The following is a great comment submitted by someone named DSH to my last post on the natural law.
I agree that the Founders most certainly did not have Aquinas in mind for the reasons given (i.e., he was a Roman Catholic).
"Natural law" is not as clear cut as its proponents suggest. Aquinas at ST, I-II, 94, ii, states that the natural law derives from the eternal law, and that the law in man is reason. If this were what most people thought was natural law, I probably wouldn't find it entirely objectionable (although still obviously false).
Pope John Paul II used the concept repeatedly in all his encyclicals, but it's clear he has Suarez's conception, not Aquinas's, in mind. His conception is that the "natural purpose of x is y" is ubiquitous.
But the whole notion of "final ends," (teleology) established in the first by Aristotle, is fuzzy category. In the human production of things, the notion is simple and obvious: I do A to obtain B results. B is the final end. A is the licit means. But is A the only means? Rarely. Too often natural law insists on only one A. That's untenable.
Since Newton, the notion of "natural ends" has dropped from salience. While we do think that the natural end of an acorn is to be an oak tree, it's also possible that its natural end is to remain an acorn or chipmunk food. Teleology, in other words, no longer has any hold in the natural/scientific realm.
Almost concurrently, David Hume raised what would later become known by G. E. Moore as the "naturalistic fallacy." Can one derive "ought" (morals/values) from "is" (fact)? The obvious answer is no. For example, the fact that the penis can insert itself into the vagina doesn't further instantiate that it must, much less "should." Celibates and homosexuals demonstrate it can either not be used at all or used in quite different ways.
I suggest that George, Finnis, et alia make the most convincing case for natural law in the past 500 years. But no matter how one constructs it, either on Aquinas's "reason" or Suarez's "natural nature," or these latter-day advocate, this bird may have wings, but it can't fly (ala Penguins). After all, isn't the function of wings and their final end is to fly; but Penguins, Ostriches, etc. have wings, but do not fly. Have they broken the natural law?
The whole concept is a confusion of thought. First, it raises the specter of essentialism (covered in the main article) that just can't be sustained. The notion of essences has largely been abandoned ever since Wittgenstein alighted onto "family resemblances." If essences qua essences are amorphous to start, then defining a particular essence is even more suspect. Is "man is a rational animal" (Boethius) man's essence? Or, "man is an emotional animal" ... "a linguistic animal" etc.
Since there are no natural ends (teleology) in natural science, why do we think there are natural ends that guide us in moral/political discoure? Ever since Aristotle's Four Causes have been reduced to one (efficient), it's no longer intelligible to talk about first, formal, and final causes, simply because these are metaphysical speculations. Natural law is entirely a metaphysical construct based on an outdated Aristotlean science.
Next, anyone who retains fondness for the natural law concept need only read G.E. Moore's "Principia Ethica" of last century to see clearly how misconceived the whole notion is. One simply cannot derive "ought" from "is." To try to do so is a categorical mistake (Ryle). Yet, that's the very heart of natural law theory.
Even the most benign interpretation (e.g., natural law = reason) doesn't add anything to the discussion. The problem with "this" kind of reason is that it tries to piggyback on facts of nature to derive moral platitudes, and so even at it's neatest divide, we're back to the naturalistic fallacy. If the matter were simply the equation as stated, then we've added absolutely nothing to the forum. Facts from nature are either explicit or implicit in every natural law moral/political claim.
As I said at the outset, George is probably the best defender of an archaic, untenable, unintelligible system of thought known as natural law theory, but to defend the indefensible gets us nowhere. Indeed, I could rattle off hundreds of deeply mistaken errors that natural law theorists have caused millions of people anxiety and ill. Just one example: Forbiddance of Contraception. It's coherent ONLY if you grant that the facts of nature can teach us moral lessons. Well, it can't. Since it can't pass initial muster, it belongs in the dustbin of history.