Aquinas’ Natural Law


Telos & the four tiers of law

Aquinas developed Natural law as a form of religious ethics. Natural law is the theory that God has designed a moral law into human nature such that we are naturally inclined to certain moral behaviours. Ethics is therefore about using reason to discover the natural law within our nature so that we can conform our actions to it in order to fulfil our purpose (telos) of glorifying God, by following his law.

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

The four tiers of Law. The ultimate source of moral goodness and thus law is God’s nature, which is the eternal law. However, that is beyond our understanding, so we have access to lesser laws that derive from the eternal law.

Eternal Law – God’s omnibenevolent nature.
Divine law – God’s revelation to humans in the Bible.
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.

The purpose of the four tiers is to show how Aquinas thinks that human law can gain its authority by deriving from the natural and divine law which themselves ultimately derive authority from God’s nature.

Aquinas was influenced by Aristotle’s views that there is a natural end to things – everything has a purpose (telos) built into it by its nature – the final cause. Whereas Aristotle thought the final cause of all things was the prime mover, Aquinas Christianised the concept and claimed that it was God. The telos/end/goal of rational beings is the good, which is God’s eternal law. God created us and our nature to have the telos of glorifying God. We can use our God given reason to figure out the inclinations of our God-designed nature. That will then tell us how we should live, i.e. ethics.

Modern science’s rejection of final causation. Francis Bacon, called the father of empiricism, argued that formal and final causation (telos) have no place in empirical science but are instead matters for metaphysics. He thought purpose was a divine matter. Regarding form, he gave the illustration of the ‘whiteness’ of snow and explained how science could investigate how snow results from air and water, but this only tells us about its efficient cause, not the form of ‘whiteness’, which is thus not a scientific matter.

Modern physics goes even further than Bacon in its rejection of final causation. A deterministic universe operating by the laws of physics seems to be completely without purpose. All supposed telos of an object can be reduced to non-teleological concepts regarding the material structure of an object. This suggests there is no basis for grounding telos in God like Aquinas did, or in grounding it as a required explanation of change in objects like Aristotle did. Modern science can explain the change and apparent purpose in the world without telos.

Science cannot rule out something like a prime mover or God which could provide telos, however.

Nonetheless, at the very least the current scientific understanding of the universe works without the need for any kind of telos. A century after Bacon, Laplace wrote a book on the workings of the universe, claiming to have ‘no need’ of the hypothesis that there is a God. More recently, Stephen Hawking made the same claim.

The Primary Precepts & Synderesis

Aquinas claims that reason is a power of the human soul and synderesis is the habit or ability of reason to discover foundational ‘first principles’ of God’s natural moral law which gives us insight into God’s intentions for human life and thus our telos.

“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.”

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.

In addition to this, synderesis tells us 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.

Arguably Aquinas is too optimistic about human nature. If you consider the terrible things that humans have done and that entire cultures have embraced, e.g. slavery and Nazism, it starts to look like human nature is not as positive as Aquinas thought. If we really had an orientation towards the good and the primary precepts accurately described our nature’s orientation, then we should not expect to find the extent of human evil we do.

However, 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.

The fact that humanity could sink so low as to produce Nazism and the holocaust is strong evidence against human nature having an orientation toward the good.

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.” 

The primary precepts are applied to situations or types of actions – a process called conscientia. 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.

Real vs apparent goods

Aquinas’ natural law ethics relies on human reason to understand and apply the natural law. Yet, human reason is fallible. It is unavoidable that sometimes our reasoning about what is morally right will err.

We might reason that something is in accord with our nature’s goal and is thus good, when really is not. A common cause of our reasoning being faulty is that we can be misled by the temptation of short-term pleasures into being blind to an actions’ long-term negative consequences for achieving our telos. For example, adultery might seem like a good idea at first if one is focused only on the short-term pleasure, yet the consequences for raising children which is our real goal might be put at risk. Such actions are called apparent goods because they only appear good to someone engaged in faulty reasoning. They are not real goods.

Aquinas’ Natural Theology

Aquinas believed that human reason could never know or understand God. However, Aquinas is a proponent of natural theology through reason which he claimed could support faith in God. Human reason can gain knowledge of God’s natural moral law through the ability of human reason to know the synderesis rule and primary precepts.

Karl Barth argued that Aquinas’ natural law theory was a false natural theology which placed a dangerous overreliance on human reason. Barth argued that if humans were able to know God or God’s morality through their own efforts, then revelation would be unnecessary. Yet, God clearly thought revelation necessary as he sent Jesus.

Barth also argued that “the finite has no capacity for the infinite”; our finite minds cannot grasp God’s infinite being. Whatever humans discover through reason is therefore not divine so to think it is must then amount to idolatry – the worship of earthly things. Barth argued idolatry can lead to worship of nations and then even to movements like the Nazis. It follows for Barth that after the corruption of the fall, human reason cannot reach God or figure out right and wrong by itself. Only faith in God’s revelation in the bible is valid.

