Eric Voegelin and Leo Strauss were both driven to reconsider the status of the knowability of the natural law by their opposition to the regnant dogmas of their day (and ours); dogmas that denied the very possibility of philosophy. Voegelin called for a “retheorization” or regaining of principles in the face of the positivistic position dominating political science. He maintained that positivism had led to what he called a “destruction of science” that caused a loss of knowledge of principles; this destruction was accomplished by the improper application of methods borrowed from the natural sciences coupled with the belief that these methods ruled out questions that could not be considered “factual” (i.e. metaphysical matters). Voegelin proceeded to attempt this retheorization by an interpretation of the complicated web of symbols, at various levels of compactness and differentiation (from ritual to myth to theory) that societies have employed to understand themselves. His study of these symbols led him to an examination of the nature of law in general and also the nature of the natural law in particular. He addressed the nature of the law and the natural law in courses on jurisprudence taught at LSU in the 1950’s (after writing The New Science of Politics); these lectures and supplementary notes were published posthumously. 
The focus of Voegelin’s treatment is primarily on the concept of law and only secondarily on the natural law. This is I believe a necessary consequence of his basic approach: the natural law appears as a moment, albeit an extended moment, in the unfolding or increased differentiation of symbols in human history. A concept of law exists in all of the “primary” social orders according to Voegelin (cosmological, anthropological/classical, and soteriological) ; but natural law is not possible in say a cosmological civilization as there is no comprehension of a difference between the existing order and the cosmic order. As differentiation proceeds, it becomes impossible to return to an earlier epoch. Voegelin argued that “the very historicity of human existence, that is the unfolding of the typical in meaningful concreteness, precludes a valid reformulation of principles through a return to a former concreteness.” Much can be learned from earlier philosophers, but their ideas cannot be simply true in the concrete circumstances of the present. Once one has left Egypt for Greece or Plato for Augustine return is impossible. Voegelin’s premises hence necessarily require a “new science of order” or politics. This is a fundamental distinction between Voegelin and Strauss; Strauss thought return was possible or that philosophers who lived in earlier ages may have been simply right. It is important to note that Voegelin did maintain that human nature was unchangeable as a basic principle.
The unchangeable character of human nature may be the reason why Voegelin does not present the natural law as a short-lived phenomenon. The natural law originates “where a concept of human nature is formed in opposition to the accepted order of society . . .” in a post-cosmological order. These conceptual descriptions of human nature are formed out of a “reaction toward injustice . . . .” These reactions arise from moments of social crisis—the birth, death, or other “epochal” moment in the life of a society.
The first example of natural law provided by Voegelin in his “Supplementary Notes” is the Aristotelian distinction between natural and legal justice. It is striking to note the robust character of the natural law in Voegelin’s account: natural justice is not the mere bare requirements of morality required for the existence of civil society; it is rather “the suitable environment for the full unfolding of human potentialities to the bios theoretikos of the mature man.” Natural justice is comprehensive; it describes the full conditions of human flourishing. Of course it is difficult to see Aristotle as a natural law thinker in the strict sense as there is no lawgiver in his account. Voegelin consistently uses the term “law” in a loose or metaphorical sense.
The natural law is said to be obligatory—it has “theoretical justification insofar as it translates the insights gained by a theory of man into the language of obligatory purposes.” Voegelin gives us two examples of how these insights are transformed into obligatory moral rules based on his understanding of Aristotle. First, man should strive to fulfill his natural end (the theoretical life); second, “society should be organized in such a manner that fulfillment of the purpose is possible, at least for those who wish to attain it.” The Aristotelian natural law would seem to demand that society be organized along the lines of the best regime described in Plato’s Republic; but then Voegelin qualifies this when he argues that “[N]atural law becomes dubious if it erects theoretically justified, paradigmatic rules of order into postulates of revolutionary reform.”
It is also the case that the natural law is not necessarily comprehensive or does not describe everything required for full human flourishing. In his description of the Thomistic doctrine, Voegelin notes that insights based on the natural law “are imperfect and must be aided by the revealed lex divina.” The natural law and the divine law are said to be parts of the “Christian philosophy of law.” The natural law can describe either a part or the whole depending on the historical period (and the related level of differentiation of symbols) in question.
While the natural law can be either comprehensive or partial, it exists always within a larger framework—one of the primary experiences of social order. This larger framework seems to determine the particular characteristics of the natural law in a given order, e.g. whether or not it is comprehensive or partial.
