The major task of this paper is to compare Aristotlelian–Thomistic theory of Natural Law with the views of philosophers in the modern period. To successfully accomplish this task, the paper critically looks at the concept or theory of Natural Law from Aristotle’s as well as Aquinas’ points of view. This is followed by a comparison of what these two philosophers said, to actually justify the claim that Aquinas’ theory of Natural Law is Aristotelian or based on that of Aristotle. The work then gives a brief history of modern philosophy, followed by a comparison of the Aristotelian-Thomistic theory of Natural Law with modern philosophy – which is the focal point of the paper.



                The concept of Natural Law appears vague. If one is asked “what is natural law”? At first instance, one’s mind goes to the laws of nature as they pertain to the discipline physics – where we talk of the laws guiding gravitation, motion, etc. In this paper we shall centre the discussion of the concept of Natural Law on Morality. For a better understanding of what natural law is, let us look at the words that make up the concept of Natural law. The word natural is derived from nature and the word law means “the whole system of rules and regulations set to guide a particular thing or group of things, or a particular person or group of persons. So putting these two ideas of law and nature together, natural law means the rules and regulations set to guide nature. In this context Natural law means the rules set to guide and direct humanity’s moral behaviour.

                The concept of Natural law has been explained by many philosophers. St. Thomas Aquinas explains it as the rules for behaviour that correspond to the features of human nature. These human natural features include: the preservation of life, the propagation of species and the inclination towards the search for truth. According to him, Natural law has two sources- the divine and the human. Benedict Spinoza (1652 - 1677) of the modern period defines Natural law as the law of necessity by which all creatures (including man) are conditioned to behave in certain ways. He explains that it is a deterministic law by which the behvaiour of all creatures (including the activities of all men) are determined by nature. On a critical research in the domain of philosophy, it is always observed that every philosophy or idea of any particular individual always has its “tap root” from preceeding philosophy or philosophers. For instance, a lot of ideas in the Medieval period are observed to have their roots in the philosophies of the Ancient period, and so does the observation go on regarding all the periods in philosophy. Following this, St. Thomas Aquinas of the Medieval period is not an exception. He builds his philosophy on that of Aristotle, and of major concern to these authors is Aquinas’ theory of Natural law which is a modification of Aristotle’s Ethics. This implies that, although the concept of natural law is attributed to St. Thomas Aquinas, it did not originate with him. On the other hand, the fact that the concept of natural law does not have its origin from Aquinas, does not mean that he did not make new contributions that are original to him. Aquinas made original use of Aristotle’s ideas. That is why his theory is a “modification” of that of Aristotle.



                Aristotle’s concept of Natural Law is embedded in his Ethics which is naturalistic. Generally, ethics is concerned with the way the whole of humanity or man in particular ought to live, and what is regarded as the good life for man. Unlike Plato who believes that the Good is found in a world different from the physical world – the world of ideas, Aristotle believes that the Good is found in man and is discovered through a rigorous study of the very nature of man. For Aristotle, by understanding the nature of man, one can know what the good life for man ought to be.  Following this, he says that a good person is one who is fulfilling his/her natural function as a person. Aristotle distinguishes between the professional life of a person and his/her moral life. For instance, one being a good doctor does not automatically mean one is a good or moral person and vice-versa. What is the natural function of a man? The natural function of man, according to Aristotle, is what distinguishes him from the rest of creation. In other words, the good life for man which is discovered through performing your natural function must be outstanding and unique to man and not shared with animals and other components of the world. For example, sensation. The idea of sensation is shared by both men and other animals. So definitely, this cannot be the good life for man. Aristotle believes that what is peculiar to humanity (man) is the principle of Rationality. He argues that the ability to reason can only be attributed to man. Reason, according to Aristotle, is an activity of the soul in accordance with virtue.

