Natural law doesn't bode well for the "Sola-Scriptura" the Bible trumps understanding of orthodox Protestantism.
Francis Schaeffer makes it here. As it relates to the context of the American Founding I am going to quote some arguments from Gary North's book "Conspiracy in Philadelphia." Note, though Dr. North is an extremist Reconstructionist, he has a PhD in history from University of California at Riverside and studied under Douglass Adair. Let his arguments in this regard rise and fall on their own merits.
Dr. North deals with Blackstone's Christian natural law synthesis. Christian Nationalists invariably cite Blackstone as authority for the "laws of nature and nature's God" because parts of his commentaries discuss the proper relationship between reason and revelation and Blackstone has a smoking gun quotation that gives revelation the edge. James Wilson, who unlike Blackstone, was actually an American Founder, also discussed the proper relationship between reason and revelation but, as I read him, is far more ambiguous on which can trump what, though he does, like Blackstone, assert their by in large agreement. Wilson also notes the two are incomplete without the other, but are supposed to work together. At one point Wilson notes:
These considerations show, that the scriptures support, confirm, and corroborate, but do not supercede the operations of reason and the moral sense. The information with regard to our duties and obligations, drawn from these different sources, ought not to run in unconnected and diminished channels: it should flow in one united stream, which, by its combined force and just direction, will impel us uniformly and effectually towards our greatest good.
As noted, Blackstone "theoretically" puts his cards more clearly on scripture's side. The passage Christian America apologists invariably stress follows:
This law of nature, being co-eval with mankind and dictated by God himself, is of course superior in obligation to any other. It is binding over all the globe, in all countries, and at all times: no human laws are of any validity, if contrary to this; and such of them as are valid derive all their force, and all their authority, mediately or immediately, from this original.
But in order to apply this to the particular exigencies of each individual, it is still necessary to have recourse to reason; whose office it is to discover, as was before observed, what the law of nature directs in every circumstance of life; by considering, what method will tend the most effectually to our own substantial happiness. And if our reason were always, as in our first ancestor before his transgression, clear and perfect, unruffled by passions, unclouded by prejudice, unimpaired by disease or intemperance, the task would be pleasant and easy; we should need no other guide but this. But every man now finds the contrary in his own experience; that his reason is corrupt, and his understanding full of ignorance and error.
....The doctrines thus delivered we call the revealed or divine law, and they are to be found only in the holy scriptures. These precepts, when revealed, are found upon comparison to be really a part of the original law of nature, as they tend in all their consequences to man's felicity. But we are not from thence to conclude that the knowlege of these truths was attainable by reason, in it's present corrupted state; since we find that, until they were revealed, they were hid from the wisdom of ages. As then the moral precepts of this law are indeed of the same original with those of the law of nature, so their intrinsic obligation is of equal strength and perpetuity. Yet undoubtedly the revealed law is (humanly speaking) of infinitely more authority than what we generally call the natural law. Because one is the law of nature, expressly declared so to be by God himself; the other is only what, by the assistance of human reason, we imagine to be that law. If we could be as certain of the latter as we are of the former, both would have an equal authority; but, till then, they can never be put in any competition together.
UPON these two foundations, the law of nature and the law of revelation, depend all human laws; that is to say, no human laws should be suffered to contradict these.
Here is Dr. North's critical commentary of Blackstone's remarks:
Having said this, he then spent four volumes describing English common law with only a few footnote references to the Bible. In the first three volumes, running almost 500 pages each, each has one footnote reference to the Bible. The fourth volume, on criminal law (Public Wrongs), has ten references. Not one of them is taken by Blackstone as authoritative for civil law; they were seen merely as historical examples. There is not a single reference to “Bible,” “Moses,” or “Revelation” in the set’s index.
