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Kelsen and Aquinas on “the Natural-Law Doctrine”



Robert P. George[1]



The fiftieth anniversary of the publication of  Hans Kelsen’s influential essay “The Natural-Law Doctrine Before the Tribunal of Science”[2] provides an occasion to revisit a work in which the leading European legal theorist of the Twentieth Century outlined and strongly criticized the tradition of natural law theorizing.  Contemporary scholars on the continent and in the English-speaking world will, no doubt, examine Kelsen’s essay from variety of angles.  I am struck, however, by the fact that it makes no reference whatsoever to the thought of the most famous and influential of all natural law theorists, namely, St. Thomas Aquinas.  Kelsen refers frequently to the writings of Grotius, Pufendorf, Hobbes, Kant, Hegel, and classical Greek philosophers; but Aquinas’s theory, or “doctrine,” of natural law is left unaddressed.  If, however, something called “the natural law doctrine” can be attributed to anyone, surely it can be attributed to Aquinas.  I propose, therefore, to consider (a) the extent to which Kelsen’s exposition of “the natural-law doctrine” captures or describes Aquinas’s account of natural law, and (b) whether Kelsen’s critique of natural-law ethics and jurisprudence tells against the teachings of Aquinas.[3]


Natural Law, Moral Truth, and Religion

Let us begin by considering sentence by sentence the opening paragraph of Kelsen’s essay.


Sentence One:  “The natural-law doctrine undertakes to supply a definitive solution to the eternal problem of justice, to answer the question as to what is right and wrong in the mutual relations of men.”[4]


Aquinas is concerned with “right and wrong” not only in “the mutual relations of men,” but in human affairs generally.  He famously argues that all acts of virtue, and not merely those ordained to the common good narrowly conceived, are the subject of natural law.[5]  Questions of justice are, to be sure, central to his thought, but they are not the only questions.  His prescriptions concern what we would call, though he did not, “self-regarding” as well as “other-regarding” conduct.[6]  The principles and norms of natural law, as Aquinas understands them, would have relevance to the man permanently stranded alone on an island.  Nevertheless, it is fair to say that Kelsen’s statement is true, so far as it goes, when applied to Aquinas’s conception of “the natural law doctrine.”


Sentence Two:  “The answer is based on the assumption that it is possible to distinguish between human behavior which is natural, that is to say which corresponds to nature because it is required by nature, and human behavior which is unnatural, hence contrary to nature and forbidden by nature.”[7]


Aquinas does, sometimes, employ the terms “natural” and “unnatural” in a morally normative sense.  However, he makes abundantly clear that human choosing and acting is “natural” or “unnatural” in such a sense precisely insofar as it is reasonable or unreasonable.[8]  Other natural law theorists have sought to infer the reasonableness or unreasonableness of a possible choice or action from judgments about its naturalness or unnaturalness.[9]  And this approach is sometimes, though mistakenly, attributed to Aquinas.[10]  The truth is, however, that for Aquinas things work precisely the other way round:  it is the reasonableness or unreasonableness of a choice or action that controls judgment as to its naturalness or unnaturalness in any morally normative sense.[11]


Sentence Three:  “This assumption implies that it is possible to deduce from nature, that is to say from the nature of man, from the nature of society, and even from the nature of things certain rules which provide an altogether adequate prescription for human behavior, that by a careful examination of the facts of nature we can find the just solution of our social problem.”[12]


Aquinas certainly assumed no such thing.  In his famous treatment of the question whether the natural law contains several precepts or only one, he says that the first principles of practical reason, which are the basic precepts of natural law, are self-evident (per se nota) and indemonstrable.[13]  As such, they are not deduced from prior judgments about nature, human nature, the nature of society, or anything else.[14]  On the contrary, practical reasoning proceeds from its own first principles.  We needn’t look to physics, metaphysics, anthropology, sociology, or any other speculative (or, to use the Aristotelian term, “theoretical”) discipline to supply them.[15]  Of course, information drawn from these disciplines, when considered in the light of practical principles, can be highly pertinent to moral inquiry.[16]  Indeed, such information is often indispensable to sound judgments of right and wrong.[17]  But, according to Aquinas, the primary principles of practical reason and basic precepts of natural law are not “deduce[d] from nature” (or anything else).


Sentence Four:  “Nature is conceived of as a legislator, the supreme legislator.”[18]


Not according to Aquinas.  True, he allows that human goods, and the norms of morality directing choice and action with respect to these goods (and their privations), would be different if human nature were different, if, that is to say, human beings were fulfilled and perfected by activities and purposes (“goods”) other than those that in fact fulfill and perfect us.[19]  And in this sense morality and its content depends on (human) nature.[20]  But precisely because we do not (and, indeed, cannot) deduce the “ought” of morality from the “is” of nature (or anything else—including God’s will),[21] it is a mistake, or so Aquinas would say, to imagine that we could discover moral truth by inquiring into the intentions or purposes of nature conceived as some sort of law giver.


Moving to the second paragraph of Kelsen’s essay, we can see even more clearly that his account of “the natural-law doctrine” is at sharp variance with what Aquinas’ teaches about the natural law:


This view presupposes that natural phenomena are directed toward an end or shaped by a purpose, that natural processes or nature conceived of as a whole are determined by final causes.  It is a thoroughly teleological view, and as such does not differ from the idea that nature is endowed with will and intelligence.  This implies that nature is a kind of superhuman personal being, an authority to which man owes obedience.[22]


Whatever views about final causes Aquinas retains from Aristotle’s thought, he certainly would reject “the idea that nature is endowed with will and intelligence.”  Nature is not, in Aquinas’s account, “a kind of superhuman personal being.”  Nor is the ground of our moral obligations a debt of obedience to the “will” of nature or, indeed, any other authority.  Unlike many later theorists of natural law, Aquinas eschewed the voluntarism implied by this conception of moral obligation.[23]  The force of practical—including moral—principles, according to Aquinas, is rational; these principles state reasons for action and restraint; to defy them is wrong inasmuch as it is unreasonable.[24]  And, in this sense, the natural law is no extrinsic imposition of an alien will—whether the “will” of nature or anything (or anybody) else.  It is, rather, intrinsic to human beings; its fundamental referents are the human goods that constitute human well-being and fulfillment and precisely as such are reasons for action.[25]

