A couple of weeks ago we took a little flak from a potential reviewer of our book, Supporting Life: The Case for a Pro-Life Economic Agenda. The chief complaints were that, one, we didn't say anything about how God was on the side of the Pro-Life movement because we can only know right from wrong (the "natural law") if God tells us — and God said so, so that's that. Two, the argument conceded too much to "those people," meaning Pro-Choice adherents.
First, not only the Catholic Church, but Aristotle (who wasn't even Greek Orthodox), teaches that the principles of the natural law are discernible by reason alone. Second, conceding too much to the opposition is a matter of opinion, so we won't spend any time on that. Today.
Frankly, the first complaint confused us, and caused more than a few eyebrows to go up when (without naming any names, of course) we mentioned it to some attorneys, priests, and professors of moral philosophy, as well as rabbis, imams, and people like that.
The general consensus substantiated what we learned by reading Mortimer Adler (the "Great Books" philosopher) and Heinrich Rommen (a student of Father Heinrich Pesch, S.J., the "solidarist philosopher"), basing the natural law on faith in any degree automatically excludes anyone from whom the gift of faith has been withheld, or who has never heard of whatever revelation is being used.
Yet the popes in the encyclicals come right out and say that the natural law is "written in the hearts of all men" (and, yes, women and children), and thus no one is exempt from the precepts of the natural law. The primary precept of the natural law, of course, is "good is to be done, evil avoided."
The belief that more than reason is needed to discern the natural law was the error specifically addressed by Pope Pius XII in Humani Generis ("Concerning Some False Opinions Threatening to Undermine the Foundations of Catholic Doctrine") in 1950: ". . . absolutely speaking, human reason by its own natural force and light can arrive at a true and certain knowledge of the one personal God, Who by His providence watches over and governs the world, and also of the natural law, which the Creator has written in our hearts." (§ 2) [Emphasis added.]
Basing the natural law in any degree on faith also, according to both Adler and Rommen, leads directly into totalitarianism, and (as Rommen put it in his book, The Natural Law, Indianapolis: Liberty Fund, Inc., 1998) "pure moral positivism, indeed to nihilism" (p. 52), as well as to the belief that it is not necessary for God to exist (p. 62).
The role of faith should be confined to illuminating and helping us overcome the obstacles that "prevent reason from making efficient and fruitful use of its natural ability." (Humani Generis, loc. cit.)