Thomas Aquinas Natural Law Essays

Natural Law is an absolutist theory because it doesn’t vary its primary precepts with circumstances. Natural law is a mixture of teleological and deontological because it has primary precepts which are to do with duty, and secondary which apply to circumstances.

Thomas Aquinas based Natural Law on Aristotle’s teaching about causality. In Aristotle Final cause and purpose are important when trying to give an explanation of a thing. Eg. the final cause of a knife is to cut. Aristotle thought this is what made a good knife. Something is good inasmuch as it fulfills its purpose. (The most important cause is the final cause which when achieved by an object it reaches perfection – because it has moved from potentiality to actuality eg. a potential A grade student becomes an actual one through application of hard work. )

The contrast with other senses of the word good can be brought out if we consider that a good knife can be used to perform a bad deed – ie. to stab a person. However, if it cut cleanly it would be good in the sense of doing what it was made for. This use of the word good is taken up in Aquinas and used in his theory.

What is clear for a knife is not so clear for humans – what is our purpose? Ultimately, God Himself is the final purpose of human beings – our goals are not merely temporal, but eternal, because we have an immortal soul. However, we also have temporal purposes, which could be summarised as to live and flourish in certain ways discoverable by reason.

Thomas Aquinas believed that Natural Law was part of a hierarchy of laws that are part of the cosmos created by God. God created everything via the Eternal Law. As God is the ultimate cause of all being, he has the highest qualities in respect of his creation, therefore it follows that he is the ultimate lawgiver. Through the Eternal Law God creatively directs all beings to a common end by endowing them with spontaneous natural inclinations to move toward their own perfection.

Aquinas thought that humans have an essential rational nature established by God. We are to apply reason to know our purpose and that allows us to choose to follow our final goal. Thus, unlike non-rational animals who just follow natural inclination unknowingly towards their good, humans can freely and knowingly co-operate with the eternal law through the use of reason. These laws are discoverable by humans to give the goods appropriate to humans such as food, shelter, knowledge, friendship etc. This discovering ‘from within’ of the eternal law by reason is called natural law – not something extra to the eternal law. Aquinas says: “the natural law is nothing else than the rational creature’s participation of the eternal law”. 

Aristotle’s idea of eudaimonia is useful here. Seeking happiness and all-round well-being – all aim to find this in life and it enables us to thrive. Aquinas thought however, that there was a difference between apparent goods and actual goods. Apparent goods are things which we desire and think will be good, but which actually take us away from our final purpose – eg. drink, drugs, gambling and so on.

Aquinas makes the presumption that people will choose good and avoid evil. This inclination is called synderesis, which actually has the sense of an intuitive grasp of first principles. What this means is that, to Aquinas, certain basic precepts are self-evident to anyone with a functioning reason and some experience of the world. For instance, ‘bodily health is a good to be sought after, and bodily illness to be avoided’.

(Then explain the five primary precepts and the secondary precepts and you’re done)


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Thomas Aquinas's Aristotelian interpretation of natural law has shaped western law and politics, although it is a minor section in the Summa Theologiae (ST II.I.94). It belongs within a comprehensive account of four levels of law (ST II.I.90-104). Eternal law is incomprehensible to us, because it is the order upon which all other order depends. We cannot think outside the laws we think with. Divine law is revealed in scripture and is meaningful only to those who accept scriptural authority. Natural law is what we have in common. It refers to our rational capacity to discern general principles in the order of nature to enable us to flourish as a species in communities, given that by nature we are social animals. Today, we might say that it is in our DNA.

Human law is the interpretation of natural law in different contexts (ST II.I.95-97). Like Aristotle, Aquinas believed that just laws relate to the species, so the collective good comes before the individual good – although in a just society, these are not in conflict. This means that law is not about individual morality, and individual vices should only be legislated against when they threaten harm to others. Unlike Aristotle, Aquinas believed that an informed conscience takes precedence over law. No individual should obey a law that he or she believes to be unjust, because laws that violate reason are not laws. Moreover, laws must have sufficient flexibility to be waived when necessary in the interests of the common good.

Natural law supports different cultures and religions, but unjust societies are those whose laws violate natural law.

Modern thinkers who appeal to natural law as a foundation for morality often lose sight of Aquinas's naturalism, presenting it as a transcendent rational capacity or divine command that overrides our natural instincts and desires. This manifests itself in the rationalist quest to conquer nature (now redounding on us in a looming environmental catastrophe), and in the Catholic church's attempt to use politics and law to impose its views on sexuality over and against changing social customs.

Aquinas argues that laws should change to reflect customs (although custom cannot change natural or divine law). I'll focus on two issues relating to this in terms of a widening gulf between the Catholic hierarchy and modern culture, including many Catholics.

Contraception: Aquinas believed that the sex act must be intended for procreation for the preservation of the species (ST II.II.153.2). He also believed that children need to be raised in a loving environment, and marriage is the proper context for this. But we now know that females are not always fertile, and sexual activity among animals seems to be less functional than he realised. The changing role of women is also a transformation in culture and custom that requires a radical rethinking of law and reproductive ethics. The prohibition of artificial birth control finds little support from this reading of natural law, particularly since it flies in the face of customary practice among many Catholics and non-Catholics.

Homosexuality: once one accepts that non-procreative, loving sex is good, the argument from natural law against homosexuality becomes untenable. What remains central from a Thomist perspective is what it means to live well as a sexual creature whose relationships reflect the love of God and respect the dignity of the human made in the image of God.

Natural law is our rational capacity to interpret the laws of nature in order to use our scientific knowledge well. It is still relevant, even if our science is very different from Aquinas's, as we see from debates about the ethical implications of the laws of evolution. The Darwinian eugenicists of the early 20th century were engaged in one kind of natural law deriving from evolutionary science, and debates on this blog about genetic altruism are another form of natural law. They follow Aquinas insofar as they express a desire to discern order and goodness rather than randomness and futility in what science reveals to us about nature. Evolutionary eugenics may be as rationally defensible as evolutionary altruism, so why do we think one is bad and the other is good? Aquinas would have said because one respects the dignity of the human made in the image of God and the other violates it, but without that perspective, the answer is less clear.

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