            Moving now more deeply into Kelsen’s second paragraph, we find him arguing as follows:


At a higher stage of religious evolution, when animism is replaced by monotheism, nature is conceived of as having been created by God and is therefore regarded as a revelation of his all powerful and just will.  If the natural-law doctrine is consistent, it must assume a religious character.  It can deduce from nature just rules of human behavior only because and so far as nature is conceived of as a revelation of God’s will, so that examining nature amounts to exploring God’s will.  As a matter of fact, there is no natural-law doctrine of any importance which has not a more or less religious character.[26]


According to Aquinas, the natural law is a “participation of the eternal law in the rational creature.”[27]  And “the eternal law” is the supreme act of (practical) reason by which an omnipotent and omnibenevolent Creator freely orders the whole of His creation.[28]  Thus, the natural law is a part of the rational plan by which God providentially governs the created order.[29]  In this sense, Aquinas’s natural-law doctrine can be regarded as having a “religious character.”  Its religious character, however, has nothing to do with any putative deduction from nature, conceived as revelatory of the will of God or anyone (or anything) else, of moral norms or other “rules of human behavior.”  There is no sense, for Aquinas, in which one “reads off” from nature (or human nature) God’s will regarding human conduct.[30]

It is worth pausing here to observe, moreover, that there is no sense in which the natural law, as the eternal law’s participation in the rational creature, is incompatible with human freedom.  The dependency of human choice and action on divine power and causality does not vitiate the human power of creative free choice.  Indeed, Aquinas interprets the Biblical teaching that man is an imago dei[31] precisely as meaning that human beings are endowed with the God-like attributes of practical rationality and freedom.


Man is said to be made in God’s image, insofar as the image implies an intelligent being endowed with free will and self-movement:  now that we have treated of the exemplar, i.e., God, and of those things which come forth from the power of God in accordance with his will, it remains for us to treat of His image, i.e., man, inasmuch as he too is the principle of his actions, as having free will and control of his actions.[32]


Thus it is that, though God directs the brute animals to their proper ends by instinct or “natural appetite,” human beings He directs to their proper ends by the God-like power of practical reason, viz., the power to understand what is humanly (including morally) good and bad and freely to choose to act in light of the reasons thus provided.[33]

            According to Aquinas, the whole of the created order is suffused with meaning and value inasmuch as it is the product of God’s free and intelligent action.  At the same time, part—though not all—of the created order also has meaning and value by virtue of the contributions of human freedom and reason (which human capacities are themselves, as parts of the created order, suffused with meaning and value by virtue of divine wisdom and free choice).[34]  This part of the created order is governed by the principles of natural law by which free and intelligent creatures order their lives according to the directives of practical reason.  In precisely this sense, the natural law is “a participation of the eternal law in the rational creature.”

            Does Aquinas’s natural law theory presuppose religious premises?  Can it be accepted only by those who presuppose God’s existence and believe that He has revealed something of his will for human beings?  I have said enough already to indicate that the answer to these questions must be “no.”  At the same time, one may not infer from the fact that principles of natural law, according to Aquinas’s account, can be understood and acted upon without appeal to religious premises that God does not exist, or that God’s existence is simply irrelevant to natural law theory.


[J]ust as the fact that a good explanation of molecular motion can be provided, without adverting to the existence of an uncreated creator of the whole state of affairs in which molecules and the laws of their motion obtain, does not of itself entail either (i) that no further explanation of that state of affairs is required or (ii) that no such further explanation is available, or (iii) that the existence of an uncreated creator is not that explanation, so too the fact that natural law can be understood, assented to, applied, and reflectively analysed without adverting to the question of the existence of God does not of itself entail either (i) that no further explanation is required for the fact that there are objective standards of good and bad and principles of reasonableness (right and wrong) or (ii) that no such further explanation is available, or (iii) that the existence of God is not that explanation.[35]


            Let us now move beyond the opening paragraph of Kelsen’s essay to his critique of “the natural-law doctrine.”  His principal objection to natural law theory is that it “obliterates the essential difference which exists between scientific laws of nature, the rules by which the science of nature describes its object, and the rules by which ethics and jurisprudence describe their objects, which are morality and law.”[36]  This objection boils down to the proposition that “it does not follow from the fact that something is, that it ought to be or to be done, or that it ought not to be or not to be done . . . there is no logical inference from the ‘is’ to the ‘ought,’ from natural reality to moral or legal value.”[37]

            That certain natural law theorists (including some who have claimed the patronage of Aquinas) have proposed to derive the “ought” of morality from the “is” of  (human) nature is true.[38]  It is equally true, however, that Aquinas is not among them, nor are his leading contemporary followers.[39]  Although Hume is widely credited with discovering the logical fallacy inherent in any attempt at such a derivation,[40] Aquinas, among other pre-modern thinkers, was quite well aware of the fallacy and sought more scrupulously than did Hume himself to avoid committing it.[41]  Accepting Aristotle’s distinction between “theoretical” (or “speculative”) and “practical” reasoning, Aquinas insisted, as we have seen, that practical reasoning proceeds from its own first principles.  He did not treat practical principles as theoretical principles that are given normative force by an act of the will.  He did not treat theoretical knowledge of human nature as providing a sufficient premise for practical knowledge, let alone for practical knowledge of moral obligation.[42]  He did not suppose that having first discovered the “facts” about human nature by way of non-practical inquiry, we then identify ethical obligations by applying a norm such as “follow nature.”[43]

            Hume and his followers, perhaps including Kelsen,[44] suppose that if  “values” cannot be derived from “facts,” then they cannot be objective (or “true”), but must, rather, be mere projections of feeling, emotion, or other subrational factors capable of motivating human behavior.  They deny that practical reasons, as such, can motivate people.  So they conclude that, unless natural law theorists commit “the naturalistic fallacy” of purporting to derive “ought” from “is,” their doctrine collapses into a form of ethical non-cognitivism.[45]  But this simply begs the question against Thomists and others who claim that we can understand, and thereby be motivated to act for the sake of, more-than-merely-instrumental practical reasons.[46]  It does a poor job of accounting for the experience of most people who, after all, often suppose that they are moved to do things (or to avoid doing things that they might otherwise do) not as a matter of brute desire, but, rather, because they perceive the worth or value, and thus the practical point, of doing (or avoiding doing) them.[47]  Moreover, it flies in the face of powerful retorsive arguments which show that any truly knowledge-seeking defense of Humean moral skepticism, or other forms of non-cognitivism, will be self-refuting inasmuch as it contradicts in practice the very claims it seeks to defend in theory.[48]