In defence of Aquinas, he is not suggesting that our finite minds can understand God’s nature or goodness (eternal law). Aquinas is only suggesting that reason can understand the natural law God created within our nature. If reason only has this goal of supporting faith in such ways, then it cannot make revealed theology unnecessary.

 Tillich defends Aquinas to a degree, arguing that Barth was too negative in denying the possibility of reason discovering anything whatsoever of the natural law.

“there is self-deception in every denial of the natural moral law … The very statement that man is estranged from his created nature presupposes an experience of the abyss between what he essentially is and what is existentially is. Even a weak or misled conscience is still a conscience, namely, the silent voice of man’s own essential nature, judging his actual being” – Tillich.

To deny that our conscience can discover the natural law is to claim that there is a gap between what we currently are and what we could be. Yet, to have an awareness of that gap is to have a conscience that is aware of its fallen state. So it is contradictory to deny the natural law. Even it now involves a weakened conscience, that still tells us at least something of the direction we have fallen in and the direction back towards righteousness.

However, whatever a weak and misled conscience discovers is surely not God’s morality. Humanity believing it has the ability to know anything of God is the same arrogance that caused 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. The arrogance of natural theology is evidence of a human inability to be humble enough to simply have faith.

Fletcher’s critique of Aquinas

Aquinas was not as aware of different cultures as we are today. As we now know thanks to modern anthropology, there are vastly different moral beliefs across cultures; this is called descriptive moral relativism. Fletcher made the argument that this could be taken as evidence that there is not an innate God-given ability of reason to discover the natural law, since then we should expect more moral agreement. Freud would argue that it is society which conditions our moral views. There either is no natural moral law or human reason is unable to discover it. So, what Aquinas thought was human nature was really just his culture.

However, there do seem to be some core similarities between the moralities of different cultures such as not killing for no reason and rules about stealing. 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 could be taken to show that moral views are determined by a universal human moral nature.

Alternative explanation: These cross-cultural similarities in moral codes might have resulted from a biologically evolved morality rather than one designed by a God, however, which would mean it is not related to a telos designed by God.

Alternatively, 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 and not or not only evolution.

The double effect

Aquinas argued that a single action can have two effects, one in accordance with the primary precepts and one in violation of them. Being a good person involves developing the kind of virtuous character which acts with the intention of following God’s natural law. Aquinas first introduced this as a way to justify killing in self-defence. Aquinas insists there are two effects; the saving of a life and the killing of a life. He claims that in such cases it can be that one effect is intended while the other effect is “beside the intention”. So, Aquinas thinks 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.

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.

Natural law is different to the Bible though. 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.

Grenade example. Arguably the double effect does not accurately capture why we find certain rule-breaking acceptable. Consider the example of a soldier throwing himself on a grenade to save his fellow soldiers. This seems to be a heroic act however it’s hard for the double effect to make sense of it, because the double effect would require the bad effect of suicide to be foreseen but besides the intention of the heroic soldier. However, that seems inaccurate because it is part of the heroism of the act that the soldier intends to sacrifice their own life. Natural law, as can be seen by the secondary precepts and the double effect, is trying to introduce flexibility into Christian ethics, but arguably it fails to introduce enough flexibility as this case shows. This prompts some to go further and accept situation ethics.

The role of virtue in Aquinas’ Natural law ethics

Aquinas thought it was important to be a virtuous person as that will secure a firmer foundation for ethical action. He was influenced by Aristotle’s account of virtue, which envisaged humans are creatures of habit such that being virtuous involves having a character which cultivates virtuous habits.

The cardinal virtues are the virtues humans have the ability to cultivate.

  • Prudence involves the practical wisdom to understand how to fit general moral principles to particular situations so as to achieve our proper end/telos. Without this virtue we will be liable to make cognitive errors when reasoning through our moral judgements.
  • Temperance involves having a moderating and controlled attitude towards bodily pleasures and other emotional desires like anger, vanity and vengeance.
  • Fortitude/Courage involves having the strength of mind required to overcome our fears when they stand in the way of achieving our good.
  • Justice involves the willingness to give people what they deserve.

The Christian theological virtues are granted to humans by God’s grace.

  • Faith is the virtue of belief in God and his revelation.
  • Hope is the virtue of the hope of heaven.
  • Charity/Love is the virtue of agape; selfless Christian love of your neighbour.

The issue of the virtue and salvation of non-Christians. Augustine, Aquinas & Pelagius on virtue and salvation. Augustine’s view of original sin and grace commits him to the view that non-Christian philosophers cannot be truly virtuous since their actions are not directed towards the goodness of the Christian God. God is the ultimate goal of all good human action and the source of all good in human life. Ultimately, all non-Christian action, no matter how seemingly good, fails to have this essential motive, only possible through the love and faith that is gifted to some people by grace.

