Natural Law


This page: full notes      A* summary notes      C/B summary notes


Natural law ethics goes back to Aristotle and his theory of telos; that everything has a nature which directs it towards a particular end goal. Aquinas Christianised this idea, adding that it is the Christian God who set a thing’s telos according to his omnibenevolent plan for the universe.

Christian ethics is most associated with the commands and precepts found in the Bible. Aquinas’ contribution was to argue that telos is also a source of Christian moral principles. Human nature has the God given ability to reason which comes with the ability both to intuitively know primary moral precepts and to apply them to moral situations and actions. Following this ‘natural law’ is thus also an essential element of living a moral life.


Aquinas was influenced by Aristotle’s views that there is a natural end to all beings. Everything has a purpose (telos) built into it by its nature. The nature of a thing determines the behaviours that are ‘natural’ to it. An acorn naturally grows into an oak tree, because of its inherent nature. Whereas Aristotle thought the final cause of all things was the prime mover, Aquinas claimed that it was the Christian God. The telos/end/goal of rational beings is the goodness of God, which for us involves glorifying God by following God’s moral law.

Ethics is therefore about using reason to discover the natural law within our nature and conforming our actions to it. God designed the universe to operate according to his divine plan by instilling telos in every being, to direct it towards its good end. Human beings are unique in that we have free will and are thus capable of either following or rebelling against the divine plan. Following God’s natural law results in flourishing (eudaimonia) both for individuals and society. Disobeying what is naturally good for us has the opposite effect.

“the light of reason is placed by nature in every man, to guide him in his acts towards his end”. – Aquinas.

The four tiers of law

The ultimate source of moral goodness and thus law is God’s omnibenevolent nature, which created and ordered the universe with a divine plan, known as the eternal law. However, that is beyond our understanding. We only have access to lesser laws that derive from the eternal law.

The eternal Law. God’s plan, built into the nature of everything which exists, according to his omnibenevolent nature.

The divine law – God’s revelation to humans in the Bible.

The natural law – The moral law God created in human nature, discoverable by human reason. 

Human law – The laws humans make which should be based on the natural and divine law. Human law gains its authority by deriving from the natural and divine law which themselves ultimately derive authority from God’s nature.

“Participation of the eternal law in the rational creature is called the natural law”. – Aquinas

The Primary Precepts & Synderesis

Reason is a power of the human soul. Synderesis is the habit or ability of reason to discover foundational ‘first principles’ of God’s natural moral law.

“the first practical principles … [belong to] a special natural habit … which we call “synderesis” … is said to incite to good, and to murmur at evil, inasmuch as through first principles we proceed to discover, and judge of what we have discovered.” – Aquinas

The first principle synderesis tells us is called the synderesis rule: that the good is what all things seek as their end/goal (telos). This means that human nature has an innate orientation to the good.

“This therefore is the principle of law: that good must be done and evil avoided. – Aquinas

Further to this, through synderesis we learn the primary precepts: worship God, live in an orderly society, reproduce, educate, protect and preserve human life and defend the innocent. These primary precepts are the articulation of the orientations in our nature toward the good; the natural inclinations of our God-designed human nature, put into the form of ethical principles by human reason. Simply having reason allows a being to intuitively know these precepts. We are all born with the ability to know them.

Secondary precepts & conscientia

“there belongs to the natural law, first, certain most general precepts, that are known to all; and secondly, certain secondary and more detailed precepts, which are, as it were, conclusions following closely from first principles.” – Aquinas.

Conscientia is the ability of reason to apply he primary precepts to situations or types of actions. The judgement we then acquire is a secondary precept. E.g euthanasia: the primary precepts don’t say anything about euthanasia exactly, but we can use our reason to apply the primary precepts to euthanasia, and realise that it goes against the primary precept of protecting and preserving human life. Arguably it even disrupts the functioning of society too. Therefore, we can conclude the secondary precept that euthanasia is wrong.

Interior & exterior acts

A physical action itself is an exterior act because it occurs outside of our mind. Our intention; what we deliberately choose to do, is the interior act because it occurs inside our mind.

The point of natural law ethics is to figure out what fulfils the telos of our nature and act on that. By doing so, we glorify God. This cannot be done without intending to do it. A good exterior act without a good interior act does not glorify God because it is not done with the intention of fulfilling the God-given goal/telos of our nature.

The act of giving money to charity is an example of a good exterior act, but is only morally good when combined with the right kind of intention, which would be an interior act. If the intention was only to be thought of as a good person, which is not the right kind of intention, then the action is not truly morally good.

