Natural Law is a long-standing and widely influential theory in ethics and legal philosophy. Because of its long and varied history, and the diversity of definitions of the term "natural," it is somewhat difficult to summarize exactly what makes a position or methodology one of natural law—at least in such a way as to neatly include all the positions and methodologies that have gone by that name. In attempting to establish a broad set of characteristics such a theory would have to possess in order to be considered natural law, it is useful then to look at the historical development of paradigmatic theories, paying attention to David Hume's advice that when trying to understand a discourse that employs the concept of "nature," we must consider what the concept is contextually being opposed to, and "the opposition will always discover the sense, in which it is taken" (Hume 2000, p. 305, n.).
In general, we can say that the traditional notion of natural law has held to the following four propositions: (1) morality is ultimately real and objective and is not relative in its primary truths to culture, subjective taste, or social agreement; (2) morality is somehow grounded in human nature, which is a specific part of the general order of nature, and is crucial for human happiness and flourishing; (3) the normative force and obligatoriness of morality is somehow the result of this grounding and may be understood using the terminology associated with a legal code; (4) the application of reason in examining human nature, and to some extent general nature, provides evidence for the specific content of our moral obligations.
Some theories, especially contemporary ones, may not clearly fit the pattern of this list. However, this speaks to a criticism that some recent "natural law" theories are not really natural law theories at all. It is in reference to the sort of positions specified above that such criticisms are made. There is also a problem in producing such a list as to whether reference should be made to God as a divine legislator of natural law. While the original and most traditional theories of natural law do rely on a theological foundation, it is characteristic of modern and contemporary versions that they do not, and therefore theism has not been listed as a basic proposition.
It is generally held that the first complete formulation of a natural law theory was a product of Stoic philosophers. It is also generally held, however, that classical Greek philosophers made significant conceptual contributions to what became natural law. Plato suggests the first, moral realist, tenet of traditional natural law theory in proposing his division of the Forms and appearances. In taking such a strong realist position, Plato provides material for the claim that goodness, or at least good order, is fundamentally real and our knowledge of it can be directly produced through reason. In dialogues such as Gorgias, Protagoras, and Phaedrus, Socrates defends a notion of objective truth and knowledge over the relativistic claims of sophists, which fits the natural law emphasis on moral realism. In the Republic, he analogizes the virtuous person to a healthy body and state, which fits the second proposition that morality is self-rewarding, tends toward happiness, and is the proper state of being. In the Laws, Plato touches upon the fourth proposition by referring to a law of nature forbidding homosexual sex as unnatural, appealing to animal behavior as evidence (836c–e).
Aristotle has an even stronger claim on influencing natural law, though his contribution is contested. One writer considers natural law his "principal legacy to Christian thought" (Hastings 2000, p. 465), whereas another believes that he "figures as a natural law thinker only ambiguously and not very helpfully" (Haakonssen 1992, p. 890). Howard P. Kainz (2004) points out that the passages in Aristotle commonly used to indicate support of natural law—"Universal law is the law of nature. For there really is, as every one to some extent divines, a natural justice and injustice that is binding on all men, even on those who have no association or covenant with each other" (pp. 6–8)—come from the Rhetoric (1373b5–1373b15), and are embedded in a section giving advice to lawyers on how to argue cases. Aristotle suggests using the rhetoric of natural law when "the written law tells against our case" but suggests that when "the written law supports our case" it is better to argue that "trying to be cleverer than the law is just what is forbidden by those codes of law that are accounted best" (Rhetoric, 1375a25–1375b25). But though Aristotle may not be as clearly a natural lawyer as some have thought, he does bequeath three important ideas that get taken up by natural law later on. First, in the Physics, Aristotle speaks at length concerning teleology—the notion that all natural objects have an end they are internally driven to fulfill (their telos) and that to understand a thing we must understand the end toward which it aims (194b15–199b30). Second, in the Nicomachean Ethics, Aristotle applies this principle to discover the end of human beings, arguing that humans, as natural, aim at some specific highest good for humans, which he defines as happiness—virtuous, rational, satisfactory activity (1097a15–1098a15). The teleology of natural objects and a complex virtuous happiness as the end of human beings will figure prominently in later natural law formulations, particularly those of Aquinas. Third, in the Politics, Aristotle argues that living in a political organization is entirely natural for humans. In fact, nature implants in us a social instinct and we can tell by the fact that humans are not individually self-sufficient that the purpose of the state is to produce well-being (1253a25–1253a35). States that work for this common well-being are genuine; states that do not are "perversions" (1279a25–1279b10).