How could this be if he was persuaded that biblical law and natural law are the same, but with biblical law so much clearer to us? Blackstone’s preliminary remarks were familiar in his era. Englishmen commonly tipped the brim of their epistemological caps to God and the Bible, but they did not take off their caps in the presence of God. They pursued their academic specialties just as Christians do today: with no systematic study of what biblical law specifically reveals regarding those disciplines. It was considered sufficient for Blackstone to have formally equated biblical law with natural law. Having done so, he could then safely ignore biblical law.
This raises another question: Was Blackstone in fact deliberately lying? In a perceptive essay by David Berman, we learn of a strategy that had been in use for over a century: combating a position by supporting it with arguments that are so weak that they in fact prove the opposite….If he was not lying, then he was naive beyond description, for his lame defense of biblical revelation greatly assisted the political triumph of the enemies of Christianity in the American colonies. pp. 22-24.
North also explains the context of how "educated" men like America's key Founders and the English Whig intellectuals they followed dealt with the reason v. revelation issue [Note: This is what's in between the ellipses in North's above quotation right before North's speculation that Blackstone might be lying.]:
This common equation of biblical law with natural law faced two monumental problems in the eighteenth century: (1) the continuing negative legacy of the English Civil War, 1642–60, in which the various Christian churches and sects had failed to agree on much of anything, a social and political experiment which ended with the restoration of Charles II; (2) the intellectual legacy of Isaac Newton, which had created a blinding illusion of the near-perfectability of reason’s ability to discern the perfect laws of nature in the physical world, and which therefore held out hope that this could also be accomplished in the moral and social realms.18 This dual legacy indicated that biblical revelation – or at least men’s understanding of that revelation – is far less certain as a guide to human action than unaided, unregenerate reason. Biblical higher criticism was a century old in English religious thought and politics by the time Blackstone wrote his Commentaries.19 Thus, by the time that the Commentaries appeared, the foundation of his defense of the superiority of biblical law to natural law – the greater clarity of biblical revelation compared to reason’s perception of natural law – was not believed by most men who called themselves educated. p. 23
And indeed, educated men Jefferson and J. Adams, in their private letters, demonstrate far more clearly than Wilson in his "Works" that they believed revelation subservient to man's reason. Wilson's Works (which were public) I admit are somewhat ambiguous on the proper relationship between reason & revelation, far more so than Blackstone. I would bet if his private letters were uncovered, we'd see something not unlike what follows from J. Adams and Jefferson on the superiority of reason over revelation.
First from Adams:
Philosophy, which is the result of reason, is the first, the original revelation of the Creator to his creature. man. When this revelation is clear and certain, by intuition or necessary inductions, no subsequent revelation, supported by prophecies or miracles, can supersede it. Philosophy is not only the love of wisdom, but the science of the universe and its cause. There is, there was, and there will be but one master of philosophy in the universe. Portions of it, in different degrees, are revealed to creatures. Philosophy looks with an impartial eye on all terrestrial religions. I have examined all, as well as my narrow sphere, my straitened means, and my busy life would allow me ; and the result is, that the Bible is the best book in the world. It contains more of my little philosophy than all the libraries I have seen; and such parts of it as I cannot reconcile to my little philosophy, I postpone for future investigation.
-- To Thomas Jefferson, Dec. 25, 1813.
Adams entire letter is worth a read, small parts of which have often been quoted out of context. Adams makes a number of notable claims: 1) That reason, not revelation is penultimate; 2) that, nonetheless, the Bible is the "best book"; and 3) that Hinduism and many exotic world religions teach the same truth as Christianity.