Whether telos exists

It is a strength of telos-based ethics that they are empirical, i.e., based on evidence. Aristotle observed that everything has a nature which inclines it towards a certain goal which he and Aquinas called its telos. It is a biological fact that certain behaviours cause an organism to flourish. Telos thus seems an empirically valid concept.

Weakness: Modern science’s rejection of final causation. Francis Bacon, called the father of empiricism, argued that only material and efficient causation were valid scientific concepts, not formal and final causation. The idea of telos is unscientific.

Aquinas and Aristotle claim every being has a unique essence which gives it a particular end/purpose. The issue is, modern science tells us that things are merely atoms moving in fields of force – i.e., material and efficient causation. The idea that entities have an ‘essence’ and thus a telos is unscientific. Physicist Sean Carroll concludes that purpose is not built into the “architecture” of the universe.

All supposed telos of a thing can be reduced to non-teleological concepts regarding its material structure and forces operating on it (material & efficient causation). There is no basis for grounding telos in God like Aquinas did, or as a required explanation of change like Aristotle did. For example, Aristotle would regard the telos of a seed as growing into a tree/bush. However, we now understand that change as resulting from the seed’s material structure which was itself caused by evolution, not anything like telos. Similarly, if there is anything in human nature which orients us towards certain behaviours, it is only because evolution programmed them into us because they happened to enable survival in our environment, not because of telos. So, Modern science can explain the world without telos. Telos is an unnecessary explanation.

Evaluation defending telos:

Polkinghorne, a modern Christian philosopher and physicist, argued that science is limited and cannot answer all questions. It can tell us the what but not the why. Science can tell us what the universe is like, but it cannot tell us why it is this way, nor why it exists. It cannot answer questions about purpose.

Polkinghorne’s argument is successful because science is limited. It cannot rule out something like a prime mover or God which could provide some kind of telos. If purpose existed, science would not be able to discover it. So, science cannot be used to dismiss the existence of purpose.

Evaluation critiquing telos:

Dawkins responds that it’s not valid to simply assume that there actually is a ‘why’. He makes an analogy: ‘what is the color of jealousy?’ That question is assuming that jealousy has a color. Similarly, just because we can ask why we and the universe exist, that doesn’t mean there actually is a purpose for it.

Dawkins’ argument is successful because it makes use of the burden of proof. Those who claim purpose exists have the burden of providing a reason to think it exists. There is no scientific basis for thinking anything other than material and efficient causation exists. Furthermore, scientists may one day actually explain ‘why’ the universe exists, but even if they don’t, that doesn’t justify a non-scientific explanation of purpose such as telos.

Universal human nature & moral dis/agreement

A strength of Natural law is that it is based on universal human nature. The primary precepts are found in the morality of all societies. For example, not killing for no reason and rules about stealing are universal. Valuing reproduction and education are also universal. Moral thinkers from different cultures came up with similar moral prescriptions such as the golden rule; to treat others as you would like to be treated, which can be found in ancient Chinese Philosophy, Hinduism, Judaism and Christianity. This suggests that moral views are influenced by a universal human moral nature. This is good evidence that we are all born with a moral orientation towards the good (telos), which is the foundation of Aquinas’ theory.

Weakness: If all humans were really born with the ability to know the primary precepts, we should expect to find more moral agreement. In fact, we find vastly different moral beliefs. Furthermore, the disagreement is not random tends to fall along cultural lines. This suggests that it is actually social conditioning which causes our moral views, not a supposed natural law in human nature. This has been argued by psychologists like Freud and Skinner. Fletcher argues this shows there is not an innate God-given ability of reason to discover a natural law. He concludes that ethics must be based on faith, not reason (Fletcher’s positivism).

Evaluation defending Aquinas:

Aquinas’ claim is merely that human nature contains an orientation towards the good, it doesn’t involve a commitment to humans actually doing more good than evil, nor to incredibly evil acts or cultures occurring infrequently. Aquinas acknowledges that there are many reasons we might fail to do good despite having an orientation towards it. These include original sin, mistakes in conscientia, lacking virtue and a corrupt culture. So, the fact that there is a core set of moral views found cross-culturally shows his theory is correct. 

Evaluation critiquing Aquinas:

Furthermore, cross-cultural morality might result merely from the basic requirement of a society to function. If anyone could kill or steal from anyone else for no reason whenever they wanted, it’s hard to see how a society could exist. That might create an existential pressure which influences the moral thinkers of a society, yielding prescriptions such as the golden rule. Cross-cultural ethics therefore has a practical reality as its basis, not God.