It would seem that a natural law doctrine is always a possibility according to Voegelin, with at least two exceptions. A concept of natural law cannot arise in a cosmological order such as ancient Egypt (where there is no distinction between the given and the true) or where positivist ideas rule metaphysical questions out simply. Since human nature is constant, it would seem that the discovery of the natural law is always a human possibility; but it would also seem that an earlier natural law doctrine (i.e. the Thomistic version) cannot be simply true. I think there are several difficulties with Voegelin’s account: there seems to be a lack of clarity about the relation of the possibility of natural law to historical circumstances; there is also a lack of clarity about what it means for the natural law to be “obligatory”; and there is a lack of an investigation of the relationship between God (and his nature) and the natural law. I believe Voegelin’s lack of attention to these particular matters is a function of the historicist orientation of his thought.
Voegelin’s researches proceeded on the basis of attempting to understand the human experiences of the transcendent and the search for order through the unfolding differentiation of symbols explaining these human experiences. Strauss, like Voegelin, also rejected positivism. But his concern for the natural law was dominated by his passionate interest in the question “what is (a) god?” Unlike Voegelin, Strauss did not pursue this question in light of human historical experience but instead principally through textual investigation. To be more specific, the Biblical revelation had altered the concerns of the philosopher: Strauss, following Maimonides, recognized that the ancients studied the governing nomoi made by wise men; the Biblical revelation meant that men were now governed by divine commands. The philosophic understanding of the confrontation of reason and revelation that was at the heart of Strauss’s project and the relation of this question to the natural law is expressed this way by Strauss in a letter to Voegelin: “I believe . . . that the theioi nomoi is the common ground of the Bible and philosophy—humanly speaking.” But he goes on to note that philosophy and the Bible reach diametrically opposed solutions to the problem presented by the multitude of divine laws.
The philosopher is confronted by the Biblical claim that there is one God and that He rules through divine commands. If the unaided human reason can ascertain any of the divine commands, then the natural law is possible. Strauss took this possibility seriously. He examined the natural law in two principal ways: he investigated the natural law in the context of the distinction between the philosopher and the theologian and in itself as a Platonic “idea” or problem. I will pass over, in the interest of time, Strauss’s deliberately tendentious account of the natural law in Natural Right and History and the interesting “genetic” account in “On Natural Law” reprinted in Studies in Platonic Political Philosophy.
Strauss presents the different aspects of the natural law as they appear from the perspectives of the philosopher and the theologian in his essay “The Law of Reason in the Kuzari.” Initially, we are told that the philosopher sees the law of reason or natural law as a set of generally accepted opinions useful for society, while the theologian sees the natural law as a set of rational rules given by God. In the course of his investigation, Strauss makes it clear that it is difficult to distinguish between the philosopher and the theologian—the philosopher can appear as a theologian and perhaps vice-versa (it is not so clear that a theologian can appear as a philosopher). The law of reason can come to light as the natural law (the “framework” of every code, which tends to be the bare minimum moral requirements for the existence of any society) or as a complete theological-political code (i.e. Plato’s Laws). The great problem of the natural law or natural morality is that it lacks demonstrable divine sanctions to make it effectual.
Strauss also conducted a thorough investigation of the natural law as what may be called a Platonic “idea” or problem—the problem specifically being “what would it take for the natural law to be knowable and effectual?” in “Locke’s Doctrine of Natural Law.” Strauss focuses on the difficulty that unaided human reason faces in demonstrating the existence of a knowable and effectual natural law. In the Halevi essay, the great problem is the lack of demonstrable knowledge of divine sanctions to support the natural law (i.e. to make it a “categorical imperative”).
The apparently insurmountable difficulty in the Locke essay is the lack of knowledge of the divine nature necessary for the existence of a coherent doctrine of natural law. For instance, there must not only be a divine lawgiver; there must also be a divine lawgiver with a certain nature. If God has the character of a simply willful being (“He shall be what He shall be”) then the natural law is not possible. Of course the deity must also be concerned about particular human actions.
Strauss also addresses the problem of divine sanction at greater length in the Locke essay. In the course of his exposition of Locke’s Essays on the Law of Nature, he raises the problem of punishment. It seems that an effectual natural law doctrine requires not only divine sanction in general but the possibility of eternal punishment (consider the Myth of Er in the Republic); eternal punishment requires knowledge of the immortality of the souls. It does not seem that demonstrable knowledge of the immortality of the souls is available.