It is commonly considered, however, that the first full-fledged description of natural law arises in Stoic philosophy. In general, Stoic philosophers were drawn to the idea that the universe is controlled by a perfectly rational and fateful principle called the logos, a concept prominent in Heraclitus's thought. The logos, as a rational principle that is creative, pervades all nature, and is reflected in human beings' ability to consciously reason and express logical relations in language, unites the metaphysical, the epistemological, and the ethical. As A. A. Long (1986) writes: "[I]t is clear that logos is something which can be heard, which serves to explain things, which is common to all" (p. 145). This unity is important for a view of reason as a law that connects nature, thought, and morality. In ethics, Zeno of Citium and other Stoics advise us to accept the logos-determined activity of the universe as right and unchangeable. It is our moral obligation to live in accordance with nature and our nature includes the instinct for self-preservation and the possession of reason (Diogenes Laertius 1925, pp. 193–197). The mostly widely cited statement of Stoic natural law, however, comes from Cicero, who wrote:
True law is right reason in agreement with nature; it is of universal application, unchanging and everlasting; it summons to duty by its commands, and averts from wrongdoing by its prohibitions. … It is a sin to try to alter this law, nor is it allowable to attempt to repeal any part of it, and it is impossible to abolish it entirely. We cannot be freed from its obligations by senate or people, and we need not look outside ourselves for an expounder or interpreter of it. And there will not be different laws at Rome and at Athens, or different laws now and in the future, but one eternal and unchangeable law will be valid for all nations and all times, and there will be one master and ruler, that is, God, over us all, for he is the author of this law, its promulgator, and its enforcing judge. Whoever is disobedient is fleeing from himself and denying his human nature, and by reason of this very fact he will suffer the worst penalties. (1928, p. 211)
In this passage, many of the traditional characteristics of natural law theory are asserted—the appeal to reason, natural ends, and universality, the lawlike features of obligation, commandment and punishment, the connection to human nature, our internal ability to determine natural law obligations through intuition, conscience, or acknowledgement of impulses, and the reliance on God as legislator. These aspects of natural law were subject to refinements and modifications at the hands of later thinkers including Roman jurists, such as Gaius, who focused on understanding natural law as the rational underpinning of positive law; Ulpian, who applied the natural law to all animals; and Gratian, who focused on natural law being spelled out as biblical commands (Kainz 2004).
With St. Thomas Aquinas (1225–1274), natural law reached a summary moment and was systematized and incorporated into the dominant Christian theological tradition of the West. Aquinas is so influential on the natural law tradition that his position is often seen as paradigmatic—a response that both limits the tradition and over-theologizes it.
Aquinas begins his discussion of the nature of law in the Summa Theologiae by defining law in general as "a rule and measure of acts, whereby man is induced to act or is restrained from acting" (Summa, Part 2, Part 1, Question 90, Answer 1), which is immediately "nothing else but a dictate of practical reason emanating from the ruler who governs a perfect community" (220.127.116.11). Proper laws always aim toward the general good and, following Aristotle, the goal of human life and thus the common good, is happiness (18.104.22.168). Aquinas then distinguishes between four types of law. Eternal law is the very idea of how things should be and has been intended in God's mind. This idea of how things should be according to God has "the nature of a law" (22.214.171.124). The natural law is essentially the way in which human beings, as rational beings, are positioned within this divinely designed order of things, directed toward fulfilling their nature in that order. Aquinas says:
Wherefore, since all things subject to Divine providence are ruled and measured by the eternal law … it is evident that all things partake somewhat of the eternal law, in so far as, namely, from its being imprinted on them, they derive their respective inclinations to their proper acts and ends. Now, among all others, the rational creature is subject to Divine providence in the most excellent way.… Wherefore it has a share of the Eternal Reason, whereby it has a natural inclination to its proper act and end: and this participation of the eternal law in the rational creature is called the natural law.… It is therefore evident that the natural law is nothing else than the rational creature's participation of the eternal law. (126.96.36.199)
Aquinas adds the categories of human law (specific determinations of practical regulations) and divine law (scriptural revelations of certain specifics). It is the relationship between natural law and eternal law that is most important here, however. As Aquinas sees it, the natural law is the way in which humans participate in the eternal law, by fulfilling our natural ends in the created order which is itself the expression of the eternal idea of God. The natural law is "imprinted" on us so that we have certain inclinations toward our ends but we also have reason, which allows us to perceive and choose to follow the imprinted inclinations in the proper way. In this sense, Aquinas frames natural law as objective, grounded in human nature, dependent ultimately on God as the creator of its content, understood through reason and through observation of our own innate tendencies and capacities.
When it comes to laying out the actual rules that the natural law prescribes, the first general principle is "good is to be done and pursued, and evil is to be avoided" which is coupled with the principle that "good has the nature of an end, and evil, the nature of a contrary, hence it is that all those things to which man has a natural inclination, are naturally apprehended by reason as being good, and … their contraries as evil" (188.8.131.52). Aquinas then develops from these two principles other precepts, including the duty of self-preservation, procreation and education of offspring, seeking knowledge of God, living in society, and avoiding offending others. It is here that Aquinas begins a popular tradition among natural law theorists of laying out a set of necessary and basic human goods.
Aquinas goes on elsewhere to develop more specific rules dictated by the natural law, for example, famously outlawing masturbation, noncoital sex, and homosexual intercourse as "contrary to the natural order of the venereal act as becoming to the human race" (184.108.40.206). It is also largely on the basis of natural law reasoning that the teaching of the Roman Catholic Church rules out contraception as ever morally permissible (Hastings 2000, Catechism of the Catholic Church 1994).
It is important to realize, however that there is no simplistic equation of the natural with the moral in Aquinas. In addressing the question of whether the natural law can be changed, he distinguishes between adding to and subtracting from the requirements of the natural law and also between primary and secondary principles of the natural law. Adding to what the natural law requires is not by itself any problem "since many things for the benefit of human life have been added over and above the natural law, both by Divine law and by human laws" (220.127.116.11). Subtracting from what the natural law requires, however, depends on what level of principle we are considering. The primary principles, such as the first precept of pursuing good and avoiding evil and the immediately derivative precepts of self-preservation, and so on, cannot be changed at all. The secondary principles, however, which are "certain detailed proximate conclusions drawn from the first principles" may be changed "in some particular cases of rare occurrence, through some special cause hindering the observance of such precepts" (18.104.22.168).