Alternatively, some of the cross-cultural similarities in moral codes might also have resulted from a biologically evolved moral sense rather than one designed by a God, which would mean they are not related to morality or telos at all.

Aquinas’ Natural theology vs Augustine & Karl Barth

A strength of Aquinas’ ethics is its basis in what seems like a realistic and balanced view of human nature as containing both good (reason & telos) but also bad (original sin). Natural law adds an engagement with autonomy to Christian ethics. Sola scriptura protestants like Calvin regard humans as mere passive receptacles for a set of biblical commands. However, Aquinas argues that God presumably gave humans reason so that they may use it.

Natural theology is the view that human reason is capable of knowing God, in this case God’s moral law. Aquinas defends this by first accepting that original sin destroyed original righteousness, meaning perfect rational self-control. However, it did not destroy our reason itself and its accompanying telos inclining us towards the good.

Only rational beings can sin. It makes no sense to say that animals could sin. Original sin made us sinners, but human nature was not reduced to the level of animals. We still have the ability to reason. Furthermore, Aquinas diverges from Augustine, claiming that concupiscence can sometimes be natural to humans, in those cases where our passions are governed by our reason. So, a comprehensive approach to Christian morality must include the use of reason to discover and act on the telos of our nature.

Weakness: Natural theology places a dangerous overreliance on human reason. Karl Barth was influenced by Augustine, who claimed that after the Fall our ability to reason become corrupted by original sin.   Barth’s argument is that is therefore dangerous to rely on human reason to know anything of God, including God’s morality.   “the finite has no capacity for the infinite” – Karl Barth.   Our finite minds cannot grasp God’s infinite being. Whatever humans discover through reason is not divine, so to think it is divine is idolatry – believing earthly things are God. Idolatry can lead to worship of nations and even to movements like the Nazis. After the corruption of the fall, human reason cannot reach God or God’s morality. That is not our telos. Only faith in God’s revelation in the bible is valid.

Final judgement defending Aquinas: Barth’s argument fails because it does not address Aquinas’ point that our reason is not always corrupted and original sin has not destroyed our natural orientation towards the good. Original sin can at most diminish our inclination towards goodness by creating a habit of acting against it. Sometimes, with God’s grace, our reason can discover knowledge of God’s existence and natural moral law. So, natural moral law and natural theology is valid.

Arguably Aquinas has a balanced and realistic view, that our nature contains both good and bad and it is up to us to choose rightly.

Final judgement critiquing Aquinas: Barth still seems correct that being corrupted by original sin makes our reasoning about God’s existence and morality also corrupted. Even if there is a natural law, we are unable to discover it reliably. The bad in our nature unfortunately means we cannot rely on the good. Whatever a weak and misled conscience discovers is too unreliable.

Humanity’s belief that it has the ability to know anything of God is the same arrogance that led Adam and Eve to disobey God. Humanity believing that it has the power to figure out right and wrong is what led to the arrogant certainty of the Nazis in their own superiority. This arrogance of natural theology is evidence of a human inability to be humble enough to solely rely on faith.

Whether Religious & Natural law ethics is outdated

A strength of Natural law ethics is its availability to everyone because all humans are born with the ability to know and apply the primary precepts. Regarding those who do not belong to Abrahamic religion the Bible says:   “Gentiles, who do not have the law, do by nature what the law requires … God’s law is written in their hearts, for their own conscience and thoughts either accuse them or tell them they are doing right” (Romans 2:14-15).   So, it is possible to follow the natural law even if you are not Christian and/or have no access to the divine law (Bible).

Weakness: Secularists often argue that biblical morality (divine law) is primitive and barbarous, showing it comes from ancient human minds, not God. J. S. Mill calls the Old Testament “Barbarous, and intended only for a barbarous people”. Freud similarly argued that religious morality reflected the “ignorant childhood days of the human race”.   Aquinas’ Natural law ethics is criticised as outdated for the same reason. Medieval society was more chaotic. Strict absolutist ethical principles were needed to prevent society from falling apart. This could explain the primary precepts. For example, it was once useful to restrict sexual behaviour to marriage, because of how economically fatal single motherhood tended to be. It was useful to simply ban all killing, because killing was much more common. It was useful to require having lots of children, because most children died.   The issue clearly is that all of these socio-economic conditions have changed. So, the primary precepts are no longer useful. Society can now afford to gradually relax the inflexibility of its rules without social order being threatened.