                Aristotle sees reason or the principle of Rationality as taking its place at the centre of morality. It is this principle, according to him, that governs and controls the activities of a moral being. But Aristotle was also faced with the inevitable problem, in morality, of what accounts for right or wrong actions. This he attributed to choice and choice he attributed to desire and reasoning. Expantiating on reason as the cause of choice, he divided the soul, which is the real man, into the rational and the irrational parts. According to him, the conflict between the rational and the irrational parts or elements in man is what raises the problems and subject matter of morality. In other words, the problem of right and wrong actions is attributed to the rational and irrational parts of the soul respectively. This means that for Aristotle right or moral actions are performed when the soul allows the rational part to govern the other parts of the body. On the other hand, wrong actions are performed when one allows one’s irrational part (i.e desire and appetite) to take control of one. For Aristotle (1932:BK.V-Vii:1-2), therefore, since men can cause revolution through their private lives, some magistracy must be set up to inspect those whose mode of living is unsuitable to the constitution, unsuited to democracy in a democracy and to a federation in a federal system. Aristotle pointed out that politics is knit to ethics. For him political science is the second half of a subject of which ethics is the first half. Aristotle describes man in biological terms as by nature a political animal who develops capabilities only in societies rightly organized for his welfare. The aim of politics, says Aristotle, is to discover first in what mode of life man’s happiness consists. And secondly to discover by what form of government and what social institutions through which that mode of life can be secured. The former question, says Aristotle, requires the study of man’s character which occupies the NicomacheanEthics and the latter question requires the study of the constitution of the state, which is treated in Politics by Aristotle. So for Aristotle politics is the sequel of Ethics, the second half of a single Treatise. This subject is covered by Plato in a single dialogue - The Republic.

                Having succeeded in achieving the principle that leads to the good life, the question now is ‘what is the good life for man?’ Following the teleological nature of Aristotle’s Ethics, he believes that what should serve as the good life for man must be an end in itself and not a means to an end. He distinguishes a virtue that is a means to an end - calling it an instrumental end, from a virtue that is an end in itself – calling it an intrinsic end. For instance, if a carpenter constructs a table for a student, that construction of the table serves as an intrinsic end for the carpenter since that is his/her function as a carpenter, but for the student, the table is an instrumental end since it only aids him/her in reading to achieve academic success which is the intrinsic end of a student. In saying what the good life of a man is, Aristotle believes it is Happiness or well being (Eudamonia) which he equated to the good. It is only happiness that has met the requirements of the good life. He avers that happiness alone is an intrinsic good and does not serve as an instrumental good to any other virtue. He defines Happiness as “a working of the soul in the way of excellence or virtue”.

                Virtue, according to Aristotle, is a habit of choosing in accordance with a mean. This implies that virtue is achieved in between two extremes. And this further implies that either of the extremes of any virtue is vice. For instance, the mean between prodigality and stinginess is what is regarded as virtue – and this is liberality, while the two extremes – prodigality (excess) and stinginess (defect) are vices. Other virtues include: justice, courage, temperance, etc.



                St. Aquinas’ (1225 - 1274) theory of Natural Law is a modification of Aristotle’s Naturalistic Ethics. Aristotle talked of man fulfilling his natural function as a man in order to achieve Happiness or the good which he regarded as intrinsic, that is, for its own sake and not as a means to an end. Upon this Aquinas built his notion of a person’s supernatural end. Aquinas believed that the complete happiness of a man is not found in man fulfilling his natural capacities or end alone but also fulfilling his supernatural end too. He talked from the view point of a Christian that human nature has both its source and ultimate end in God. On this note Aquinas believed that the right choices could be made by the help of the will in collaboration with the power of reason (intellect). Aquinas also believed that the will alone cannot make the right choice since it is not all choices that are right. The intellect must guide the will. To put this in his words, Aquinas (1987:184-186) said that “the will by itself cannot always make the right move; the intellect must be the guide… The will represents a person’s appétite  for the good and right whereas the intellect has the function and capacity for apprehending the general or universal meaning of what is good”.       His notion of a person’s supernatural end now comes up as an additional prerequisite for achieving complete or perfect Happiness. In otherwords, the will and the intellect alone are not sufficient if one has to attain perfect Happiness. This is because, according to him, the whole of human nature is understood in relation to God. Aquinas said that in fulfilling one’s supernatural end, one needs God’s grace and revealed truth.