Kelsen’s claim that “from the point of view of science the natural-law doctrine is based on the logical fallacy of an inference from the ‘is’ to the ‘ought’”[49] simply has no force whatsoever against “the natural-law doctrine” as it is understood by Aquinas.  For Aquinas’s theory of natural law proposes no such inference.  When Kelsen goes on to say that “[t]he norms allegedly deduced from nature are—in truth—tacitly presupposed, and are based on subjective values, which are presented as the intentions of nature as a legislator,”[50] again his critique has no applicability to Aquinas.  To be sure, the critique itself, it seems, tacitly presupposes the Humean idea that all “values” are subjective, viz., that people cannot be aware of and act on more-than-merely-instrumental reasons as such, and this is in direct contradiction to Aquinas’s view.  But insofar as the truth of the Humean idea is not obvious—indeed, that idea is, at best, highly problematic—Kelsen’s marshaling of the idea in his critique of “the natural-law doctrine” need trouble no one interested in defending Aquinas or his natural-law doctrine.  If the idea is to be marshaled effectively against Aquinas and his contemporary followers, then its proponents must, among other things, provide a plausible account of common moral experience with which it is apparently incompatible, and they must come to terms with the problems of retorsion that appear, at least, to render any intellectually serious defense of the idea self-refuting.


Natural Law and Positive Law

In the second section of his essay, Kelsen focuses on the natural law-doctrine of the relationship between natural and positive law.  His central claim against the doctrine here is that it renders the positive law “superfluous.”[51]


Faced by the existence of a just ordering of society, intelligible in nature, the activity of positive-law makers is tantamount to a foolish effort to supply artificial illumination in bright sunshine.[52]


Yet, he insists:


[N]one of the followers of this doctrine had the courage to be consistent.  None of them has declared that the existence of natural law makes the establishment of positive law superfluous.  On the contrary.  All of them insist upon the necessity of positive law.  In fact, one of the most essential functions of all natural-law doctrines is to justify the establishment of positive law or the existence of the state competent to establish positive law.  In performing this function most of the doctrines entangle themselves in a highly characteristic contradiction.  On the one hand they maintain that human nature is the source of natural law, which implies that human nature must be basically good.  On the other hand they can justify the necessity of positive law with its coercive machinery only by the badness of man.[53]


Here, I believe, Kelsen offers a spectacularly poor argument (or pair of muddled together arguments).  It likely tells against no historically important natural law theorist.  It certainly casts no doubt on Aquinas’s theory.  We have already seen that Kelsen’s particular account of human nature as the “source of natural law” in natural-law doctrines has no applicability to Aquinas’s teaching.  Again, though it is true that, for Aquinas, human goods are what they are because human nature is constituted as it is, there is no sense in which Aquinas proposes to deduce knowledge of human goods—practical knowledge—from methodologically antecedent—theoretical—knowledge of human nature.  The first principles of practical reason and basic precepts of natural law, which direct choice and action to the goods of knowledge, friendship, and other more-than-merely instrumental reasons for action, far from being inferred from anthropological, historical, metaphysical, theological, or any other theoretical premises, are grasped in non-inferential acts of understanding whereby “the practical intellect”—one’s single intelligence directed towards answering the question what is to be chosen and done— grasps the intelligible point of a possible action in its promise to instantiate a human benefit, viz., something (e.g., knowledge, friendship) humanly fulfilling and, as such, worthwhile for its own sake.[54]

Now, the fact that there are goods for human beings which, as such, provide reasons for action, does not entail that there are no bads; on the contrary, the privations of human goods (e.g., ignorance, muddleheadedness, misunderstanding, animosity) are bads that provide reasons (which may or may not in any particular case be conclusive) for people to avoid them, where possible.[55]  Nor does the ability of human beings to understand certain ends or purposes as humanly fulfilling, and, as such, good entail that human beings cannot choose in ways that are incompatible with the integral directiveness of the human goods, viz. immorally.  Indeed, one can, for the sake of a certain good or the instantiation of goods in certain persons, choose in ways that unreasonably damage or shortchange other goods or treat other persons unfairly.[56]  Any such choice will be unreasonable inasmuch as one’s reason for it was in truth defeated by a conclusive (moral) reason against it.  But a defeated reason remains a reason—for unreasonable choices are not necessarily utterly irrational—albeit one that can be acted for by a person who, at some level, at least, understands the wrongfulness of his deed, only on the basis of emotional motives that compete with and cut back upon or fetter reason.[57]

            One need not suppose that people are inherently “bad” in order to acknowledge the evident truth that human emotions, when inadequately integrated in the human personality, can motivate people to perform immoral acts.  This is by no means to suggest that emotions are themselves inherently bad or ought somehow to be gotten rid of.  (Indeed, in the properly integrated personality emotions support morally upright choosing.)  It is only to say that people can be emotionally motivated to do things that are contrary to the integral directiveness of human goods—sometimes for the sake of genuine, albeit partial, human goods to which they are deeply committed or attached.[58]  And this fact about human beings is, in part, what calls for and justifies “the establishment of positive law” and “the existence of the state competent to establish positive law.”[59]

            At the same time, it is important to see that in Aquinas’s account of the matter, positive law would remain necessary even in a human society in which people could always be counted upon to do the morally right thing.  This is because any society—even a “society of saints”—needs law, and a system of law making, to provide authoritative stipulations for the coordination of  actions for the sake of the common good. [60]  Of course, in such a society laws against murder, rape, theft and other morally wrongful acts would be unnecessary, and punishment and other coercive features of real life legal systems would have no place since, ex hypothesi, no one would willfully fail to abide by the law’s just and authoritative stipulations.  But the vast majority of laws by which people—particularly in complex modern societies—are governed in their daily lives as citizens would remain pertinent.