Here is Dr. Gregg Frazer's commentary on the letter:
In context, he has just said: “Philosophy, which is the result of reason, is the first, the original revelation of the Creator to his creature, man. … no subsequent revelation, supported by prophecies or miracles, can supersede it.” [the latter refers, of course, to the Bible and its inferiority to philosophy] He goes on to say: “Philosophy looks with an impartial eye on all terrestrial religions” and then talks about the Bible further. About the Bible, he then says: “such parts of it as I cannot reconcile to my little philosophy, I postpone for future investigation.” He then talks about Joseph Priestley (his spiritual mentor) and about various religious systems he and Priestley have encountered, including Zoroastrianism, Confucianism, Plato, the Brahmins, and then the Shastra — and the quoted commentary on the Shastra. A paragraph later, he says “these doctrines, sublime, if ever there were any sublime, Pythagoras learned in India, and taught them to Zaleucus and his other disciples.” Earlier in the same letter, he said: “The preamble to the laws of Zaleucus, which is all that remains, is as orthodox as Christian theology as Priestley’s ….” This is critical because Priestley is Adams’s (& Jefferson’s) spiritual mentor and because the laws of Zaleucus were supposedly handed down to pagans from Athena! SO YOU SEE THAT HE SPECIFICALLY INCLUDED CHRISTIANITY IN THE COMPARISON! Further, if a set of laws supposedly handed down from Athena 600 years before the birth of Christ can be considered “Christian” — what real meaning does the term have for Adams?
And Jefferson's exaltation of reason:
Fix reason firmly in her seat, and call to her tribunal every fact, every opinion. Question with boldness even the existence of a God; because, if there be one, he must more approve of the homage of reason, than that of blindfolded fear. You will naturally examine first, the religion of your own country. Read the Bible, then as you would read Livy or Tacitus. The facts which are within the ordinary course of nature, you will believe on the authority of the writer, as you do those of the same kind in Livy & Tacitus. The testimony of the writer weighs in their favor, in one scale, and their not being against the laws of nature, does not weigh against them. But those facts in the Bible which contradict the laws of nature, must be examined with more care, and under a variety of faces. Here you must recur to the pretensions of the writer to inspiration from God. Examine upon what evidence his pretensions are founded, and whether that evidence is so strong, as that its falsehood would be more improbable than a change in the laws of nature, in the case he relates. For example, in the book of Joshua, we are told, the sun stood still several hours. Were we to read that fact in Livy or Tacitus, we should class it with their showers of blood, speaking of statues, beasts, &c. But it is said, that the writer of that book was inspired. Examine, therefore, candidly, what evidence there is of his having been inspired. The pretension is entitled to your inquiry, because millions believe it. On the other hand, you are astronomer enough to know how contrary it is to the law of nature that a body revolving on its axis, as the earth does, should have stopped, should not, by that sudden stoppage, have prostrated animals, trees, buildings, and should after a certain time gave resumed its revolution, & that without a second general prostration. Is this arrest of the earth's motion, or the evidence which affirms it, most within the law of probabilities?
-- To Peter Carr, August 10, 1787.
A passage from James Wilson's Works intimates such an understanding:
The law of nature is immutable; not by the effect of an arbitrary disposition, but because it has its foundation in the nature, constitution, and mutual relations of men and things. While these continue to be the same, it must continue to be the same also. This immutability of nature's laws has nothing in it repugnant to the supreme power of an all-perfect Being. Since he himself is the author of our constitution; he cannot but command or forbid such things as are necessarily agreeable or disagreeable to this very constitution. He is under the glorious necessity of not contradicting himself. This necessity, far from limiting or diminishing his perfections, adds to their external character, and points out their excellency.
Finally let me address the fact that these "educated" men sometimes quoted from the Bible as though it were a history book. Men like Ben Franklin and James Wilson did indeed sometimes quote the Bible as though its history were true and better respected it as a history book than today's intellectuals do. However, they still didn't necessarily believe it infallible or *the* authoritative source of history, just one of many valid sources. As Dr. North put it:
I am not arguing that Englishmen trusted a priori reason as the sole guide to human institutions; they also placed great weight on historical experience. My point is only that they placed almost zero practical weight on Old Testament law and experience, and when they cited the Old Testament, they did so because it was merely one historical source among many. p. 24 footnote 21.