                Aquinas also talked on virtue. According to him, virtue or goodness consists in making the right choices, and the right choices he regards as the mean between two extremes. He mentioned the dominant natural virtues to be courage, temperance, justice, and prudence. In addition to these virtues, the natural end of a person is further realized through humanity’s knowledge of the natural law or the moral law. Aquinas defined Natural Law as the rules for behaviour that correspond to the features of human nature. These features include: preservation of life, the propagation of species, ensuring peace and order in the society. The orderliness of these activities or features of human nature necessitated the division of Aquinas’ Natural or Moral Law into four namely: Eternal Law, Natural Law, Human Law, and Divine Law (Aquinas 1987:184-186). The Eternal Law, according to St. Aquinas, is the law from God who is the Divine Reason. This law is used to govern the whole universe as a community. By the articulation of this law (the Eternal Law), Aquinas counters the argument of the school of thought known as Deism. Deism is a philosophical school of thought that holds that God created the universe and abandoned it after creation. Aquinas (1987:84-186) further argues that “since the Divine Reason’s conception of things is not subject to time but eternal … therefore it is that this kind of law (the Eternal Law) must be called Eternal”.



                The main purpose of this segment of the paper is to compare what Aristotle says in his Ethics with what St. Aquinas has in his natural law to see whether or not they are in concordance, and use this synthesis, which will now serve as the Aristotelian – Thomistic theory of natural law, to compare with modern philosophy. Both Aristotle and Aquinas believe in the principle of love and Hate as what guides the irrational part of the soul. They believe that since the appetitive part of the soul is affected or influenced by things outside of the self, there are two basic ways in which it reaches to them. This is through love and hate otherwise called “the concupiscent and the irascible passions”. Love (concupiscent) leads one to desire things and people, while hate (irascible) leads one to avoid or destroy them.

                Although Aquinas went further to talk of the supernatural, they both have a naturalistic view of, or approach to, morality. They believe that man can achieve his natural end through fulfilling his natural activities or function. Aquinas modified this by saying that our end is complete when man fulfills also his supernatural functions. Their morality is also teleological in nature. They believe that to every thing or every action there is an end or a purpose. The problem of evil, to them, is a matter of choice. Both of them hold that it is what man chooses to do that he does. This makes them freewillists. The concept of choice they attribute to reason. When the rational part of the soul controls a man, he makes right choices, but when he is controlled by the appetitive part, he makes wrong choices which are evil. To both of them, Happiness, is the highest goal. To them Happiness is the only intrinsic good – it is not an instrumental good to any other. They opine that what every man seeks is happiness. Aristotle’s concept of Happiness is “Eudamonia” meaning well-being. Aquinas agreed with Aristotle as regards the theory of moderation (the Golden Mean). Like Aristotle, Aquinas believes that virtue is achieved at the middle of two extremes – excess and defect.

                Like was said earlier, Aristotle and Aquinas are both freewillists. They believe in the doctrine of freewillism as against determinism. For them man is free and should be responsible for every action he takes. They uphold both Natural and conventional justice. Natural justice is from nature. It is related to the law given by nature, while conventional justice is related to the rules and regulations by human beings to govern our relationship with others.




                Modern philosophy is the philosophy that spans between 14th century to 18th century AD. The modern period of western philosophy began with the Renaissance, also known as the period of the enlightenment. This period was unique because of the new philosophical and scientific turn of mind that greeted the era. During this period, great thinkers emerged who distinguished themselves and their thought system from the dogmatic beliefs of the medieval period of philosophy which preceded the modern period.