Evaluation defending Aquinas:

Conservative Catholics often argue that natural law is not outdated because it serves an important function without which society flourishes less. They argue that secular liberal western culture is ethically retrograde because of its abandonment of traditional moral principles like the primary precepts. This shows that we really do need to follow God’s natural law in order to flourish.

Marriages are fewer and less successful. Mental illness increases. Rates of etcetc

People are no longer united by an ethic of devoting our lives to something greater than ourselves. Self-interest and materialistic consumerism is all modern society has to offer by way of meaning and purpose.

“[excluding] God, religion and virtue from public life leads ultimately to a truncated vision of man and of society and thus to a ‘reductive vision of the person and his destiny’”. – Pope Benedict XIV.

Here, Benedict XVI references an encyclical called “Caritas in Veritate”, where he argued that while there is indeed religious fanaticism which runs against religious freedom, the promotion of atheism can deprive people of “spiritual and human resources”. The atheist worldview is that we are a “lost atom in a random universe”, in which case we can grow and evolve, but not really develop morally.

“ideological rejection of God and an atheism of indifference, oblivious to the Creator and at risk of becoming equally oblivious to human values, constitute some of the chief obstacles to development today. A humanism which excludes God is an inhuman humanism. Only a humanism open to the Absolute can guide us in the promotion and building of forms of social and civic life — structures, institutions, culture and ethos — without exposing us to the risk of becoming ensnared by the fashions of the moment.” – Pope Benedict XIV.

So, religious and natural law ethics is not outdated but is a vital societal anchor for morality, meaning and purpose.

Evaluation critiquing Aquinas:

Natural law ethics is outdated because Aquinas’ theory was actually a reaction to his socio-economic context and since that has changed, Natural law is no longer relevant.

Aquinas thought that he discovered the primary precepts through human reason, as God designed. However, it’s a simpler explanation that Aquinas was simply intuiting what was good for people in his socio-economic condition. The idea that the resulting principles actually came from God was only in his imagination.

The great strength of religion as a form of social organisation is also its greatest weakness. By telling people that its ethical precepts (such as the primary precepts or sanctity of life) come from God it creates a strong motivation to follow them. Yet, because those precepts are imagined to come from an eternal being, they become inflexible and painstakingly difficult to progress. This makes them increasingly outdated.

The double effect

A single action can have two effects, one in accordance with the primary precepts and one in violation of them. Aquinas claims that such actions can be justified the good effect is intended while the bad effect is “beside the intention”. This is because being a good person involves developing the kind of virtuous character which acts with the intention of following God’s natural law.

Aquinas illustrated this with killing in self-defence. There are two effects; the saving of a life and the killing of a life. Killing someone, which clearly violates the primary precept of preserving human life, can be justified so long as it is an effect which is a secondary effect beside the intention of an action whose other effect was intended and was in accordance with the primary precepts.

There are four generally accepted conditions in modern Catholicism for an action to be justified by the double effect:

The intentionality condition. The good effect must be intended and the bad effect must be ‘besides the intention’. Aquinas illustrated the double effect with the example of killing someone in self-defence. So long as you intended to save your own life, then it is morally permissible to kill someone in self-defence. The bad effect is ‘besides the intention’. 

The proportionality condition. The good effect must be at least equivalent to the bad effect. Saving your life is equivalent to ending the life of the attacker. You can’t use more force than is necessary to save your life – there must be proportionality there too. 

The means-end condition. The bad effect and the good effect must both be brought about immediately – at the same time. Otherwise, the person would be using a bad effect as a means to bring about a good effect – which is not permissible.

The double effect only applies to actions which have two effects – one good, one bad – where both effects are brought about immediately.

The nature of the act condition. The action must be either morally good, indifferent or neutral. Acts such as lying or killing an innocent person can never be justifiable. An attacker would not count as an innocent person.

Whether the double effect is unbiblical

A strength of the double effect is that it helps to resolve seemingly disparate biblical themes. Jesus’ commands were not merely about following certain rules, but also about having the right moral intention and virtue (E.g. sermon on the mount). The double effect provides important clarity to Christian ethics by showing the relation between the important moral elements of intention and following the moral law. Good intention is important, not to the degree of justifying pure violations of the law, but when involved in an action that has a good effect it can justify permitting a bad side effect. 

Weakness: the double effect is unbiblical. Some theologians reject the double effect as unbiblical because God’s commandments are presented as absolute and not dependent on someone’s intention. For those theologians, the distinction between intended effects of actions and merely foreseen effects “beside” the intention has no morally relevant significance. It’s not that intention has no relevance in traditional Christian ethics. Most theologians accept that people are not immoral for consequences of their actions which they could not have foreseen which violate God’s commands. For example if you decide to drive your car at the time a drunk person happened to be out and you ran them over, that would not be considered your fault even through it was an effect of your action. However if you could foresee a bad consequence, the fact that it was a secondary effect beside the effect you did intend doesn’t justify it for theologians who take this view.