With this added layer of complexity, it is incumbent upon people to use their reason and to attend to circumstances in order to determine what is and is not permissible according to the secondary principles. For example, in the pursuit of procreation, it might seem eminently natural for men to have multiple wives, yet the tradition of the church is for monogamy. How to decide this question? Aquinas argues that marriage has a primary end of producing and raising children, but also a secondary end of a social function within a community:
Accordingly plurality of wives neither wholly destroys nor in any way hinders the first end of marriage, since one man is sufficient to get children of several wives. … But though it does not destroy the second end, it hinders it considerably for there cannot be peace in a family where several wives are joined to one husband, since one husband cannot suffice to satisfy the requisitions of several wives.… (3.suppl.65.1)
Thus according to a Thomistic reading of natural law, noncoital sex to the point of climax may never be permitted but a plurality of wives might be permitted if the material resources of the husband and culture made it workable.
Finally, for understanding the immense influence of Thomistic natural law, it is important to note that human law relies on natural law for its justification and authority. While human law may add various requirements in specifics (tax codes, civil regulations, etc.) it may not subtract from primary principles. Therefore, human laws are subject to a comparative test for their justification and authority. If they conflict with the natural law, they are not just, and not true law. Aquinas says:
Now in human affairs a thing is said to be just, from being right, according to the rule of reason. But the first rule of reason is the law of nature, as is clear from what has been stated above. … Consequently every human law has just so much of the nature of law, as it is derived from the law of nature. But if in any point it deflects from the law of nature, it is no longer a law but a perversion of law (22.214.171.124).
As with Cicero years before, this idea makes it possible to judge human laws as unjust and nonobligatory, and opens the way for the possibility of just revolutions against unjust states and human laws.
Aquinas's analysis of natural law set the stage for an ongoing debate over the nature of the relationship between morality, God's will, and God's intellect. For Aquinas, the eternal law, which was expressed in material creation, was found in God's intellect, God's perfect reason. As such, natural law was not simply an edict of God's will, as divine command theorists would argue, but rather was the automatic rational relationship between a created, purposeful order and the rational beings within that order. Presumably, if God had created a different type of purposeful world than he did, there would still automatically be a derived natural law that applied to that world as a function of reason, though its specific content would be different than the existing world. In this sense, God is bound by reason, and the natural law is the immediate rational product of created order. As Aquinas writes: "the natural law is something appointed by reason, just as a proposition is a work of reason" (Summa, Part 2, Part 1, Question 94, Answer 1).
One position, credited to Gregory of Rimini, took from this view that the natural law simply illuminated which actions and goals were intrinsically good and which were intrinsically evil. As such, the natural law "demonstrates" but is not literally a law in the sense of being legislated. As Francisco Suárez (1548–1617) encapsulates it in his influential De legibus, Gregory's position is "that the natural law is not a preceptive law … since it is not the indication of the will of some superior; but that, on the contrary, it is a law indicating what should be done, and what should be avoided, what of its own nature is intrinsically good and necessary, and what is intrinsically evil" (1944, p. 189). Another group of theologians called voluntarists, including to various degrees Bonaventure, Duns Scotus, and most prominently, William of Ockham, were defenders of the notion that the natural law was the product of God's will, not his intellect. As such, God could make the natural law, and thus morality, be anything he wished. Suárez writes: "This is the view one ascribes to William of Occam … inasmuch as he says that no act is wicked save in so far as it is forbidden by God and that there is no act incapable of becoming a good act if commanded by God" (p. 190).
Suárez himself, however, takes a middle course between the "intellectualist" and "voluntarist" positions, which he sees as being consistent with Aquinas. Suárez claims that the natural law not only demonstrates what is intrinsically good and evil but also "contains its own prohibition of evil and command of good" (p. 191). As indicating intrinsic good and evil, the natural law cannot be said to be simply willed by God. However, this does not mean that there is no divine command to follow the natural law on top of whatever rational obligation we might have to follow it. In fact, "it is revealed by the light of natural understanding, that God is offended by sins committed in contravention of the natural law, and that the judgments and the punishment of those sins pertain to Him" (p. 207). What this means is that although right reason can show us the intrinsic moral status of actions, and somehow produces some binding moral force, it is natural law's necessary connection to (but not identity with) the divine law that provides commanding obligation. Suárez writes:
The binding force of the natural law constitutes a true obligation; and that obligation is a good in its own way, existing in point of fact; therefore, this same obligation must proceed from the divine will, which decrees that men shall be bound to obey that which right reason dictates. … Therefore, although the additional obligation imposed by the natural law is derived from the divine will, in so far as it is properly a preceptive obligation, nevertheless … that will presupposes a judgment as to the evil of falsehood, for example, or similar judgments (pp. 196-197; 199).
Suárez thus describes a natural law that is both morally independent of God's Will but always joined by willed legislation to follow it.