                As far as this paper is concerned, we shall limit our discussion to a facet of this period – the period when the rationalist school of thought as well as the empiricist flourished. This period featured men such as Rene Descartes, Benedict Spinoza, and Willhelm Leibniz – all of the rationalist school of thought, and John Locke, Bishop George Berkeley, and David Hume – the empiricists. Rationalism, on the one hand, holds that knowledge is got through reason. The rationalists discard the senses as sources of knowledge and believe that certain knowledge is achieved only through reason. Descartes (1596 - 1650) as the founder of this position, and other rationalists (Spinoza and Leibniz) sought to formulate clear rational principles that could be organized into a system of truths from which accurate information about the world could be deduced. Empiricism, on the other hand, is a school of thought that acknowledges the primacy of experience and seeks to show that all human knowledge is dependent on experience. The notable empiricists as already mentioned above are John Locke (1632-1704), George Berkeley (1685-1753), and David Hume (1711-1776). They are all called the British empiricists. The empiricists debunk the rationalists’ belief in innate ideas, saying that the mind is like a “tabula rasa” at birth and that it is experience that imprints knowledge on the mind. The aim of this brief history is to enable us familiarize ourselves with the philosophers whose philosophical postulations we shall discuss in the light of the Aristotelian–Thomistic theory of Natural Law.



                In line with Aquinas’ notion of revealed truth, Rene Descartes believed in revealed truths as above our intelligence, and opined that for one to be successful in thinking about them, it is necessary to have some extraordinary assistance from above and to be more than a mere man. Descartes had a universal view of the concept of Natural Law. According to him, the physical universe is governed by rigid and unchanging laws of nature. These laws govern all the activities of the physical world, including those of animals and all the organs of the human body. For him, humans are like machines and their activities are automatic products of the physical laws of nature. His position is a deterministic one.

                Spinoza disagrees with Aristotle and Aquinas as regards their theological perspectives. For him, there is no purpose or end. He argues that the idea of purpose in nature is simply a human invention. All things including human actions, for Spinoza, are a modification of God. Spinoza is a determinist, unlike Aristotle and Aquinas. He argues that there is no such thing as human freedom or free actions.  As regards the notion of good and bad or right and wrong actions, Spinoza says that what satisfies our desires is regarded as good or right, and what frustrates our desires is bad or wrong. In otherwords, there is nothing that can be generally regarded as good or bad. These concepts depend on our emotions. This is against the view of Aristotle and Aquinas who see rightness or wrongness from a universalistic point of view, and attribute rightness or wrongness to reason. Spinoza argues in line with Aristotle that virtue is a life lived in accordance with reason, he advocated reason as the director of human activities. Unlike Aristotle and Aquinas who believe that evil occurs as a result of man being a free moral agent that makes choices, Spinoza holds that there is no evil. According to him we feel that a thing is evil because of our lack of understanding of the nature of that thing. Spinoza, in line with Aristotle and Aquinas, believes in the organization of a civil society. He observes that man is by nature selfish and if he is allowed to pursue his selfish interest without restriction, the society would be in chaos. Thus, an organized civil society is necessary. This can only be achieved through the making of laws. The concept of Natural Law, according to Spinoza, is the law of necessity by which all creatures (including man) are conditioned to behave in certain ways. It is a deterministic law by which the behaviour of all creatures (including the activities of all men) are determined by nature. Spinoza’s concept of Natural Law is therefore quite different from that of St. Thomas Aquinas for whom natural Law is the Moral Law, not a law of necessity.

                Leibniz (1646-1716) talks about the harmonious nature of the universe. This hamony he attributes to God.  This position of Leibniz could be interpreted in the light of Aquinas’ notion of the Eternal Law. In other words, Aquinas’ “Eternal Law” expresses God’s rulership over nature, and Leibniz’s harmonious nature of the universe which he attributes to God, could be seen in the same vein. Leibniz calls this world “the best of all possible worlds”. As regards the problem and source of evil, Leibniz disagrees with Aristotle and Aquinas. Whereas Aristotle attributes the source of evil to choice, Leibniz attributes it to the nature of man. For him, there is a bit of imperfection in the world because that is how God created it. If God had given the universe His nature of full perfection, then He would be equating the world to Himself.




