FROM BOTH A CLASSICAL AND CHRISTIAN perspective, a Natural Law theory will require a combination of four elements. It will require an epistemology (a theory of knowledge) that is "realist," that is, one which maintains that objective reality is communicated or translated from the object to the subject in a manner that is both reliable and true. Unlike the Kantian critique, it insists we are able to know the "ding an sich," the thing in itself. It will require a metaphysical understanding of nature that sees nature as have an "end," a reason, a blueprint in it. It will suppose a natural theology. In other words, it will acknowledge a Divine Creator and Orderer of the natural world. It will suppose that man is free and rational, and must use these faculties, and not only impulse, in knowing and doing good. Simply put, it will require that man (i) know (ii) himself and his nature, (iii) that that nature has a purpose or end, placed there by God, which informs him of the good, and (iv) that he is free, both in his reason and will, to do that good. Without these, any theory of law and morality, even one given the title natural law, will, at best, limp or falter. These requirements of a classical natural law are well-summarized by John Courtney Murray, S.J.John Courtney Murray, S.J., We Hold These Truths: Catholic Reflections on the American Proposition (Lanham: Sheed & Ward, 1960), 327-28.
Natural law supposes a realist epistemology, that asserts the real to be the measure of knowledge, and also asserts the possibility of intelligence reaching the real, i.e., the nature of things--in the case, the nature of man as a unitary and constant concept beneath all individual differences. Secondly, it supposes a metaphysic of nature, especially the idea that nature is a teleological concept, that the "form" of the thing is its "final cause," the goal of its becoming; in the case, that there is a natural inclination in man to become what in nature and destination he is--to achieve the fullness of his own being. Thirdly, it supposes a natural theology, asserting that there is a God, Who is eternal Reason, Nous, at the summit of the order of being, Who is the author of all nature, and Who wills that the order of nature be fulfilled in all its purposes, as these are inherent in the natures found in the order. Finally, it supposes a morality, especially the principle that for man, a rational being, the order of nature is not an order of necessity, to be fulfilled blindly, but an order of reason and therefore of freedom. The order of being that confronts his intelligence is an order of "oughtness" for his will; the moral order is a prolongation of the metaphysical order into the dimensions of human freedom.
The philosopher Kant denied the first. In different ways, René Descartes and David Hume denied the second. Nietzsche denied the third. Calvin denied the fourth. These are some of the enemies of the Natural Law.