            Thus, Aquinas holds that positive law is necessary both because actual human beings sometimes need the threat of punishment to deter them from doing what the natural law already proscribes (or require them to do what it prescribes) as a matter of basic justice and because authoritative stipulations are frequently needed to coordinate action for sake of  the common good.[61]  And he further holds that all just positive laws—including laws that are purely norms of co-ordination—are derived, in some sense, from the natural law.[62]  The task of the legislator, he suggests, is to give effect to relevant principles of natural law in the shape of principles and norms of positive law for the governance of human society.

This work of giving effect to the principles of natural law is accomplished in two distinct ways, two forms of “derivation.”  Some laws, such as those prohibiting murder, rape, theft, and other grave injustices which are straightforwardly contrary to natural law, are derived from the natural law by a process akin to the deduction of demonstrable conclusions from general premises in the sciences.[63]  Other positive laws, however, cannot be derived from the natural law in so direct and straightforward a fashion.  Where law is required to resolve a co-ordination problem, it is often the case that a variety of possible solutions, all having certain incommensurable advantages and disadvantages, are rationally available as options.  One solution, however, must be authoritatively chosen by the legislator if the problem is to be solved.  Consider, for example, the regulation of highway traffic.  From the basic principle of natural law which identifies human health and safety as goods to be preserved, together with the empirical fact that unregulated driving, even among motorists of impeccable goodwill, places these human goods in jeopardy, it follows that a scheme of regulation (co-ordination) is necessary for the common good.  Yet typically various reasonable, but incompatible, schemes are possible.  For the sake of the common good, then, the relevant law making authority must stipulate that one from among the various possible schemes shall be given the force of law.  In selecting a scheme, the law makers operate not by any process analogous to the deduction of demonstrable conclusions from premises, but, rather, by a process of choosing between reasonable, yet incompatible, options—a process that Aquinas refers to as determinatio.[64]

Laws that come into being as determinationes, according to Aquinas, have their binding force “not from reason alone,” but also from “having been laid down” by valid law making authority.[65]  Although it is the case that but for the law’s enactment no one would be under any general moral duty to behave as it requires, and despite the fact that the law maker(s) could, compatibly with the requirements of natural law, have stipulated a different requirement or set of requirements, “its directiveness derives not only from the fact of its creation by some recognized source of law (legislation, judicial decision, custom, etc.), but also from its rational connection with some principle or precept of morality.”[66]

It is entirely clear, then, that the existence of natural law, as Aquinas conceives it, does not render positive law otiose.  On the contrary, Aquinas quite reasonably views positive law, and the institutions of government that enjoy the power of law making, to be indispensable to the common good of any society—even a hypothetical society of saints.  They are themselves, as it were, requirements of natural law.[67]  Although the binding force of (just) positive law always depends in part on its derivation from principles of natural law, the positive law, in Aquinas’s account of it, is no mere emanation or simple reflection of those principles.  Indeed, insofar as human law is a matter of determinatio, law makers enjoy a measure of rational creative freedom that Aquinas himself analogizes to that of  “the craftsman [or, as we might say, architect] [who] needs to determine the general form of a house to some particular shape,”[68] yet who may design the structure, compatibly with the purposes it is meant to serve, to any of a vast number of possible shapes.  The existence of this freedom in no way entails the utter independence of positive law from natural law (any more than the creative freedom of the architect entails the complete independence of his determinationes from the general principles of architecture which must be observed if a house is to be structurally sound and otherwise suitable for purposes of habitation, or from the governing terms of his commission).  But it also marks the reasons of principle which Aquinas has for completely rejecting, as he does, the notions ascribed by Kelsen to “natural law doctrine”—that, given the reality of natural law, positive law is “superfluous.”


Natural Law, Unjust Law, and Resistance to Tyranny

The third section of  “The Natural-Law Doctrine”[69] introduces Kelsen’s version of a familiar charge against natural law theory, namely, its alleged merging of the categories of “moral” and “legal” such that either (i) all positive laws are morally good, or (ii) morally bad laws are in no meaningful sense truly laws.  The section opens with the following sally against theorists of natural law:


If the positive law is, as all followers of the natural-law doctrine assert, valid only so far as it corresponds to the natural law, any norm created by custom or stipulated by a human legislator which is contrary to the law of nature must be considered null and void.  This is the inevitable consequence of the theory which admits the possibility of positive law as a normative system inferior to natural law.  The extent to which a writer abides by this consequence is a test of his sincerity.  Very few stand this test.[70]


Those who fail the test—the vast majority—are driven inexorably, Kelsen suggests, into the opposite position, namely, that “conflict between positive and natural law, although theoretically possible, is practically excluded.”[71]  Indeed, Kelsen goes so far as to allege that “the natural-law doctrine has no other function than to justify the positive law—any positive law established by an effective government.”[72]  So, in effect, natural law theory, which begins by opening up in theory the possibility of the radical moral critique of regimes of positive law and government, ends by functioning in practice as an ideological apologetic for existing regimes—whatever they happen to be.

Kelsen’s principal targets here are Hobbes and Pufendorf, who, he alleges, despite their differences in other important respects, and notwithstanding Pufendorf’s critique of Hobbes’s straightforward identification of positive with natural law, hold in common the view that the natural law must serve in the end to justify virtually any extant regime of positive law.  Kelsen argues, moreover, that “there is a principle advocated by all leading representatives of the natural-law doctrine, by which a conflict between the natural and the positive—if at all admitted as possible—is deprived of any effect that could be dangerous to the established legal authority:  it is the dogma that under the law of nature there is no or only a restricted right of resistance.”[73]  Is such an inherently “conservative” view justly attributable to Aquinas?

