Natural Law


Key Points

Theory in detail

Everything has a purpose

Ethics is the struggle to determine what is right or wrong, or ‘good’ and ‘bad’.  Some ethical theories are hedonistic – they say that pleasure (and the absence of pain) are the only ultimately ‘good’ ends towards which to aim.  Some Christian ethicists argue that following God’s will – as revealed through prayer, scriptures and prophecy – is the ultimate good.

The theory of Natural Law was put forward by Aristotle but championed by Thomas Aquinas (1225-74). Natural Law has elements of both of these approaches. Man desires happiness, but for Aquinas this means fulfilling our purpose as humans. He said, in Summa Theologica, "whatever man desires, he desires it under the aspect of good." Fulfilling our purpose is the only ‘good’ for humans.

We will see that Aquinas first asks what our human nature is, and then looks at the rules that can be derived from this.

The purpose of humans - the Primary Precepts

There is a single guiding principle that sums up our nature:

"good is to be done and pursued, and evil is to be avoided." All other precepts of the natural law are based upon this.

Aquinas looks at what is 'good' for humans, saying that humans share part of their nature with all natural things, part with animals, and part of our human nature is particular to us. Aquinas said:

Although textbooks talk of five Primary Precepts, and some resources on this site reflect this, Bernard Hoose revealed, over lunch at an Ethics conference, his frustration with this tendency. He felt there were only three, as can be seen above. Read Summa Theologica yourself, and you may feel that Aquinas is not giving an exhaustive list, but simply some examples of "self evident principles" perceived by reason.

A mnemonic for these might be PREGS:

Teleology and Deontology

In 1930, CD Broad contrasted teleology and deontology in an attempt to categorise ethical theories. By teleology, he meant theories where "the rightness or wrongness of an action is always determined by its tendency to produce consequences which are intrinsically good or bad". According to Broad, deontological theories hold that "such and such a kind of action would always be right (or wrong) in such and such circumstances, no matter what its consequences might be". In essence, teleology is concerned with good and bad, deontology with right and wrong.

This distinction, and these definitions, are seen by many ethicists as unhelpful, but they are on most syllabuses. Broad admitted that "most theories are actually mixed", and we can see this in Natural Law.

In defining the Primary Precepts, Aquinas was stating 'self-evident principles' that are universal and absolute - they are part of our very nature as humans. This sounds deontological. However, looking at Aristotle's notion of telos as excellence, we see that the Primary Precepts are not concerned with actions themselves, but with our telos or purpose. As such, the Primary Precepts are actually teleological. For Aquinas, man's final purpose (telos) is happiness with God (beatitudo), something for which we all have an innate desire. The Primary Precepts are our natural inclinations that guide us towards this final purpose.

From the general principles, practical reason enables us to derive secondary precepts. These are rules that govern our specific actions. The secondary precepts are what makes Natural Law appear deontological. They concern rules for our actions, for example "Goods entrusted to another should be restored to their owner". If I am looking after your money, I should not give it away to a charity, even if doing so would bring about some good. It would be the wrong sort of action. I have a duty to return to you what I was entrusted with.

However, this is not always an absolute duty, and this causes confusion in understanding Natural Law Using the above example, Aquinas says:

it is right and true for all to act according to reason : and from this principle it follows as a proper conclusion, that goods entrusted to another should be restored to their owner. Now this is true for the majority of cases: 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.

This doesn't mean that we do not have a duty to return goods entrusted to us, but that there may be conflicting duties that prevent us from doing so.

Secondary Precepts

Secondary Precepts are rules derived from Primary Precepts using practical reason. In some cases, they refer to an action that is unnatural, and is therefore always wrong. Certain actions were seen by Aquinas to be contrary to human nature. Reason would then give us absolute secondary precepts that would always hold. For example, Aquinas felt that masturbation went against the natural end (telos) of sex, which is procreation. This means that 'Do not masturbate' is an absolute secondary precept. However, in modern infertility treatment, masturbation might be used to assist procreation through artificial insemination by a husband. On this issue, Natural Law theorists disagree about whether masturbation is unnatural, and therefore disagree about the secondary precept 'Do not masturbate'.

Aquinas gives examples (in Summa Theologica) as illustrations of those actions that are wrong in and of themselves because they contradict the primary precepts of natural law:

These are all examples of deonotological, absolutist secondary precepts derived from the self-evident, universal teleological Primary Precepts. Other examples include the Ten Commandments.

Some secondary precepts are deontological (concerning actions rather than ends, and related to our specific duties), but not absolutist. An example is given above.

‘Efficient’ and ‘Final’ Causes

This is Aristotle’s distinction between what gets things done (efficient cause) and the end product (final cause).   With humans, it is the accomplishment of the end product that equates to ‘good’.  An example is sexuality – an efficient cause of sex is enjoyment: because humans enjoy sex, the species has survived through procreation.  However, the final cause of sex (the thing God designed it for) is procreation.   Therefore sex is only good if procreation is possible.

Put another way, the efficient cause is a statement of fact or a description.  If we ask why people have sex, we might talk about attraction, psychological needs etc.    The final cause is a matter of intent – what was God’s purpose behind sex?  The final cause assumes a rational mind behind creation, and as such moves from descriptive ethics (saying what is there) to normative ethics (statements about what should or should not be the case).

Another example – did the soldier shoot well?  The efficient cause deals with the set of events around the shooting – did he aim well, was the shot effective, did the target die?  These are descriptive points, and clearly don’t tell us about the morality of the shooting.   When we look into this area – was it right to kill? -  we are evaluating his intent, and are asking about the final cause.  We can then look at whether that cause is consistent with God’s design for human beings.   We may decide that killing innocent people goes against God’s design for us, so it is always wrong to kill innocent people.

Real and Apparent Goods

Aquinas argued that the self should be maintained.  As a result, Natural Law supports certain virtues (prudence, justice, fortitude and temperance) that allow the self to fulfil its purpose.  Similarly there are many vices (the seven deadly sins) that must be avoided as they prevent the individual from being what God intended them to be.

Following a ‘real’ good will result in the preservation or improvement of self, getting nearer to the ‘ideal human nature’ that God had planned.  There are many apparent goods that may be pleasurable (e.g. drugs) but ultimately lead us to fall short of our potential.   Reason is used to determine the ‘real’ goods.


Aquinas believed in life after death, which leads to a different understanding of God’s plan for humans.  Natural Law can be upheld by atheists, but there seems no good reason for keeping to Natural Law without God.   Aquinas holds that the one goal of human life should be ‘the vision of God which is promised in the next life’.  This is why humans were made, and should be at the centre of Natural Law thinking.

Casuistry and Double Effect

Casuistry, from the Latin for 'case', refers to the process of applying principles to individual cases. In the Roman Catholic Church, this means applying the universal principles of Natural Law to specific situations.   This is done in a logical way, as some principles have logical consequences.  For example, if it is in principle wrong to kill innocent human beings, it follows that bombing civilian targets (such as Dresden in WW2) is wrong.  However, if it is accepted that killing in self defence is okay, we could justify an air attack on Afghanistan on these grounds.  Innocent people might die, but that is not the aim of the action, so the doctrine of double effect comes in to play. 

Double effect refers to situations where there is an intended outcome and another significant but unintentional outcome.  According to Natural Law, it is our intentions that are important, not the consequences of our actions. Double effect would not allow you to perform an action where an unintended outcome had devestating effects. The unintended effect has to be PROPORTIONATE. What this actually means, critics say, is that Natural Law becomes like Utilitarianism.

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