We are going to be looking in this video and blog post at the idea of natural law through the work of the legal philosopher whose name has become most closely associated with natural law: Thomas Aquinas. Aquinas wrote the Summa Theologiae in a tightly structured way: he begins with a question, which Aquinas then breaks down into a series of sub-questions called articles. For each sub-question, Aquinas begins by giving the wrong answer, then he may quote an authority who supports the right answer, and then he will proceed to explain what he thinks is the right answer and to deal with all the objections to his argument. Having established what is the right answer, he then moves on to the next sub-question and repeats the method. Once all the sub-questions have been answered, then he moves on to the next question. Although this method is highly logical, the answers Aquinas gives and the ways in which they link together are not always straightforward. Using an analogy from chess, one commentor has said about Aquinas that ‘his ideas move more like knights than rooks and are no less difficult to corner.’

Although Aquinas’s name has become synonymous with the idea of natural law, it is only a small part of his architecture of law. Aquinas asked 25 questions about law, of which eight deal with the Old Law, nine deal with the New Law, three deal with human law, and only one deals with the natural law.

The question on natural law is broken down into six sub-questions, which we will explore in the remainder of this post.

1. What is the natural law?

The natural law are those moral principles which are known to us through a properly formed conscience. The other word Aquinas uses “synderesis” may be one which is new to you. Synderesis refers to the innate ability of the mind to know the first principles of natural law and moral reasoning. It’s often described as an intuitive, instinctive knowledge of good and evil, guiding human actions towards the good. 

In an earlier question, 91, Aquinas argues that human beings know the natural law in ways that are linked both to our reason and to our will. Aquinas claims that there are certain principles that are known naturally, and those principles form the basis for right reasoning about what is good for ourselves, good for those we love, and good for society. Aquinas also claims that our appetites direct us towards things which are good: we want to eat because eating is good for the health of our bodies, we want to have sex because sexual reproduction ensures the survival of the human species, and so on.

What Aquinas has in mind may become clearer as we move on to the second question he asks:

2. What are the precepts of the natural law?

If we are going to do any sort of reasoning, we have to start somewhere. For Aquinas, we have to start with first principles. Those first principles, Aquinas believes, are self-evident. By self-evident, he means that they are indemonstrable but they require no demonstration. One cannot reasonably deny them.

Have you ever tried to have an argument with a three year old? Three year old children are good at asking one question, again and again. The question is: “But why?” When their parents are telling them to do something they do not want to do, the three year old child will ask “But why?” Whatever answer the parent gives to that question, the three year old child’s next question will be “But why?” Very soon the exasperated parent ends up saying: “Because I told you so”.

The first self-evident principle Aquinas puts forward is that good is to be done and ensured, and evil is to be avoided. Aquinas says that 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.

This is, like Kant’s categorical imperative, a purely formal principle. What we need to know is what things are good for human beings and what things are evil. Here Aquinas appeals to our natural inclinations. Our natural inclinations, the things we desire, are naturally apprehended by our reason as good, so that our desires and our reason ought to align to lead us to pursue the things which are good for human beings and to avoid the things which are bad for human beings.

Aquinas does not stop there, however. He suggests that human beings have natural inclinations at three different levels: natural inclinations simply because we are alive, natural inclinations because we are animals, and natural inclinations because we have reason.

The natural inclination we have simply because we are alive is our survival instinct. Aquinas says that by reason of this inclination, whatever is a means of preserving human life, and of warding off its obstacles, belongs to the natural law.

The natural inclinations we share with other animals include sexual intercourse, education of off-spring and so forth.

The natural inclinations we have because we have reason include, according to Aquinas, a natural inclination to know the truth about God, and to live in society. From this Aquinas derives principles such as the desire to shun ignorance and the desire to avoid offending those among whom one has to live, and other such things regarding the above inclination.

At this point, Aquinas has set out the key elements of his argument for natural law. The natural law is the set of moral precepts which we cannot sensibly deny and which should shape our reasoning and our desires.