A commonplace criticism of Aquinas is that his evident endorsement of Augustine’s statement that “an unjust law seems not to be a law”[74] shows that he is guilty of merging the categories of “legal” and “moral” in such a way as to render it analytically impossible for positive law and natural law to be in conflict.[75]  (Of course, Kelsen himself does not consider Aquinas’s specific treatment of the relationship of natural to positive law in the essay here under review; Kelsen could not plausibly deny, however, that Aquinas’s treatment falls within what he says “all followers of the natural-law doctrine assert.”)  We have already seen that Aquinas’s account of the derivation of positive law from natural law is complex and, in certain respects, quite subtle.  Still further complexities and subtleties can be brought into focus if we consider the context of Aquinas’s endorsement of Augustine’s statement.  It will become clear that Aquinas’s conception of “law” and “legality” is every bit as rich and highly nuanced as the conceptions advanced by modern analytical legal philosophers.  To be sure, Aquinas does not go very far in carrying out the analytical work of explicitly identifying the respects in which concrete instances of the phenomenon of human law can deviate from “law” in a social-theoretical “focal” or “paradigmatic” sense (a sense in part built up out of consideration of concrete instances, albeit from an “internal” viewpoint that itself requires the application of critical-practical intelligence[76]) while still retaining constitutive features of the concept of law.  But he deploys the term “law” in an appropriately flexible way to take into account the differences between the demands of (i) intrasystemic legal analysis or argumentation (e.g., in the context of professional legal advocacy or judging); (ii) what we would call “descriptive” social theory (e.g., “sociology of law”); and (iii) fully critical (i.e., “normative,” “moral,” conscience-informing) discourse.

That Aquinas believed that laws could be, and indeed sometimes were, unjust is evident both from his many explicit references to unjust laws and from the very considerable attention he devoted to the problem of legal injustice.  Central to his reflections was precisely the question whether, and, if so, how and to what extent, did unjust laws bind in conscience those subject to them to obey.[77]   It is clear that Aquinas believed that human positive law creates a moral duty of obedience even where the conduct it commands (or prohibits) would, in the absence of the law, i.e., morally, as a matter of natural law, be optional.  This critical-moral belief in the power of positive law to create (or, where moral obligation already exists, reinforce) moral obligation naturally suggests the question whether this power (and the duties that are imposed by its exercise on those subject to it) is absolute or defeasible.  If defeasible, under what conditions is it defeated?

To answer this question, it is necessary to press the critical-moral analysis.  What is the source of the power in the first place?  Plainly it is the capacity of law to serve the cause of justice and the common good by, for example, coordinating behavior to make possible the fuller and/or fairer realization of human goods by the community as a whole.  But, then, from the critical-moral viewpoint, laws that, due to their injustice, damage, rather than serve, the common good, lack the central justifying quality of  law.  Their law-creating power (and the duties they purport to impose) is, thus, weakened or defeated.  Unjust laws are, Aquinas says, “not so much laws as acts of violence.”[78]  As violations of justice and the common good, they lack the moral force of law; they bind in conscience, if at all, only to the extent that one is under an obligation not to bring about bad side effects that would, in the particular circumstances, likely result from one’s defiance of the law (e.g., causing “demoralization or disorder,”[79] as by undermining respect for law in a basically just legal system, or unfairly shifting the burdens of a certain unjust law onto the shoulders of innocent fellow citizens[80]).  That is to say, unjust laws bind in conscience, if at all, not per se, but only per accidens.  They are laws, not “simpliciter,” or, as we might say, in the “focal” or “paradigmatic” sense, but only in a derivative or secondary sense (“secundum quid”).

            Nothing in Aquinas’s legal theory suggests that the injustice of a law renders it something other than a law (or “legally binding”) for purposes of intrasystemic juristic analysis and argumentation.  True, he counsels judges, where possible, to interpret and apply laws in such a way as to avoid unjust results where, as best they can tell, the law makers did not foresee circumstances in which a strict application of the rule they laid down would result in injustice, and where they would, had they foreseen such circumstances, have crafted the rule differently.[81]  But even here he does not appeal to the proposition that the injustice likely to result from an application of the rule strictly according to its terms nullifies those terms from the legal point of view.

Nor does Aquinas say or imply anything that would suggest treating Augustine’s comment that “an unjust law seems not to be a law” as relevant to social-theoretical (or historical) investigations of what is (or was) treated as law and legally binding in the legal system of any given culture (however admirable or otherwise from the critical-moral viewpoint).  So, for example, though H.L.A. Hart was among those who misunderstood Aquinas and his stream of the natural law tradition on precisely this point, no follower of Aquinas should suppose that Hart’s “descriptive sociology” of law errs by treating as laws (and legal systems) various social norms (and social norm-generating institutions) that fulfill the criteria or conditions for legality or legal validity of Hart’s concept of law, despite the fact that his social-theoretical enterprise (reasonably!) prescinds to a considerable extent (indeed, it seeks to prescind as far as possible) from critical moral evaluation of laws and legal systems.  The criticism Hart’s work invites from a natural law perspective has nothing to do with his willingness to treat unjust laws as laws; it has rather to do with his unwillingness to follow through on the logic of his own method and his insight into the necessity of adopting or reproducing and internal point of view—a method which, if followed through, will identify the focal or paradigmatic case of law as just law—law that serves the common good—and the focal or paradigmatic case of the internal (or “legal”) point of view as the viewpoint of someone who understands law and the legal system as valuable (and legal rules as, ordinarily, binding in conscience) because (or insofar as) they are just—and, qua just, serve the common good.[82]

            With this background in mind, let us address directly the question whether Aquinas (“like all followers of the natural-law doctrine”) embraces some principle which excludes or effectively restricts the right of people to resist tyranny or gravely unjust regimes of law.  Throughout his writings, Aquinas grapples with the problem of tyranny and, indeed, with the question of the legitimacy of tyrannicide.  In his early work, as well in his most mature writings, he defends the proposition, not only that the unjust acts of tyrants are devoid of moral authority, but that they constitute a kind of criminality which can justify revolutionary violence for the sake of the common good and even tyrannicide as a kind of resistance to, and/or just punishment of, the tyrant.

It is true that one work, De Regno ad Regem Cypri, a theological treatise from Aquinas’s middle period written to inform the conscience of a Christian king, suggests disapproval of tyrannicide.  This work is probably authentic, or, at least, substantially so, though some responsible commentators have doubted Aquinas’s authorship.  However, that may be, even Finnis, who treats the work as probably authentic, warns that it is “never a fully reliable and satisfactory source for the opinion of Aquinas.”[83]  The warning seems particularly apt with respect to the question of tyrannicide in view of the inconsistency of the teaching of De Regno both with earlier and later works of unquestioned authenticity and great clarity.

