglect of ontology has practical consequences for ethical debate, such as when arguing whether a given creature, such as a comatose person, is a human being.
I believe the last point might be an overstatement, as Robert P. George has made some detailed ontological arguments in his writings despite his theoretical missteps. Oderberg, I think, echoes other critics of the Grinnis school such as Russell Hittinger.
Oderberg makes some interesting additional comments about science. First, he is not waiting for an empirical discovery of human nature, mainly because he thinks such a method tries to acquire knowledge about a higher ontological order from a lower ontological order. Likewise, natural law theory assumes we already have enough knowledge through the law written on our heart. It won't do the layman any good to wait for science, or even philosophers, to get around to hammering out basic ethical principles. (I add, this is especially so regarding science since scientific inquiry is permeated by the fact-value distinction.)
Oderberg also discusses teleology, a phenomenon considered illusory by most scientists today. He claims we can't just focus only on human teleology, but we must also account for other directed movements in nature while not falling into panpsychism. As an example which I can't quite fully comprehend, he cites the ice-water-vapor cycle evident in nature. Here Oderberg neglects a problem with teleology for Catholic natural lawyers, since there is no natural end for the human person. Rather, God is his end.
Altogether, a worthy lecture. Oderberg's web site will provide significant further reading, and at first glance it threatens to usurp Notre Dame professor Alfred Freddoso's page as my favorite philosopher's personal web site.