I suppose I learned about the Roman Catholic "natural law" tradition in a philosophy or theology class as an undergraduate at the University of Notre Dame in the 1950s, but I don't remember much (anything?) about it. As I look now at the on-line Catholic Encyclopedia's entry on "natural law," I think I can understand why not much of it sank in way back then. It strikes me as so much gobbledygook.
I quote: "According to St. Thomas, the natural law is 'nothing else than the rational creature's participation in the eternal law.' The eternal law is God's wisdom, inasmuch as it is the directive norm of all movement and action."
So natural law is not a law in any sense like, say, Newton's laws of motion or Boyle's law of gases. It is rather a natural creature's free obedience to God-given supernatural laws. Or something.
My first significant encounter with "natural law" came as a graduate student at UCLA (University of California at Los Angeles) when my newly-married spouse and I went to a "round-table discussion" at the Newman Club (a club for Catholic students) on contraception. Well, it wasn't much of a round table at all, but rather the palsy-walsy chaplain instructing us on the fine points of natural law. The bottom line: Any form of contraception but the rhythm method is against the natural law. And sinful.
As it turned out, this wasn't a problem because my wife and I were eager to start a family, and by the time we were finished having children, we were finished with the Church's magisterium too.
In the meantime, I was studying real natural law, in the physics building across the campus, and then (again) at Notre Dame. Here was natural law of a different sort. One didn't choose to obey or disobey the law of gravity, say. If one stepped out of a fourth floor window, one accelerated at 9.8 meters-per-second-squared until one smacked into the sidewalk. The law of gravity carried no moral implications. It was eternal (presumably). If it was God's wisdom, we can thank Newton's apple for the revelation.
This was about the time when young Catholics were assertively questioning the Church's authority on such matters, and progressive theologians were not far behind. In 1968, Pope Paul VI tried to put a cap on discussion with the encyclical Humanae Vitae, overruling the majority report of a pontifical commission on the matter. The encyclical affirmed "laws written into the actual nature of man and of woman."
Paul wrote: "The Church, nevertheless, in urging men to the observance of the precepts of the natural law, which it interprets by its constant doctrine, teaches that each and every marital act must of necessity retain its intrinsic relationship to the procreation of human life."
And so the encyclical stands yet, unrevised, a deeply immoral (in my estimation) reaffirmation of a "natural law" that is neither natural nor law, that 98 percent of sexually-active Catholics with ready access to artificial contraception have the good sense to ignore, and that has caused untold human misery.