Tyranny, for Aquinas, is paradigmatically rule (whether by the one, the few, or the many) in the private interests, or for the private ends, of the ruler or rulers at the expense of the common (i.e., public) good.[84]  The tyrant, in effect, uses, rather than serves, those over whom he exercises power and for whose sake (from the critical-moral viewpoint) public authority exists.[85]  Aquinas’s earlier writings distinguish between two types of tyranny:  (i) the tyranny of those who abuse authority they legitimately acquired and hold; and (ii) the tyranny of those who obtained and hold power by usurpation.  He suggests that usurping tyrants—as, in effect, parties making war against the political community—may legitimately be resisted and even killed by anyone who has the effective power to do so.[86]  By contrast, where legitimate rule has degenerated into tyranny, the tyrant(s) are entitled to something like what we might call “due process of law.”  It is up to other public officials, operating as such, and not (ordinarily) to private citizens, to overthrow their regimes and, if necessary, bring them personally to trial and punishment (including, where appropriate, capital punishment).[87]

It is noteworthy that in his most mature writings Aquinas, as Finnis observes, “seems to have lost interest in the contrast between usurpers and other kinds of tyrant.”[88]  In the Summa Theologiae, he treats tyranny of any kind as an essentially criminal type of rule—indeed, a form of sedition—that can justify the revolutionary action of the people and the punishment of the tyrant(s).[89]  (I say “can” justify since, as always for Aquinas, a final moral judgment as to the justice of resort to force must take into account the impact of likely unintended bad side effects.  Otherwise morally permissible revolutionary action might, in any particular case, be unjust to innocent third parties who would, in the circumstances, be made unfairly to bear the burdens of such side effects.)

Aquinas’s “natural-law doctrine,” then, does not subscribe to the principle (advocated, according to Kelsen, by all leading representatives of the natural-law doctrine) which “deprives of any effect” conflicts between positive and natural law “which could be dangerous to the established legal authority.”  Although Aquinas does not treat the right of revolution in the face of tyranny as absolute, he plainly does not embrace Kelsen’s alleged “dogma that under the law of nature there is no or only a restricted right of resistance.”  Tyrants—not least those who came to power by legal means and govern by issuing and enforcing laws (lex tyrannica)[90]—must look elsewhere than to Aquinas for moral arguments designed to insulate them from insurrection and punishment for their misrule.  Nothing in his thought merges natural and positive law in such a way as to confer upon positive law an automatic conformity to the requirements of natural law.  On the contrary, according to Aquinas, the positive law of any regime, and those rulers who create and enforce it, stand under the judgment of natural law.  Tyrannical rule is a “perversion” of law,[91] and, as such, far from creating a duty of obedience, gives rise to a (prima facie) right of resistance to the uttermost.



            Despite his sometimes sweeping statements about its substance, and what “all” of its principal exponents have held, we have seen that Kelsen’s exposition of “the natural-law doctrine” has virtually no points of contact with Aquinas’s thought.  Hence, Kelsen’s critique of the doctrine has little or no applicability to Thomistic natural law theory.  Neither Aquinas’s theory of the identification of natural law principles, nor his account of their relation to divine power and to positive law, nor his views regarding their implications for the problems of legal injustice and tyranny, are captured in Kelsen’s exposition and critique.  Kelsen did well, one might conclude, to avoid mentioning Aquinas if he was to insist on describing “the natural-law doctrine” as he did.  Still, it is odd, to say the least, for the “tribunal of science” to have left unheard and unmentioned the thought of so central an exponent of the natural law tradition.




[1] McCormick Professor of Jurisprudence, Princeton University.  The author gratefully acknowledges the generous support of the Earhart Foundation.


[2] Originally published in The Western Political Quarterly, December, 1949.  Reprinted in What is Justice?  Justice, Law, and Politics in the Mirror of Science:  Collected Essays by Hans Kelsen (Berkeley and Los Angeles:  University of California Press, 1957) , pp.137-173.


[3] I shall, for the most part, refrain from commenting on the accuracy of Kelsen’s attributions to other natural-law thinkers of the various propositions he asserts to be constitutive of, or in some sense integral to, “the natural-law doctrine.”  For what it is worth, my view is that Kelsen’s essay is on this score a “mixed bag.” 


[4] Id. at 137.


[5] Summa Theologiae I-II, q. 94, a. 3.


[6] Aquinas is hardly oblivious to the distinction:  see Summa Theologiae I-II, q. 91, a. 4, deploying the distinction in teaching that it is unwise for human law to prohibit every act of vice.


[7] Kelsen, “The Natural-Law Doctrine, p. 137.


[8] See Scriptum Super Libros Sententiarum Petri Lombardiensis IV, d.2, q. 1, a. 4, sol. 1 ad 2 (“moral precepts are in accord with human nature because they are requirements of natural reason”), and Summa Theologiae I-II, q. 71, a. 2c (“virtues . . . are in accordance with human nature just insofar as they are in line with reason; vices are against human nature just insofar as they are against the order of reasonableness”).


[9] See, for example, Thomas J. Higgins, Man as Man:  The Science and Art of Ethics (Milwaukee:  Bruce Publishing Co., 1958), esp. pp. 49-69, 88-100, 120-126.


[10] Id.  See also Lloyd L. Weinreb, Natural Law and Justice (Cambridge, Mass.:  Harvard University Press, 1987), p. 33 (“natural law [according to Aquinas] directs us to fulfill our natural inclinations”).


[11] For a careful and amply documented explanation of this critical point, see John Finnis, Aquinas:  Moral, Political, and Legal Theory (Oxford:  Oxford University Press, 1998), pp. 90-94.


[12] Kelsen, “The Natural-Law Doctrine,” p. 137.


[13] Summa Theologiae I-II, q. 94, a. 2.  On the (much misunderstood) meaning of “self-evidence” in Thomistic ethical theory, see Robert P. George, “Recent Criticism of Natural Law Theory,” University of Chicago Law Review, Vol. 55 (1988), pp. 1371-1429, at 1387-1389 and 1413.


[14] See Germain Grisez, “The First Principle of Practical Reason:  A Commentary on the Summa Theologiae, 1-2, Question 94, Article 2,” Natural Law Forum, Vol. 10 (1965), pp. 168-201.


[15] See Finnis, Aquinas, pp. 90-94.


[16] See George, “Recent Criticism of Natural Law Theory,” pp. 1412-1414.


[17] So, for example, knowledge of the facts of human embryogenesis and intrauterine human development is critical to a proper application of moral principles to the question of abortion.


