In their natural law analysis of politics, both Mortimer Adler and Heinrich Rommen claimed that abandoning the Aristotelian/Thomist understanding of the natural law as the basis for human positive law leads inevitably to tyranny. To this we can add the Aristotelian philosophy of Moses Maimonides and Ibn Khaldûn.
This is because the Aristotelian understanding of the natural law is that the natural law is based on God's Nature. Because God is (by definition) a perfect Being, His Nature is "self-realized" in His Intellect, that is, in His Reason. As Christians and adherents of many other faiths would put it, "God is Love." That is, God's Nature is so perfect that He cannot be less than what He is — the embodiment of absolutely pure perfection.
Paradoxically, God the Supreme Being cannot do everything. There is one thing that God cannot do. That is to be less than God, i.e., embody an imperfection. An imperfection is a contradiction in a perfect being. God cannot, therefore, contradict Himself in any way, or He would, ipso facto, not be God. This might seem an esoteric or unimportant point to make, but it is at the heart of the problem with the moral relativism that is, in turn, the foundation for Keynesian economics.
It all goes back to definition, as we might expect. That is, what is the natural law? How we answer that question determines in large measure our view of reality. This was one of the most burning questions of the Middle Ages. No, really. The definition we use of the natural law affects how we view God, religion, the State, the family, even (or especially, for our purposes) money, credit, banking and finance.
To try and be brief, if we define the natural law as God's Nature, self-realized in His Intellect (and therefore discernible by reason alone), we cannot go around changing that definition, even when it proves to be inconvenient or embarrassing. If life, liberty and property are natural rights — as the common consent of humanity has agreed for millennia — then we cannot change the definitions of life, liberty and property without claiming to subordinate God Himself to our personal will — our private opinion.
Abandoning the understanding of the natural law based on God's Nature self-realized in His Intellect, and adopting our opinion as to what constitutes God's Will effectively abolishes the natural law. To apply to all of humanity, the precepts of the natural law must be "written in the heart of every man."
Humanity being the animal that reasons, that means the law must be discernible by reason alone. It cannot be tied to any article of faith, as Pius XII pointed out in Humani Generis, even if faith is necessary for reason to make "efficient and fruitful use of its natural ability."
Opening by stating that the principles of Christian culture are today being attacked on all sides, the pope identified the modern rejection of the natural law based on God's Nature, self-realized in His Intellect as first among the "false opinions threatening to undermine the foundations of Catholic doctrine." As Pius XII explained,
"[A]bsolutely 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." (Humani Generis, § 2.)