Aquinas then turns to his third question:

3. Whether all acts of virtue are prescribed by the natural law?

As a Christian theologian and a monk, Aquinas’s argument about how our natural desires and our reason help us to identify the natural law runs into an objection. The objection is this: you say that the survival instinct is part of the natural law, what about martyrs? You say that sexual reproduction is part of the natural law, what about Jesus, Paul, and all the Christian monks and nuns? Aquinas’s answer is that not all virtuous acts are prescribed by the natural law: for many things are done virtuously, to which nature does not incline at first; but which, through the inquiry of reason, have been found by men to be conducive to well-living. He could go on to say, but does not at this point, that although the natural law identifies the things which it is good to pursue in general, specific people may have individual vocations which call them to self-denial. The way Aquinas puts it is that owing to the various conditions of men, certain acts are virtuous for some, as being proportionate and becoming to them, while they are vicious for others, as being out of proportion to them.

Aquinas returns to the main theme in his fourth question:

4. Whether the natural law is the same in all?

In answering this question, we have an example of Aquinas appealing to an authority in support of his answer. Isidore, was a seventh century Christian scholar who wrote: The natural law is common to all nations. What Isidore and Aquinas both hold is that there is a universal human nature, and that all human beings must acknowledge the same self-evident principles and see the reasonableness of the same desires.

Aquinas distinguishes, however, between two kinds of reasoning. One kind of reason is speculation, the other is practical reasoning. Speculative reason is busied chiefly with necessary things, which cannot be otherwise than they are. Therefore, if we do our speculative reasoning correctly, we will always come to same conclusions, because those conclusions are necessarily and universally true. In order to demonstrate the proper conclusions of speculative reason, Aquinas gives the example of a triangle: the truth is the same for all, but is not equally known to all: thus it is true for all that the three angles of a triangle are together equal to two right angles (180 degrees), although it is not known to all.

Practical reason, on the other hand, is busied with contingent matters, about which human actions are concerned: and consequently, although there is necessity in the general principles, the more we descend to matters of detail, the more frequently we encounter defects or differences. If we do our practical reasoning correctly, we should arrive at principles which Aquinas calls common notions. When it comes to the application of those common notions, what Aquinas calls matters of detail, it is much harder to discern what is the right thing to do.

The example Aquinas gives here is: should goods entrusted to another be restored to their owner? His answer is: generally speaking yes, but it may happen in a particular case that it would be injurious, and therefore unreasonable, to restore goods held in trust; for instance if they are claimed for the purpose of fighting against one’s country. And this principle will be found to fail the more, according as we descend further into detail, e.g., if one were to say that goods held in trust should be restored with such and such a guarantee, or in such and such a way; because the greater the number of conditions added, the greater the number of ways in which the principle may fail, so that it be not right to restore or not to restore.

Aquinas’s fifth question is whether the natural law is always the same everywhere or whether the natural law can change.

5. Whether it is changeable?

Aquinas’s answer here is somewhat surprising. Although the first principles of the natural law do not change, Aquinas is prepared to accept that things may be added to the natural law if it can be demonstrated that they are for the benefit of human life. Here, Aquinas’s discussion is a little confused. What he is really doing is identifying that although the natural law is necessary, it is not sufficient to enable human communities to flourish. Therefore, he says, the written law is given for the correction of the natural law, either because it supplies what was wanting to the natural law; or because the natural law was perverted in the hearts of some men, as to certain matters, so that they esteemed those things good which are naturally evil; which perversion stood in need of correction.

Aquinas’s last question is:

6. Whether the natural law can be abolished from the heart of man?

There is a lot at stake in this question. Aquinas recognised, as the Reformation would later insist, that we do not always reason rightly and that our desires are not always orientated towards things which are good for us. However, he also thought that guilt depends on breaking a law which you knew or ought to have known.

Aquinas’s answer to the question of whether the natural law can be forgotten by human beings is that there belong 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. As to those general principles, the natural law, in the abstract, can nowise be blotted out from men’s hearts. But it is blotted out in the case of a particular action, in so far as reason is hindered from applying the general principle to a particular point of practice, on account of concupiscence or some other passion, as stated above. So, for example, cheating on your spouse is bad, but people who do cheat on their spouse usually try to rationalise their actions.

When it comes to the secondary precepts of natural law, Aquinas recognises that the natural law can be blotted out from the human heart, either by evil persuasions, just as in speculative matters errors occur in respect of necessary conclusions; or by vicious customs and corrupt habits, as among some men, theft, and even unnatural vices, as the Apostle Paul states in Romans chapter 1, were not esteemed sinful. Aquinas thinks that individuals and even whole societies can forget or mistake parts of the natural law but can never deny the natural law in its entirety.