The concerns over the actual obligations implied by natural law made their way into important political and cultural disputes, including the formal debate between the theologians Juan Ginés de Sepúlveda (1494–1573) and Bartolomé de Las Casas (1474–1566) over the treatment of Native Americans by the Spanish kings. Sepúlveda, appealing to Aristotle (who claimed slavery was justified by nature in his Politics), Aquinas, and Augustine, argued that the Native Americans were "barbaric … ignorant, unreasoning … sunk in vice … cruel, and are of such character that, as nature teaches, they are to be governed by the will of others" concluding "that the Indians are obliged by the natural law to obey those who are outstanding in virtue … This is the natural order, which the eternal and divine law commands to be observed … " (Las Casas 1992, p. 11-12). Las Casas, defender of the natives, relies partially on natural law ideals by arguing that the leaders of a community are obligated to seek the common good and waging war does not seek that end, and also that the Indians are not unreasoning but instead have rational, though still incorrect, defenses of their barbaric practices. For the most part, however, he gives consequentialist arguments as to why war should not be waged, arguing that war will produce much more harm than good.
Hugo Grotius (1583–1645) is variously credited with being the "father of modern natural law," the "father of natural rights," and the "father of international law." While Grotius spends most of his writing analyzing the nature of international war and its adjudication, his appeal to natural law leads in several influential directions. First, Grotius rejects the skeptical view (typified by classical Greek opponent of natural law, Carneades [c. 214–129 BCE]) that humans and all animals are simply driven by self-interest and that therefore all laws have their source in individual expediency, which may change as conditions do. Instead, Grotius argues in his Prole-gomena to the Law of War and Peace, humans have "an impelling desire for society, that is, for the social life—not of any and every sort, but peaceful, and organized … this social trend the Stoics called 'sociableness'" (Prolegomena 6). Grotius indicates that this innate sympathy and desire for peace is central: "This maintenance of the social order, which we have roughly sketched, and which is consonant with human intelligence, is the source of law properly so called. To this sphere of law belong the abstaining from that which is another's, the restoration to another of anything of his which we may have … the obligation to fulfill promises" (Prolegomena, pp. 8-9). In addition to sociableness, humans also have the rational power to discriminate between alternative actions and can choose what will actually "follow the direction of a well-tempered judgment, being neither led astray by fear or the allurement of immediate pleasure, nor carried away by rash impulse. Whatever is clearly at variance with such judgment is understood to be contrary also to the law of nature, that is, to the nature of man" (Prolegomena, p. 10).
Second, and largely because of this innate sociality and intelligence, Grotius claims that "what we have been saying would have a degree of validity even if we should conceded that which cannot be conceded without the utmost wickedness, that there is no God, or that the affairs of men are of no concern to him." (Prolegomena, p. 10). While Grotius was not the first to conceptually detach the natural law from God, his arguments lead to a significant shift in natural law language, making it easier to talk about natural law as intrinsically part of being human rather than something that reflects a divine idea. In fact, Grotius's later clarification on the importance of God's will—"the law of nature … proceeding as it does from the essential traits implanted in man, can nevertheless be rightly attributed to God because of his having willed that such traits exist in us"—ends up showcasing more the belief that human nature immediately provides the law, whatever the ultimate source of human nature (Prolegomena, p. 11). This move will permit the disconnection of God and natural morality, while making the source of obligation to follow the law a significant problem.
Third, the shift away from specifically religious natural law is made even more rhetorically available because of Grotius's development of the concept of natural rights. In The Rights of War and Peace, he first describes the term "right" as signifying what is just or at least not unjust, but then he goes on to say that "there is another signification of the word RIGHT … which relates directly to the person. In which sense, RIGHT is a moral quality annexed to the person, justly entitling him to possess some particular privilege, or to perform some particular act" (1901, p. 19). While the idea that individuals can possess moral qualities that produce privileges and impose duties on others has many conceptual problems, the upshot is that it allows for a discourse of human rights that steers clear of theological connections.
The emphasis on the social nature of human beings becomes central at this point, informing as it does both the content and general character of natural law. Some will agree with Grotius that humans have a natural sociability; some will argue that humans are naturally individualistic self-maximizers who are sociable only for practicality's sake. But the philosophical import of this talk is that even though modern philosophers will generally agree that there is a more or less fixed human nature and will continue to use the phrase "natural law," they may mean significantly different things by it.
For example, Thomas Hobbes (1588–1679) argues that nature has provided humans with certain set traits, including rough physical and intellectual equality. Out of this equality come roughly equal hopes of attaining the objects of desire and thus competition over goods, resources, and honor. With no limitations on such competition, violence ensues and a "war of every man against every man" arises. In analyzing a way out of this situation, Hobbes discusses "rights of nature," "laws of nature," and other phrases associated with the natural law tradition. Yet, when we read what Hobbes says about the character of natural laws, something seems to have changed. Hobbes says that
a law of nature, (lex naturalis ) is a precept, or general rule, found out by reason, by which a man is forbidden to do, that, which is destructive of his life. … and consequently it is a precept, or general rule of reason, that every man ought to endeavor peace … and when he cannot obtain it … seek and use all helps and advantages of war. … From this fundamental law of nature … is derived a second law; that a man be willing, when others are so too … to lay down this right to all things. (1988, pp. 86–87)
What appears to be happening here, in spite of some of the language used, is not that humans have a natural law moral obligation to seek peace, in the way Grotius might have envisioned, but rather that reason teaches us that our self-interest cannot be satisfied unless we agree with each other to give up some of our liberties and make social contracts. This means that the "law of nature" is not an objective moral obligation, but rather a pure practical realization of what we have to do in order to achieve our goals. Although it is tricky to try to use contemporary language here, it seems as if Hobbes's natural law is more about factual psychological principles and pragmatic planning. He agrees with traditional natural law theorists that we have a human nature and self-preservation is the first trait of that nature, but he sees the implications of that fact to have more to do with the satisfaction of desire than moral obligation.
