Russell Hittinger {respuesta a James Skillen}: These words are very tricky, especially if you go through the three foci of natural law. For instance, the noun in its nominative case—natura—is not the best one theologically. The adverb—naturaliter—works better. It's not merely coincidental that the Church Fathers and even Aquinas used the adverb wherever possible to avoid the suggestion that nature is a law. They weren't stupid. They knew you can't just predicate a law of nature, except in the metaphorical sense. But-nature can be a mo de of promulgation by a divine legislator.

In modern terms, the fact that something is internal would mean that it is part of the very essence of the thing it is internal to. Aquinas has long discussions of this issue; in short, na tural law is psychologically internal, but metaphysically extrinsic to the person. So, for instance, desire is internal, but the final cause isn't.

By analogy, for Thomas, the natural law, insofar as it's real law, is an extrinsic principle. By the way, Thomas mentions three extrinsic principles of action: diabolus, lex, gratia —the devil, law, and grace. The natural law is extrinsic, but not external; whereas positive law is extrinsic and almost always external. Aquinas makes that distinction in. his commentary on Aristotle's Metaphysics, but almost no one ever talks about it. It is crucial in Aquinas to sort out what these words mean so that you don't get lost in them. Because indeed, when you use nature in the nominative case, it can look as though you are predicating a law of nature. At that point you're in big trouble theologically. You may not be in big trouble epistemologically—just as you can go on counting while having an absolutely terrible theory of mathematics, I suppose. You can go on reasoning about this body of law while having a terrible theory about it. I would predict, though, that you won't go on reasoning well for very long. At a certain point the mind is going to want another reflective, account of what it's doing. If we should start thinking of nature or the human mind as the law, we are in big trouble.

Natural Law and Catholic Moral Theology, p. 47 y 48