However, it is difficult to believe that non-Christian philosophers never displayed true virtue. For example, when Socrates chose to die for his beliefs rather than be exiled, it’s difficult to argue that he wasn’t displaying the virtue of courage. Pelagius makes this argument. He points out that there were good people that did good actions in the old testament, such as Noah and Lot, who existed before Jesus came and dispensed God’s grace. Pelagius also points to the example of ancient ‘pagan philosophers’ who were:

“lovers of justice no less than knowledge”. “When, I ask you, do these good qualities pleasing to God come to men who are strangers to him … unless it be from the good of nature”.

“Scripture itself is the work of human minds, recording the example and teaching of Jesus Thus it is not what you believe that matters; it is how you respond with your heart and your actions. It is not believing in Christ that matters; it is becoming like him.”

Aristotle’s works on ethics were not fully available until Aquinas’ time, long after Augustine. They contained undeniably exceptional analysis of how virtue can be learned through education and cultivated through habit. Augustine claimed that humans can only be virtuous if they are infused by God. Aquinas claims that Augustine is correct –  but only about the Christian virtues of faith, hope and love. The cardinal virtues of prudence, justice, temperance and courage can be acquired by developing our natural capabilities without intervening help from God. These cardinal virtues are not sufficient for salvation, however. So, Aquinas does not go as Pelagius seemed to – that non-Christians who were virtuous could merit salvation.

Those who follow Augustine would claim that Aquinas underestimates original sin. For many Christians Aquinas’ natural law is a middle ground between the extremes of Augustine and Pelagius. Others would argue that Pelagius is truer to the implications of an omnibenevolent God than Aquinas, however.

Interior & exterior acts

Aquinas draws a distinction between the action itself, which is an exterior act because it occurs outside of our mind, and our intention which is the interior act because it occurs inside our mind.

The point of natural law ethics is for us to figure out what fulfils the telos of our human nature and then act on it. 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 was not truly morally good.

Application of Aquinas’ natural law to abortion and voluntary euthanasia


  • Abortion goes against the primary precepts to preserve human life and reproduction.
  • Abortion also goes against the divine law (thou shalt not kill)
  • Abortion is also argued to go against the stability of society by some because, as mother Theresa put it: ‘if a mother can kill her own child in her own womb, what is left to stop us from killing one another?’.
  • Some Catholics argue that abortion could sometimes be justified by the double effect however, in cases where the mother’s life is threatened by a medical emergency. As long as the intention is to save the life of the mother, killing the baby can be justified.
  • The normal reasons for allowing abortion, that the parents cannot afford a child or even that the pregnancy was the result of rape, are not relevant to natural law theory.

Voluntary euthanasia

  • Voluntary euthanasia goes against the primary precepts to preserve human life.
  • Voluntary euthanasia also goes against the divine law (thou shalt not kill)
  • Voluntary euthanasia is argued to go against the stability of society because of the slippery slope argument – that if we allow it in cases like terminal illness, we will soon allow it for incurable physical conditions, then incurable mental conditions, then babies with terrible medical conditions, then homeless people, then depressed people, then minorities and eventually we will basically be Hitler.
  • However, some Catholics argue that voluntary euthanasia could be justified if it is passive, i.e. does not involve an action that kills the person but merely involves the withdrawal or suspension of medical support – if continued medical treatment would be burdensome or disproportionate.

Evaluation of Natural law and application of Natural law

  • Natural law is inflexible and leads to unnecessary suffering. For example, it would not allow someone to get an abortion even if they had been raped, nor would they allow euthanasia even if someone had a terminal illness and was going to die soon anyway after extreme pain.
  • However, Aquinas would argue that our purpose is not to avoid all pain and only do what we want – our purpose is to follow God’s natural law in order to have the right relationship with God and get into heaven.
  • Fletcher argued that there is no natural law, or that our minds are unable to know it – because if human reason really were able to discover the primary precepts, how come different cultures have such different moral views? E.g. Some allow abortion and euthanasia and others do not.
  • However, the cultural differences could be due to original sin, lack of virtue or corrupt culture. There is still a core set of moral views that is universal to all cultures – laws against killing & disorder like stealing, encouraging of procreation and education. The primary precepts seem universal.
  • Protestants often argue that we should not use human reason to try and understand anything about God, including God’s morality, because our reason is corrupted by original sin and thus unreliable. We should only have faith in the Bible to get our moral views. For example, the double effect can be used to justify abortion and passive voluntary euthanasia which they would argue goes against the Bible and is an example of arrogant human reason thinking it knows better than God.
  • However, Aquinas would respond that our reason is God-given and designed by God to be able to know the primary precepts. God would not have given us this ability to know the natural law unless God intended us to use it. So, we should.