St Thomas Aquinas explained his theory of natural law, which shares many characteristics with other versions of natural law, in Summa 2i. 94.2.  He explained how natural laws are discoverable through RATIO, the human intellect, which is made up of SPECULATIVE REASON and PRACTICAL REASON.  Speculative reason reveals abstract notions and principles, such as “good is that which all things seek after,” then practical reason applies these to real life, developing moral precepts for us to follow.  For example, “this is the first precept of [natural] law, that “good is to be done and pursued, and evil is to be avoided.” Aquinas goes on to explain how “all other precepts of the natural law are based upon this: so that whatever the practical reason naturally apprehends as man’s good (or evil) belongs to the precepts of the natural law as something to be done or avoided.” He elaborated on the precepts of natural law, listing and explaining primary and secondary precepts, explaining how “Since… 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 consequently as objects of pursuit, and their contraries as evil, and objects of avoidance. Wherefore according to the order of natural inclinations, is the order of the precepts of the natural law.” Primary precepts are thus established as “objects of pursuit” or prescriptions and secondary precepts as “objects of avoidance” or prohibitions.  For examples, “whatever is a means of preserving human life, and of warding off its obstacles, belongs to the natural law…” meaning that the preservation of human life is a primary precept and “do not murder” is a secondary precept. Aquinas also lists as primary precepts sexual intercourse, the education of offspring, knowing the truth about God and shunning ignorance as well as living in society, avoiding offending those amongst whom one must live.  Overall, the five primary precepts are not the most important part of natural law.

Firstly, the five primary precepts are not the most important part of natural law because they are derived from the first precept “good is to be done and pursued, and evil is to be avoided”, and different scholars have derived different lists of primary precepts, which have gone on to produce different lists of secondary precepts and disagreements in how natural law guides people in practical situations. Aquinas discussed whether or not his list of primary precepts was infallible in Summa 2i. 94. 2 and concluded that the first precept “good is to be done and pursued, and evil is to be avoided” is infallible, because it is so closely related to the Primary Principle of speculative reason “good is that which all things seek after” and because speculative reason cannot be wrong, its objects being necessary truths.  Nevertheless, the primary precepts of the preservation of human life, sexual intercourse, the education of offspring, knowing the truth about God, shunning ignorance and living in society are known through synderesis, which must develop its understanding of these precepts as it is formed.  While nothing the synderesis suggests can be wrong, the list of primary precepts it provides might be incomplete, which explains why some societies seem unaware of some basic human goods.  Aquinas explains how it is possible to be unaware that living in society and avoiding offending neighbours is a primary precept, “thus formerly, theft, although it is expressly contrary to the natural law, was not considered wrong among the Germans, as Julius Caesar relates (De Bello Gall. vi).”  This would explain why later versions of natural law have listed the primary precepts differently, and particularly why some have added to Aquinas’ five.  For example, John Finnis lists seven basic human goods, including play and aesthetic experience.  This could be because Finnis’ synderesis has developed further than Aquinas’ did, coming to recognise the essential importance of fun and beauty to human flourishing in a way that Aquinas never did.  It follows that the fact that the list of five primary precepts provided by Aquinas, and indeed other lists in other versions of Natural Law, differ and might be incomplete suggests that they cannot be the most important part of natural law.

Secondly, the primary precepts are general injunctions and need to be translated into secondary precepts in order to provide useful moral guidance. Whereas the primary precepts are positive, secondary precepts are negative and tell people what they must not do.  For example, preserve human life is a primary precept and “do not murder” is a secondary precept.  Yet there are often multiple secondary precepts derived from each primary precept.  For example, the primary precept of living in society would lead to a huge number of secondary prohibitions, ranging from do not steal to do not lie to do not be jealous or aggressive… It follows that the majority of moral rules that people live by in practice are secondary and not primary precepts, suggesting that the secondary precepts are more important than the primary precepts.  Of course, secondary precepts are all derived from the primary precepts, so could not exist without the primary precepts and would differ if the primary precepts differed.  This might suggest that the primary precepts are more important than the secondary precepts, and yet for most people their moral awareness begins with the conscientia speaking against certain actions in a negative way, such as by warning people not to cheat or deceive.  Although it is true that the conscientia does this because it is seeking to apply the primary precepts to specific situations, in practice people are usually aware of the secondary precepts before or even without being aware of the primary precepts, which suggests that in fact the secondary precepts are more important.

On the other hand, Proportionalists like Bernard Hoose would argue that the primary precepts of Natural Law are more important than the secondary precepts because there are sometimes proportional justifications for breaking secondary precepts, but never for breaking primary precepts.  For example, the primary precept of preserving human life might lead to the secondary precept, don’t run inside… because running causes accidents and people might get hurt… but in the case of an explosion it might be proportionally justified to run, risking accidents, when this is the only way to save lives. Similarly, the primary precept of preserving human life might lead to a secondary prohibition against abortion, but in the case of an ectopic pregnancy where inaction would lead to the death of both mother and child, it might be proportionally justified to abort in order to preserve the maximum number of human lives. Hoose points out that even Aquinas uses proportional reasoning to justify war, capital punishment and even the toleration of prostitution… so surely this suggests that primary precepts are more important than secondary precepts, given that secondary precepts can be broken in the pursuit of good, but not primary precepts.  Nevertheless, while Hoose is right that proportional reasoning in the writing of Aquinas and later scholars suggests that secondary precepts are less important than primary precepts, proportionalism has been condemned by the Roman Catholic Church, the main adopter of Natural Law.  The Church sees proportionalism as a slippery slope towards situation ethics, utilitarianism and antinomianism in ethics. Instead, the Church has chosen to adopt a Heteronomous Ethic, with the Magisterium using Natural Law alongside Scripture and Tradition to develop Church Teachings which are Secondary Precepts.  These are then used as the basis for encyclicals, Catholic instruction and advice as well as for formation. It follows that for ordinary Roman Catholics secondary precepts are more important than Primary Precepts, because they are encouraged to follow Church Teaching rather than to try to develop it!  While the Church accepts Aquinas’ teaching about conscience, it emphasises the importance of formation so that the Synderesis and Conscientia guide Catholics to follow Church teaching in all matters, rather than really to think for themselves.  This suggests that in practice secondary precepts are in fact more important than primary principles, even though this might not be true to Aquinas’ intention.

In conclusion, the five primary precepts are not the most important part of natural law. As has been established, the list of five primary precepts might be incomplete and is certainly disputed, while also depending on the key precept “good is to be done and evil avoided”.  Further, secondary precepts make more difference in peoples’ lives today than primary precepts.