[18] Kelsen, “The Natural-Law Doctrine,” p. 137.


[19] See John Finnis, Natural Law and Natural Rights (Oxford:  Clarendon Press, 1980), p. 34.


[20] See Robert P. George, “Natural Law and Human Nature” in Robert P. George (ed.), Natural Law Theory:  Contemporary Essays (Oxford:  Clarendon Press, 1992), pp. 31-41.


[21] See Finnis, Aquinas, p. 90.


[22] Kelsen, “The Natural-Law Doctrine,” p. 137.


[23] On the impact of voluntarism on Christian moral theology after Aquinas, see Germain Grisez, The Way of the Lord Jesus, Vol. I:  Christian Moral Principles (Chicago:  Franciscan Herald Press, 1983), pp. 12-13.  In the same work, Grisez provides a powerful critique of voluntarism and defense of the authentically Thomistic alternative account of moral obligation as a kind of rational necessity: see, especially, pp. 103-105.  See also Finnis, Natural Law and Natural Rights, pp. 42-48 and 337-343.


[24] See Finnis, Aquinas, pp. 79-86.


[25] Id.


[26] Kelsen, “The Natural-Law Doctrine,” p. 138.


[27] Summa Theologiae I-II, q. 91, a. 2.


[28] Summa Theologiae I-II, q. 91, a. 1.


[29] Finnis sums up Aquinas’s teaching on “the eternal law” as follows:  “God envisages and freely chooses the whole order of things, prescribing (so to speak) that order by impressing its principles (the ‘laws of physics’, the ‘laws of logic’, and so forth) onto or into the various orders of created entity and process.  And this act is to the common benefit of the whole (and thus of its parts).  So we can think of this supreme act of government as legislative, and its rational content as a law which, like its author, is timeless (even though that content is freely chosen, not necessary, and regulates creatures which are all within time).”  Aquinas, p. 307 (notes omitted).


[30] Id. at 309.


[31] Genesis 1:27.


[32] Summa Theologiae I-II, Prologue.


[33] Summa Theologiae I-II, q. 91, a. 2.


[34] These points are explained more fully in George, “Recent Criticism of Natural Law Theory,” pp. 1384-1385.  Unfortunately, a key line on page 1384 was omitted by the printer in the version of this essay that appeared in the University of Chicago Law Review.  It would be better, therefore, for readers to consult the revised version appearing as Chapter Two of  Robert P. George, In Defense of Natural Law (Oxford:  Clarendon Press, 1999), sec. C.


[35] Finnis, Natural Law and Natural Rights, p. 49.


[36] Kelsen, “The Natural-Law Doctrine,” p. 139.


[37] Id. at 140.


[38] The origins of this approach to natural law theory are not in Aquinas, but, rather, in later writings such as those of the early seventeenth century Spanish Jesuit moral and political thinker Francisco Suarez.  See especially his De Legibus ac de Deo Legislator (1612), Book I, c. 5 and Book II, c. 6.  For a useful account of Suarez’s influence, and valuable critique of his approach to natural law theory, see Finnis, Natural Law and Natural Rights, pp. 43-47, 54-57, 337-343, and 347-350.


[39] Germain Grisez and John Finnis, for example, among other leading contemporary moral philosophers and theologians working broadly within the Thomistic tradition, explicitly reject as logically illicit any proposal to derive “ought” from “is.”  See Grisez, The Way of the Lord Jesus, Vol. I:  Christian Moral Principles, p. 105; and Finnis, Natural Law and Natural Rights, p. 33-36.


[40] For what is often taken to be Hume’s statement of discovery of the logical fallacy, see A Treatise of Human Nature (1740), Book III, part I, sec. 1.


[41] On Hume’s lack of care in this regard, see Finnis, Natural Law and Natural Rights, pp. 37-38, n. 43.


[42] Indeed, something very much like the reverse is true.  A complete (theoretical) account of human nature presupposes practical knowledge (a set of “value judgments”) which provides data for theoretical inquiry, understanding, and judgment.  Aquinas adheres to the Aristotelian methodological (and epistemological) principle according to which we come to know human nature by knowing human potentialities; these we know by knowing human acts; and these we know by knowing their objects, viz., the more-than-merely instrumental goods (bona) to which the self-evident and indemonstrable first principles of practical reason direct human choice and action.  See Finnis, Aquinas, pp. 90-91.


[43] All of this is made abundantly clear in Germain Grisez’s “The First Principle of Practical Reason:  A Commentary on the Summa Theologiae,” 1-2, Question 94, Article 2.”  This unsurpassed textual study corrects many common misunderstandings of Aquinas’s theory of natural law, including, notably, the idea that Thomistic ethical theory purports to deduce the “ought” of morality from the “is” of (human) nature.  See also Grisez’s later treatment of the matter in The Way of the Lord Jesus, Vol. I:  Christian Moral Principles, pp. 103-105, and, at 112, observing that “St. Thomas was careful to explain that practical conclusions always must be resolved into practical principles which are distinct from and irreducible to theoretical ones.”


[44] See “The Natural-Law Doctrine,” p. 141.


[45] See Jeffrey Goldsworthy, “Fact and Value in the New Natural Law Theory,” American Journal of Jurisprudence, Vol. 41 (1996), pp. 21-46.


[46] See Robert P. George, “A Defense of the New Natural Law Theory,” American Journal of Jurisprudence, Vol. 41 (1996), pp. 47-61.  A revised version of this essay, free of multiple printer’s errors that make it difficult for readers to grasp the sense of several sentences in the original, appears as Chapter One of George, In Defense of Natural Law.


[47] Id.


[48] See Finnis, Aquinas, pp. 58-61.


[49] Kelsen, “The Natural-Law Doctrine,” p, 141,


[50] Id.


[51] Id. at 142.


[52] Id.


[53] Id. (notes omitted).


[54] Summa Theologiae I-II, q. 94, a. 2.


[55] Id. wherein Aquinas formulates the first and most general principle of practical reason as “good is to be done and pursued and bad is to be avoided” (emphasis supplied).  On the proper interpretation of this principle and, particularly, the meaning of “good” and “bad” as including what is worthwhile and the privation of the worthwhile generally, and not (merely) what is morally right and wrong, see Grisez, “The First Principle of Practical Reason:  A Commentary on the Summa Theologiae, 1-2, Question 94, Article 2.”