                The moral and political philosophy of John Locke (1632-1704) has a lot in common with the Aristotelian-Thomistic theory of Natural Law. The concept of Locke’s “Law of Opinion” can be seen as carrying the same meaning with Aquinas’ concept of “the human law”. In both laws there is this idea of laws made by humans (the society) which are expected to express or conform to the Eternal Law (in Aquinas’ case) or Divine Law (in Locke’s case). Both Aquinas and Locke talk of the Divine Law and see it as the law of God which is to direct humanity towards achieving God’s purpose. According to Locke the law of opinion and the civil law should be made to conform to the divine law. This is also what Aquinas holds–that the natural law and the human law should have be expression of the Eternal Law.

                Locke in line with Aristotle and Aquinas also sees the highest good that is, what men aims at as Happiness. For him, conformity to the law of opinion, which is still an expression of the divine law, is virtue. The above views of Locke are founded on Locke’s acceptance of the existence of substance. Locke accepted metaphysics as the foundation of morality and law. But empiricism assumed an extreme posture in the hands of David Hume. Hume exalted experience and demoted reason, earlier exalted by the rationalists. According to Hume, man is a bundle of perceptions and reason is a slave of the passions. In the words of Iwe (1985:143) “This empiricistic attitude to and interpretation life refuses to perceive that the fundamental and universal principles of ethics are grounded on the essential and universal nature of man”. To guard against the dangers inherent in Hume’s empiricistic attitude to the interpretation of life, this paper emphasizes, in agreement with Iwe, that the fundamental and universal principles of ethics must be grounded on the essential and universal nature of man. David Hume’s erroneous view that “man is a bundle of perceptions” arose from Hume’s erroneous rejection of metaphysics. The logic underlying the radical rejection of metaphysics by Hume is mistaken and needs to be corrected. Hume fails to understand that reality is not limited to what can be perceived with the senses. Reality is too complex to be limited to the empiricists’ approaches to its study or means of knowing of. Hume’s criticism of metaphysics based on the empiricist’s principle, is mistaken. It restricts the concept of truth about matters of fact in the world to sense experience. According to Omoregbe (1996:130),

Hume erroneously conceived man as a purely material being, ignoring the spiritual dimension in man. He emphatically denied the existence of any spiritual element (the soul) in man. He, therefore, had only a partial conception of truth, experience and knowledge. His criticism of metaphysics derives precisely from these partial and erroneous conceptions. 


For Iroegbu (1995:179),

Hume’s lopsided view of knowledge and reality is a philosophical negativity. His proposal of vigorous sensism as an alternative to our natural and acquired scientific, metaphysical and socio-cultural deposits creates more problems than it resolves. It leaves us in the make-shift, sandy subjectivism of dry empiricism and vague association.

Hume’s errors in rejecting metaphysics will be even clearer from our following discussion of the metaphysical foundation of ethics and law.




                Ethics has to do with the norms of human behaviour. Being is the foundation of goodness; metaphysics is the foundation of ethics and an ethical proposition can validly be derived from an ontological proposition about human nature. The idea that being is the foundation of goodness is implicit in the thoughts of some philosophers in the Ancient, Medieval and Modern times. A succinct examination of the relationship between being and goodness in the thoughts of some of these philosophers is pertinent.

                Being as the foundation of goodness is true both in the non-moral sphere as well as in the moral sphere. It is being that determines goodness and is the measure of goodness. “In the non-

moral sphere a thing is said to be good if it is what it ought to be” (Omoregbe 1996:116). An artifact it defined in terms of the purpose it is meant to serve, and if it serves this purpose it is said to be good because it is what it ought to be. In the moral sphere, being remains the foundation of goodness, for actions are judged as morally good or bad in terms of their relations to the being of other people. Actions that enhance the being of other people are good, while those that adversely affect the being of other people are evil actions. It is therefore, with reference to being, that is, the being of other people that actions are judged as good or as evil, for being is the foundation of goodness. The moral imperative is ultimately the imperative to enhance being and to refrain from whatever is detrimental to being. Thus, being is the basis of morality. The ultimate goal of morality is the enhancement of one’s being, the full actualization of one’s being. This, of course, presupposes that man’s being is not yet a closed book, a finished product. Man, in other words, is a self-developing being, and morality is a process through which he carries out this task of self-development, self-enhancement and self-realization. Like Aristotle will tell us, morality is the process whereby man progressively actualizes the potentialities of his being, until he eventually attains the fullness of his being. The moral imperative, then becomes an imperative urging man to strive towards the fullness of his being.