This seems even clearer when Hobbes reductively defines human rights of nature as liberties to act and then defines liberties as merely "the absence of external impediments" (p. 86) and then later says that "where no covenant hath preceded, there hath no right been transferred, and every man has right to every thing; and consequently, no action can be unjust" (p. 95). Contrasting sharply with the traditional natural law claim that theft, for example, is immoral, Hobbes argues that theft only has meaning, and only becomes wrong, after social covenants are set up describing it as so. So here we see a case where the language of natural law is used but the substance is one of self-interested prudence. It is not surprising here that Hobbes's phrase, "state of nature," describes a dangerous environment that reason must be used to change. Our natural state is one of horror; our happy and peaceful state is one of artifice produced by reason.
Samuel Pufendorf (1632–1694) takes an approach both similar and somewhat more traditional. He agrees that humans are naturally self-interested and likely to engage in warlike activity to acquire the things they want. However, humans also seem to go beyond nature in excessive pursuit of the basics nature has provided them—lusting more than is necessary for procreation, seeking clothes more for show than for necessity, desiring tasty food far beyond what we need for nutrition (1991, p. 34). In a vein similar to Hobbes, Pufendorf writes:
Man, then, is an animal with an intense concern for his own preservation … incapable of protection without the help of his fellows.… Equally, however, he is at the same time malicious, aggressive, easily provoked, and as willing as he is able to inflict harm on others. The conclusion is: in order to be safe, it is necessary for him to be sociable.… The laws of this sociality … are called natural laws. On this basis it is evident that the fundamental natural law is: every man ought to do as much as he can to cultivate and preserve sociality. (p. 35)
Here, though the term "natural law" and "ought" are used, they seem to be used prudentially, not as objective moral terms. However, Pufendorf recognizes, as did Suárez, this divide between self-preserving practicality and moral obligation and brings God back in to secure obligation. "Though these precepts have a clear utility, they get the force of law only upon the presuppositions that God exists and rules all things by His providence, and that He has enjoined the human race to observe as laws those dictates of reason which He has Himself promulgated by the force of the innate light. For otherwise though they might be observed for their utility, like the prescriptions doctors give to regulate health, they would not be laws" (p. 36). Thus, Pufendorf reverts to a modified form of divine command theory in order to fasten down the lawfulness of natural law.
John Locke (1632–1704), the most important social contract theorist after Hobbes, forms yet another subtle synthesis that ends up making natural law a moral constraint on the sorts of social contracts we can legitimately produce. As in that of Hobbes, in Locke's state of nature humans have the ability to do whatever they want but unlike Hobbes, they do not have the right to do whatever they want. Locke writes:
Yet he has not liberty to destroy himself, or so much as any creature in his possession, but where some nobler use, than its bare preservation call for it. The state of nature has a law of nature to govern it, which obliges every one: And reason, which is that law, teaches all mankind, who will but consult it, that being all equal and independent, no one ought to harm another in his life, health, liberty or possessions. For men being all the workmanship of one omnipotent and infinitely wise Maker … they are his property … made to last during his, not another's pleasure. (Locke 1960, p. 271)
So we see here that even in the state of nature there is a natural law that provides a minimum moral code, namely, not to interfere with another's body, freedom, or property, and as Locke later lays out, the natural law also provides each person the authority to enforce and punish violations of this natural law (the abuse of which leads to the need to develop an unbiased state through social contract).
What is a bit uncertain here is the role of reason and God. In one sense, Locke says that reason is the natural law, which suggests a kind of prudential characterization, but he also says that it is the fact of our being the property of God that obliges us not to harm each other, which suggest a divine origin of obligation. However, it may be that reason teaches us first the moral principle that property is sacrosanct and that this principle is what informs us that as God's property we do not have the right to harm others. Locke also says that what makes a criminal is that he chooses to live by some other rule than reason, but then states that reason "is that measure God has set to the actions of men, for their mutual security" (Locke 1960, p. 272). In his constant appeal to reason for determining the specific obligations the natural law requires of us, however, Locke seems to work with the idea that reason both teaches us the content of moral truth instrumentally (we consult it), and is the natural law itself in some way.
With these sort of modifications, revisions, and perhaps even reversals, it is not surprising that natural law as a general ethical theory began to wane and by the eighteenth and nineteenth centuries, concerns about ethical theory shifted to debates among social contract theorists, skeptics, moral sense theorists, Kantians, and utilitarians. While the early social contract theorists still used the language of natural law, other philosophers clearly challenged the language and theory explicitly.
David Hume (1711–1776) famously maintained that it is a simple logical mistake to think you can "derive" a moral obligation from a biological or psychological fact (the is/ought distinction) and argued that because of the divergent definitions of the term "natural" that "nothing can be more unphilosophical than those systems which assert, that virtue is the same with what is natural, and vice with what is unnatural" (2000, pp. 302, 305).