[56] See Germain Grisez, Joseph Boyle, and John Finnis, “Practical Principles, Moral Truth, and Ultimate Ends,” American Journal of Jurisprudence, Vol. 32 (1987), pp. 99-151, at 123-125.


[57] Id.


[58] Id.


[59] This is plainly Aquinas’s view as set forth in Summa Theologiae I-II, q. 95, a.1 (addressing the question “Was it useful for laws to be framed by men?”); see also Summa Theologiae I-II, q. 96, a. 5.


[60] See Finnis’s explanation of Aquinas’s position in Natural Law and Natural Rights, p. 28, and Aquinas, pp. 35-37.  Consistently with Aquinas’s legal theory (and Aristotle’s thought:  see Nicomachean Ethics, V, 7), contemporary analytical jurisprudence has emphasized and valuably explored the importance of law in providing authoritative, and thus binding, solutions to problems of co-ordinating human behavior for the sake of the common good.  See especially Edna Ullman-Margalit, The Emergence of Norms (Oxford: Clarendon Press, 1977).  For a summary of some important contemporary work in this area, with particular attention to the question of a prima facie (defeasible) moral obligation to obey law, see John Finnis, “Law as Co-ordination, Ratio Juris, Vol. 2 (1989), pp. 97-104.


[61] Summa Theologiae I-II, q. 96, a. 4; Finnis, Aquinas, pp. 248 at n. 148, and 265 at n. 66.


[62] Summa Theologiae I-II, q. 95, a. 2.


[63] Id.


[64] Id.  On Aquinas’s theory of determinatio, see Robert P. George, “Natural Law and Positive Law,” in Robert P. George (ed.), The Autonomy of Law:  Essays on Legal Positivism (Oxford:  Clarendon Press, 1996), pp. 321-334, at 327-330; and Finnis, Natural Law and Natural Rights 281-290 and 294-296; and Aquinas, 266-274.


[65] Summa Theologiae I-II, q. 104, a. 1.


[66] Finnis, Aquinas, p. 267 (note omitted).


[67] I have had occasion elsewhere to explain the point as follows:  “It is meaningful and correct to say that the legislator (including the judge to the extent that the judge in the jurisdiction in question exercises a measure of law-creating power) makes the natural law effective for his community by deriving the positive law from the natural law.  The natural law itself requires that such a derivation be accomplished and that someone (or a group or institution) be authorized to accomplish it.”  George, “Natural Law and Positive Law,” p. 329-330.


[68] Summa Theologiae I-II, q. 95, a. 2.  Also see Finnis, Aquinas, p. 309 at n. 69, citing Summa Contra Gentilies III, 97.



[69] The remaining sections (four through six) of Kelsen’s essay focus mainly on problems of state power and private property.   He is particularly concerned with post-Grotian thought, especially that of Locke, Comte, Spencer, Hegel, and Marx.  He claims that “the most outstanding champions of natural law, from Grotius to Kant, have done their best to prove that private property is a sacred right conferred by divine nature upon man” (p. 153).  While some of the issues he raises could fruitfully be explored in light of Aquinas’s teachings, I shall not conduct that exploration in the present essay.  For a sound exposition of the Thomistic natural law doctrine of property, see Joseph M. Boyle, Jr., “Natural Law, Ownership, and the World’s Resources,” Journal of Value Inquiry, Vol. 23 (1989), pp. 191-207.


[70] Kelsen, “The Natural-Law Doctrine,” p. 144.


[71] Id. at 145.


[72] Id.


[73] Id. at 148 (emphasis supplied).


[74] Summa Theologiae I-II, q. 96, a. 4, quoting Augustine, De Libero Arbitrio I, c. 11.


[75] So, for example, Arthur C. Danto flatly ascribes to “the Thomistic defenders of natural law” the belief that “there cannot be an unjust law.”  “Human Nature and Natural Law,” in Sidney Hook (ed.), Law and Philosophy (New York:  --, 1964), pp. --- at 187; to similar effect, see H.L.A. Hart, The Concept of Law (Oxford:  Clarendon Press, 1961), pp. 205 and 206.


[76] See Hart, The Concept of Law, pp. 59-60, 86-88, 95-96, 113, 197, and 226; Joseph Raz, Practical Reason and Norms, 2nd edition (Princeton:  Princeton University Press, 1990), pp. 171, 177 ; and Finnis, Natural Law and Natural Rights, pp. 11-19.


[77] Summa Theologiae I-II, q. 96, a. 4.


[78] Id.


[79] Id.  Note, however, that, according to Aquinas, one may never obey a law requiring one to do something unjust or otherwise morally wrong.  And sometimes disobedience is required to avoid causing (or contributing to) “demoralization or disorder.”  (On issues relevant to the translation of Aquinas’s phrase “scandalum vel turbatio,” see Finnis, Aquinas, pp. 223 at n. 23, 273 at n. 112, and 274 at n. d.)


[80] See Summa Theologiae II-II, q. 60, a. 5.


[81] Id.


[82] This criticism of Hart (and Raz) is carefully developed by Finnis:  see Natural Law and Natural Rights, pp. 12-18.  On Hart’s misinterpretation of Aquinas on these matters, see id., ch. XII.


[83] Finnis, Aquinas, p. 288; also pp. 228 and 254 at n. d.


[84] Summa Theologiae II-II, q. 42, a. 2.


[85] Summa Theologiae I-II, q. 105, a. 1.  Note Aquinas’s claim (at ad 5) that tyrants “prey[] on their subjects” and rule them “as though they were . . . slaves.”


[86] See Scriptum Super Libros Sententiarum Petri Lombardiensis II, d. 44, q.2, a.2.


[87] On the distinction between usurping tyrants and legitimate rulers who degenerate into tyranny, see Finnis, Aquinas, 289-290 and the sources cited therein.


[88] Id. at 290, citing Summa Theologiae II-II, q. 42, a. 2 and q. 104, a. 6.


[89] Summa Theologiae II-II, q. 42, a. 2.


[90] Summa Theologiae I-II, q. 92, a. 1.


[91] Summa Theologiae I-II, q. 92, a. 1.



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