Evaluation defending Natural law:

This criticism is unsuccessful because Natural law is different to the Bible. The Bible might be inflexible, but that is the divine law. The natural law in our nature is more flexible because it is in the form of very general precepts which require application and the telos of the natural law is glorifying God, which requires that it be our intention to glorify God – thus showing how intention is relevant.

Evaluation criticising Natural law:

This weakness is successful because it shows natural law is trying to add flexibility to inflexible biblical law – e.g. thou shalt not kill. Self-defence, passive euthanasia, even perhaps abortion could be justified by the double effect. The natural and divine law do not cover separate areas but cross-over and therefore conflict on this point of inflexibility. Christians must choose the Bible over Natural law.

Proportionalism & the double effect

A strength of Natural law is its flexibility due to the doctrine of the double effect. This has been used by modern Catholics to allow, for example, passive euthanasia, abortion to save the life of the mother (though this is complex and controversial), and contraception to prevent the spread of AIDS.

Weakness: B. Hoose developed proportionalism and argues it is stronger then traditional natural law because it is more ethically coherent and introduces even greater flexibility. The only valid condition in the double effect is proportionality. The nature of the act and means-end condition are invalid. A morally right action cannot be composed of some morally evil parts and some morally good parts. Their argument is that moral evil is moral evil, it could never be a component of moral goodness.

Morally right action a composite. It is composed of whether it follows the precepts of natural law and if not, whether there was a proportionate reason for doing it which was intended. A proportionate reason is involves the intention to bring about greater value than disvalue.

The result is a form of natural law ethics which is far more flexible. Euthanasia, abortion, genetic engineering, anything natural law said to be wrong could in principle be right depending on whether there is a proportionate reason for them in a particular situation.

Evaluation defending Natural law:

John Paul II defends Natural law ethics, arguing that proportionalism is not a valid development because it misunderstands the objective/intention required for ethical action.

“Acting is morally good when the choices of freedom are in conformity with man’s true good and thus express the voluntary ordering of the person towards his ultimate end” – John Paul II

Under natural law, we intentionally act on the moral law discovered in our nature by reason. John Paul II argues that Proportionalism misdirects our goal/intention towards the balance of ontic goods over evils produced by our action.

“The morality of the human act depends primarily and fundamentally on the “object” rationally chosen by the deliberate will, as is borne out by the insightful analysis, still valid today, made by Saint Thomas.” – John Paul II.

Evaluation criticising Natural law:

Defenders of traditional Natural law like John Paul II assume that our ultimate end is simply to follow the precepts of natural law in a ridged deontological way.

Calculating the ontic goods over evils of our actions could actually be part of our ultimate end.

Even Aquinas accepted that his list of primary precepts was not final but could be added to. The project of understanding the telos of our nature is ongoing. Developments like those of proportionalism cannot be dismissed simply because they differ with the traditional approach.

Possible exam questions for Natural law 

Does natural law provide a helpful method of moral decision-making?
Assess Aquinas’s natural law ethics.

Can judging something as right or wrong be based on whether it achieves its telos?
Does human nature have an orientation towards the good?
‘Ethics can be derived from human nature’ – How far do you agree?
Assess Aquinas’ claim that there is a tier of natural law between human and divine.
How ethical are the primary precepts?
Is there a moral law of God within human nature that is discoverable by reason?”
Evaluate Aquinas’ view that human law should be related to the natural law.

Is the universe designed with a telos?
How ethically valid is the doctrine of the double effect?
Does the doctrine of the double effect justify actions like killing in self-defence?
If human nature is sinful, can natural law theory work?
Analyse Aquinas’ four tiers of law.
“The eternal, divine and human laws are the only valid laws” – Discuss.
Critically assess Aquinas’ religious development of Aristotle’s concept of telos.

Suggested extra reading and media links

Youtube playlist for this topic:

Suggested books for this topic:

Suggested pages on other websites for this topic:

Quick links

Year 12 ethics topics:
Natural Law.
Situation ethics. Kantian ethics. Utilitarianism.
Euthanasia. Business ethics. 

Year 13 ethics topics:
Conscience. Sexual ethics. 

OCR Philosophy
OCR Christianity
OCR essay structure
OCR list of possible exam questions