Immanuel Kant (1724–1804) sought moral obligation in the realm of pure reason and repudiated any connection of actual contingent human psychology with moral truth. He argued in Groundwork of the Metaphysics of Morals that
everyone must grant that a law, if it is to hold morally, that is, as a ground of an obligation, must carry with it absolute necessity; that, for example, the command 'thou shalt not lie' does not only hold for human beings, as if other rational beings did not have to heed it … ; that, therefore, the ground of obligation here must not be sought in the nature of the human being or in the circumstances of the world in which he is placed, but a priori in concepts of pure reason. (1997, pp. 2–3)
It is worth noting, however, that one of Kant's formulations of the categorical imperative is "Act as if the maxim of your action were to become by your will a universal law of nature" (p. 31). In spite of this phrasing, this is not natural law theory. What Kant is talking about is the understanding of a law of nature as a Newtonian universal regularity and is asking us to consider whether we could logically will our maxims to have such a universal character. He writes: "The universality of law in accordance with which effects take place constitutes what is properly called nature in the most general sense … that is, the existence of things insofar as it is determined in accordance with universal laws." (p. 31) and later comments that "We must be able to will that a maxim of our action become a universal law. … Some actions are so constituted that their maxim cannot even be thought without contradiction as a universal law of nature" (p. 33).
John Stuart Mill (1806–1873) criticized the entire project of trying to couple morality with nature, arguing that virtually all of our actions alter nature in some way and that an attempt to imitate nature would have us follow a guide of cruelty (1969, pp. 373–402). Of course, Mill here is arguing against the claim that we should look to nature in the large sense as a guide to behavior rather than specifically paying attention to the narrower concept of human nature (something he did pay attention to), which indicates how the concept of "nature" as a more narrow moral guide was being used by the 1800s.
Finally, John Austin (1790–1859), the founder of modern legal positivism, argued that law
may be said to be a rule laid down for the guidance of an intelligent being by an intelligent being having power over him. … in the largest meaning which it has … the term law embraces the following objects:—Laws set by God to his human creatures, and laws set by men to men. The whole or a portion of the laws set by God to men is frequently styled the law of nature, or natural law: being, in truth, the only natural law of which it is possible to speak without a metaphor. … But, rejecting the appellation law of nature as ambiguous and misleading, I name those laws or rules … the Divine law, or the law of God. (2004, p. 24)
This command theory of law undermined the position that an obligation to act followed from anything other than sheer power and thus reduced natural law to nothing more than a confusing way of referring to divine command.
In the twentieth century there was a revival of interest in natural law, as seen in the works of Jacques Maritain, Elizabeth Anscombe, Yves Simon, Ralph McInerny, Russell Hittinger, Robert George, Peter Geach, Anthony Kenny, and Alisdair McIntyre. In large part, the new attention to natural law was spurred by the Catholic Church's teachings on social and moral issues, including Pope Paul VI's encyclical letter Humanae Vitae (1968), which drew on Aquinas's moral theories to condemn artificial birth control. Prominent among the theological and philosophical defenders of the church's natural law teaching on contraception, abortion, homosexuality, and healthcare (though not necessarily following in the Thomistic tradition) were Germain Grisez and John Finnis. Grisez published an influential commentary on Aquinas's natural law system in 1965, which inspired John Finnis's work, culminating in Natural Law and Natural Rights (1980).
The heart of that book is Finnis's list of basic human goods, including life, knowledge, play, aesthetic experience, sociability (friendship), practical reasonableness (intelligently choosing and affecting one's own life), and religion (concern with transcendence) (1980, pp. 85–90). These are not moral goods, but more basically goods-for-us. It is our fundamental and self-evident awareness of these basic goods that creates moral choices for us—what are we to do? How are we to use our practical reasonableness to decide what to do? Finnis then attempts to use a "natural law method" of ethics, while still only using modern (presumably not natural law) terminology, to show purely through logic and other self-evident truths what we ought to do (p. 103). He argues for a set of basic requirements of practical reasonableness, which include a coherent, rational plan for our lives, no arbitrary preferences among either the basic goods or among persons, detachment and commitment, choosing efficient methods to achieve good, a limited attention to preference satisfaction (excluding such things as theft and murder), seeking the common good, following conscience, and perhaps the most controversial principle, "one should not choose to do any act which of itself does nothing but damage or impede a realization or participation of any one or more of the basic forms of human good." (p. 118).
It is this latter principle that Finnis believes rules out any consequentialist reasoning. Consequentialist ethics, he argues, is irrational because goods cannot possibly be measured, and therefore the ends never justify the means where the means includes damaging a basic good. Once he rules out consequentialism, the principle that a basic good cannot be impeded is "self-evident" and the moral rule can be summarized as "Do not choose directly against a basic value" (pp. 119, 123). This formulation of natural law begins with empirical claims about what things it is in our nature to value and then logically tries to come to our obligations. However, with no legislator to provide the traditional source of obligation (such as Suárez's and Pufendorf's God) there remains the question of whether this theory is actually a natural law theory. Finnis himself tells us that, like scientific laws, which are actually only metaphorically laws, "'Natural law'—the set of principles of practical reasonableness in ordering human life and human community—is only analogically law" (p. 280).
Finnis seems to think that reason by itself provides obligation, but it is not clear how this is supposed to occur. Reason can help us discover what desired ends we find in our psychological constitutions and can help us determine instrumentally how to achieve those ends, but how does reason create an obligation to pursue any end?
This question of whether their theory is properly called natural law theory also follows the most prominent twentieth century legal theorists. Lon Fuller (1964) describes a set of eight requirements that civil law must meet in order to be considered genuine law—requirements such as generality, noncontradictoriness, and nonretroactivity. In this, he is appealing to a set of objective conditions that one may subject civil laws to as a test for true lawfulness, but he emphasizes that this test is procedural rather than substantive (Bix 1996). Ronald Dworkin (1967, 1986) argues that principles of values always govern how we produce and interpret civil laws, and so there is no fundamental separation of the realms of law and morality, but this could be essentially a descriptive claim and does not imply that there is a self-evident objective moral order to which civil laws must adhere in order to provide obligations. It is perhaps primarily in the sense of providing opposition to legal positivism that these theories are classified as natural law theories.