                The foregoing explanation sheds light on the famous “is” “ought” problem raised by Hume which questions the validity of the transition from “is” propositions (propositions of fact) to “ought” propositions (propositions enjoining obligation). With the problem raised, Hume is obviously referring to ethicists who base their ethics on human nature. But if man were a finished product, the “is” “ought” dichotomy would not exist and the problem of deriving normative propositions from descriptive propositions would not arise. To say that man is incomplete amounts to saying that something is lacking in him. And to say that something is lacking in human nature is a proposition of fact (a descriptive proposition), which, however, implies a normative proposition. Such a normative proposition indicates what ought to be done to provide what is lacking. Thus if being is the foundation of goodness, then metaphysics is the foundation of ethics (Omoregbe 1996:116). That being the case, an ethical proposition can legitimately follow from an ontological proposition. 



                Nature is the model which man tries to imitate in various ways. Nature remains the standard, the ideal. In the same way positive law is an imitation of nature as it is an imitation of natural law. Legal justice is an imitation of natural justice. Now the concepts of natural law and natural justice or natural right are grounded in metaphysics. Hence Hume’s empiricism which is hostile to metaphysics is also hostile to the very idea of natural law because it is a concept grounded in metaphysics. Moreover, the concepts of justice and right imply the concept of law since there cannot be rights or justice without law. Natural rights are always granted by the natural law while natural justice is justice granted according to natural law.

                The essential features of law is obligation as against command, held by command theorists of law. This obligation which is of a moral nature, accounts for the sanctity of law and the reverence it deserves Law is inseparable from morality because it has its own inner morality as well as moral conditions, which it has to fulfill in order to be valid. To separate law from morality is to strip law naked and deprive it of its obligatory feature as well as its sacred feature, both of which come from the natural law. If the essential feature of law which gives it its sacred character is from the natural law, it follows obviously, that the natural law itself is really law in the true sense of the word. “But the natural law is grounded in metaphysics, its foundation is metaphysics” (Omoregbe 1996:109). Metaphysics is the foundation of law, without it and without the concept of natural law, the concept of law cannot be correctly understood or fully explained.



                It is obviously logical to argue that the Aristotelian-Thomistic theory of Natural Law has much in common with the philosophical postulations (especially politically and morally) of the modern era. This has only been exemplified in the comparison between the theory of Natural Law, on the one hand, and the rationalist and the empiricist philosophers, on the other hand.

                From our presentation in this paper, we have seen the logic of David Hume, the most radical empiricist, who rejected metaphysics and by extension rejected the Natural Law theory which has metaphysics as its foundation. We saw that Hume is in error. Hume argues that man is a bundle of perception. This is a great error. He sees man as a purely material being, forgetting that man is a multifaceted being. Man is not only material but also spiritual. Hume has a limited notion of truth. He sees it as only related to matters of fact and experience. But there are metaphysical truths, just as there are metaphysical experiences; spiritual truths, just as there are spiritual experiences. The premise of Hume’s criticism of metaphysics, namely that man is a bundle perceptions made up of impressions and ideas, is faulty as was demonstrated above, and the whole argument or criticism collapses with the faulty premise. An argument based on error collapses with the faulty premise once the error is made public. This paper insists that metaphysics is the basis of legal values. The erroneous rejection of metaphysics by Hume and his followers, has in law, resulted to legal positivism which rejects the natural law and by implication the natural rights of man. The natural law is a law of universal reason, which should form the basis of positive law. The natural law gives positive law its obligatory nature and its rejection is an unpardonable error.





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