I believe that Strauss concludes that the natural law is, to borrow a distinction he employs from Maimonides, dialectically probable rather than demonstrably true (this accounts for what he describes as his “preference” for the classical natural right teaching, which does not depend on knowledge of divine lawgiver and hence is more readily demonstrable to the unassisted human reason). Strauss focused on problems that Voegelin did not emphasize or address due to the particular understanding of Socratic philosophy he undertook to recover. Socratic rationalism in Strauss’s understanding demanded a relentless investigation of the “ideas” as compared to Voegelin’s rich but more general attempt to understand the symbols employed by philosophers, theologians, and statesmen as they attempt to describe their experiences of the transcendent and relate them to social order. Strauss considered the question of the obligatory character of the natural law and the relation of the divine nature to a moral law in a way that Voegelin did not; this was based on Strauss’s opposition to Voegelin on the status of history,the question of the difference between reason and revelation, and the possibility of Socratic political philosophy.
 See Voegelin, The New Science of Politics, 2-3.
 See for instance Ibid, 27-28. One wonders if there is some relationship between what Voegelin describes as the movement from compactness to differentiation that goes from ritual, to myth, to theory and the Comtean historical framework (myth/metaphysics/positivism). Voegelin rejects both positivism and historicism, but I believe his thought evidences an ambivalent assessment of both schools of thought.
 See Voegelin, The Nature of the Law and Related Writings, 43-44, 78.
 Ibid., 81.
 Voegelin, The New Science of Politics, 2, 156-158; see Faith and Political Philosophy, 82-83 (Voegelin’s letter of April 22nd, 1951).
 See Faith and Political Philosophy, 82-83 (Voegelin’s letter of April 22, 1951) and 91 (Strauss’s reply of June 4th, 1951).
 Voegelin, The Nature of the Law, 78.
 Ibid., 80.
 Ibid., 71; The New Science of Politics, 41.
 Voegelin, The Nature of the Law, 81. Compare with Strauss, who differentiates the different senses of natural justice, especially in his chapter on classical natural right in Natural Right and History and his essay “The Law of Reason in the Kuzari” in Persecution and the Art of Writing. One wonders about the relationship between society and the philosopher in Voegelin’s account at this juncture. Strauss certainly emphasized the disjunction between the city and the philosopher more than Voegelin does here. Voegelin’s conflation of the mature man or spoudaios with the theoretical life of the philosopher is also questionable.
 Compare Strauss, Natural Right and History, 135-146 and 156-163.
 Compare Strauss, “On Natural Law” in Studies in Platonic Political Philosophy, 138-143.
 Voegelin, The Nature of the Law, 81-82.
 Ibid., 82. Voegelin rightly notes that the best regime of Politics VII-VIII was not presented as a model of reform by Aristotle.
 Ibid., 81.
 Ibid., 79.
 Ibid., 71. The primary experiences of social order involve participation in cosmic order (i.e. Egypt); participation in the revealed will (Israel); participation of the psyche in the divine logos (Greek philosophy); and participation in the spirit of Christ (the corpus mysticum). Voegelin only describes the natural law existing in the Greek and Medieval (mystical body) experiences of social order, but he does not explicitly say that the Israelitic order excludes the natural law (compare with Strauss, “The Origins of Natural Right” in Natural Right and History).
 I believe Strauss would reject the idea that an earlier doctrine could not be true; but the natural law is in some sense historically contingent. Plato and Aristotle did not have doctrines of natural law for instance. See Strauss, “On Natural Law”, 138ff.
 See Strauss, City and Man, end.
 See Faith and Political Philosophy, 74, 76,78, 80-84.
 See Strauss, “Maimonides’ Statement on Political Science” in What Is Political Philosophy?, 156 and context.
 See Faith and Political Philosophy, 78 (Letter of February 25th, 1951 of Strauss to Voegelin).
 In the interest of time, I am not discussing the well-known but (deliberately) tendentious account of the natural law in Natural Right and History or the interesting and rich “genetic” account in “On Natural Law” in Studies in Platonic Political Philosophy.
 Strauss admittedly relies on “assumptions” in his discussion of the Thomistic natural law doctrine in Natural Right and History. See NRH 163. Also contrast Strauss’s account of the inflexibility of the Thomistic position on NRH 164 with his description of Thomas’ position in “On Natural Law” (142-143).
 Strauss, Persecution and the Art of Writing, 96-98. St. Thomas is the representative of the theologians; Marsilius is the principal representative of the philosophers (coupled with Maimonides and Averroes).
 Ibid., 140.
 The final version of this essay appeared as chapter 8 of What Is Political Philosophy? The original, more readable essay was in the APSR.
 In the Halevi essay, Strauss does not address this issue at length. He merely describes the assumptions of the philosopher and the theologian regarding God and His relation to the law of reason or natural law. See Strauss, Persecution and the Art of Writing, 114ff.
 See Strauss, “Locke’s Doctrine of Natural Law” in What Is Political Philosophy?, 206-209 and Strauss, “Jerusalem and Athens” in Studies in Platonic Political Philosophy, 162-163 with 151.
 Ibid., 210.