Connections to Other Ethical Theories
While natural law is its own set of theories, the differences between it and other ethical theories are often exaggerated and oversimplified. There are significant connections and shared assumptions. For example, although Kant explicitly rejects appealing to empirical facts about human nature to determine the moral law, he begins his moral philosophy with a teleological principle widely held by natural law theorists, stating in Groundwork that "in the natural constitution of an organized being … we assume as a principle that there will be found in it no instrument for some end other than what is also most appropriate to that end and best adapted to it" (p. 8). Unlike natural lawyers, however, he concludes from this that the job of reason cannot be to produce happiness, because instinct would best accomplish that. Instead, reason's purpose is to produce a good will. Kant does connect nature and law through teleology though by claiming in Idea for a Universal History from a Cosmopolitan Point of View that "If we gave up this fundamental principle, we no longer have a lawful but an aimless course of nature" (1963, p. 13), and concluding that "The greatest problem for the human race, to the solution of which Nature drives man, Is the achievement of a universal civic society which administers law among men" (p. 16). In view of these commitments, it might be said that Kant shares with the Stoics a view of the metaphysical and epistemological aspects of the natural law, but not the essential moral aspects.
Mill, for all his criticisms of the use of the term "natural" in moral theory (Nature), is as quick as a natural law theorist to point to empirical facts about human psychology:
The only proof capable of being given that an object is visible, is that people actually see it … the sole evidence it is possible to produce that anything is desirable, is that people do actually desire it … No reason can be given why the general happiness is desirable, except that each person, so far as he believes it to be attainable, desires his own happiness. (1998, p. 168)
He is quick also to appeal to our consciences as guides: "The internal sanction of duty … is one and the same—a feeling in our mind; a pain, more or less intense, attendant on violation of duty. … This feeling, when disinterested … is the essence of Conscience" (p. 161).
And of course, given the natural law emphasis on the pursuit of happiness, the importance of developing character traits which lend themselves to happiness and flourishing, the fundamental desire for self-preservation, the practical need to interact with others, and the ability to apprehend our obligations through internal self-observation, we see strong shared assumptions with virtue theory, social contract theory, and intuitionism.
Problems for Natural Law
As seen through its historical development, the primary arguments for natural law have been that it is warranted theologically, that nature or human nature somehow imply that we should act in certain ways, that reason itself simply shows us the self-evident truth of natural law, and that it is necessarily practical that we act in certain ways given our nature. Criticisms have been leveled against these arguments and other aspects of natural law theory.
First, concerns about religion: If natural law theory relies on the existence of God, then proof of God must be forthcoming before we can move on to moral metaphysics—a complicated task. However, this point would only obviously apply to those versions of natural law which require God for moral obligation and some versions of natural law do not make this assumption. Problems do arise, though for relating natural law to divine command theory. For example, if, as Grotius argues, innate human traits have been directly willed in to us by God, then God's will is the source of moral obligation and thus natural law may be only a thin technical layer between human obligation and divine command theory. If, as Aquinas seems to think, some sort of natural law would proceed automatically from whatever world God created, irrespective of God's will, then this sort of moral relationship seems to be at least as fundamental and necessary as God—a point about which voluntarists are concerned.
Philip Quinn (2000), for example, actually emphasizes the divine command elements of Aquinas's thought, arguing that the Summa 's exoneration of Abraham in the sacrifice of Isaac story (Summa, Part 2, Part 1, Question 100, Answer 8, Reply 3) shows that Aquinas believed "the slaying of Isaac by Abraham, which would be wrong in the absence of the divine command, will not be wrong in its presence if Abraham obeys it" (p. 62). The issue is fundamentally about whether natural moral obligations are products of pure reason, and whether this implies there is some truth or reality that does not depend entirely on God.
Second, concerns about relativism: Just as voluntarist divine command theory is often seen as a type of moral relativism because God could (in some views at least) have made anything a moral obligation, the apparent natural law assumption that morality depends on the actual contingent facts of biology and psychology seems to make morality relative to species (rather than culture or the individual, as traditional relativisms argue). This is not a practical problem for determining obligations when there is only one sapient species to consider, but for ethicists such as Kant, morality could only be said to be truly objective if it was necessary for all possible rational beings.
Third, concerns about is and ought: Hume pointed out that many attempts at moral philosophy make a near-imperceptible shift from the way things are to the way things should be—a move that logically requires connecting premises often not given (Hume 2000, p. 302). This criticism has been analyzed at great length (Hudson 1969). Natural law may be an attempt to breach the is/ought divide, but historically it often either does nothing to supply the connection, or supplies it arbitrarily, or tries to supply the connection simply by appealing to reason. It is unclear, however, how reason is supposed to produce moral obligation. It may be true, for example, that choosing a short-term pleasure over a long-term basic good interferes with comprehensive happiness, and thus may in one sense be called unreasonable or irrational. But this sense of "unreasonable" is more a matter of acknowledging empirical constraints on what will actually satisfy our desires, health, or continued existence rather than serving as any sort of logical proof of a moral obligation.
Instrumentally, reason can help us to satisfy the desires and inclinations we do in fact naturally have, but it is not clear how reason is supposed to indicate that we should try to satisfy them. There is nothing formally illogical about not satisfying desires we have or securing our own health and happiness. For versions of natural law that retain God as a moral lawmaker, this problem seems to be avoided because obligation can been seen in a positivist sense as legislated—but then this Ockamist or Austinian approach returns us to the problem of whether natural law simply reduces to divine command theory.
Fourth, concerns about the goodness of nature: There is the assumption in natural law that human nature is fundamentally good (even though flawed), which legitimates our appeal to it. This is an inheritance of Christian theology, even for those versions of natural law that argue for no dependence on God. Other explanations, less committed to design and eternal law formulations of the world's development, see aspects of human nature as more adventitious and thus less morally authoritative. Human traits are not necessarily here because they are supposed to be but because they survived. As a result, many inherent traits may be prone to producing what we think of as evil acts and ends. As Mill writes in "Nature":
With regard to this particular hypothesis, that all natural impulses, all propensities sufficiently universal and sufficiently spontaneous to be capable of passing for instincts, must exist for good ends … this is of course true of the majority of them, for the species could not have continued to exist unless most of its inclinations had been directed to things needful or useful for its preservation. But unless the instincts can be reduced to a very small number indeed, it must be allowed that we have also bad instincts which it should be the aim of education not simply to regulate, but to extirpate. … among them one which they call destructiveness: an instinct to destroy for destruction's sake. I can conceive no good reason for preserving this. (p. 398)
Fifth, and related to the fourth, concerns about best explanation: One of the key purposes of natural law ethics, particularly in its modern versions, is to oppose the idea that there is no human nature, or that human nature is so widely divergent that no cultural or moral norms can be said to be better or worse than any other. In this sense, natural law is opposed to cultural moral relativism, behaviorist environmental determinism, and postmodern social constructivism. However, natural law is not the only theory that holds there is a human nature, that can produce a list of basic human goods, pays attention to biology and psychology, and opposes relativism. To some extent Rawlsian contractarianism does this, but in a way even more related to natural law, evolutionary ethics does as well.
Evolutionary ethics can take seriously the claim that the moral law is "written on our hearts" and that we only need our conscience to apprehend it. As Grotius defended the existence of the natural law by pointing to widespread regularities in moral beliefs (1957, pp. 25–26), evolutionary theorists defend the existence of an evolved moral sense, which explains cross-cultural similarity in moral emotions such as guilt and shame, and cross-culturally widespread moral restrictions on murder, betrayal, and sexual infidelity. But there is a difference.
Just as evolutionary theory covered much of the same territory as the argument from Design for the existence of God, but could explain both complexity and the existence of "imperfections" such as vestigial organs (having given up a perfect designer and therefore eliminating any expectation of perfect design), evolutionary ethics can explain both the widespread facts of human cooperation and widespread selfish violations of moral norms (having given up a perfect moral inculcator and therefore eliminating any expectation of perfect moral inculcation).
Even for nonreligious versions of the natural law, there remains the idea that our consciences and innate natures are essentially good and trustworthy and thus have some difficulty explaining why warmongering, murder, lying, addiction, and rape are both so self-evidently bad and so persistent. It seems to some then that evolutionary ethics does a better job of explaining human moral nature and human immoral nature. Of course, evolutionary ethics is at heart descriptive, arguing that moral attitudes are simply what have been successful at replication over time and not that they represent any objective moral truth (anymore than our bodies reflect imperfectly some infallible objective body). This is indeed a disadvantage if one is in search of moral prescriptions, but evolutionary ethicists can attempt moral prescription as well, having at first glance no lesser or greater obstacle to overcome in moving from facts to obligations than natural law theorists (Rachels 2000).
Natural law theory is still active as an applied ethics (forming as it does the foundation of the Catholic Church's moral philosophy). It is also still active in some academic investigations, generating numerous titles each year in ethics and legal philosophy. It is safe to say, however, that it is a minority position in mainstream academic ethics, at least in its traditional form, and typically appeals mostly to ethicists of particular religious bents. However, the descendants (or perhaps distant cousins?) of natural law theory thrive in the form of natural rights or human rights theory, which form the backbone of much of the world's international moral discourse—particularly when criticizing a particular state's or culture's practices. Practically speaking, though, much of the rhetoric concerning natural law in its more explicit and narrow sense (in appeals to naturalness and unnaturalness) is spent on ethical issues of sexuality and reproduction, leading some critics to claim that debates over sexual morality are actually the last stand for popular traditional natural law appeals (Mohr 2005, pp. 122–123).
See also Anscombe, Gertrude Elizabeth Margaret; Aristotle; Augustine, St.; Austin, John; Bonaventure, St.; Carneades; Cicero, Marcus Tullius; Consequentialism; Duns Scotus, John; Dworkin, Ronald; Gregory of Rimini; Grotius, Hugo; Heraclitus of Ephesus; Hobbes, Thomas; Hume, David; Kant, Immanuel; Laws of Nature; Legal Positivism; Locke, John; Logos; Maritain, Jacques; Medieval Philosophy; Mill, John Stuart; Moral Realism; Peace, War, and Philosophy; Philosophy of Law, History of; Philosophy of Law, Problems of; Plato; Pufendorf, Samuel von; Rawls, John; Rights; Social Contract; Socrates; Sophists; Stoicism; Suárez, Francisco; Thomas Aquinas, St.; Thomism; William of Ockham; Zeno of Citium.
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