The natural law theory goes as far back as the 5th century BC, from a time when Antigone of Sophocles drew distinctions between written laws of state and unwritten laws; this was a very important movement. The Natural Law theory is rooted in the philosophy of Aristotle, from the 4th Century BC. Aristotle's view was that everything in life serves a purpose and as such distinguished efficient causes from final causes. 'Efficient' causes are those processes that get things done, for example, I use tools to create a figure from wood, whilst the 'final' cause is the end product such as, the figure I have created from wood using my tools.
As such he saw that every object and every action has a final purpose, telos, and this determines its 'good'. Thus if we understand what the final good of an organism is then we will understand the necessary or natural processes by means of which it will reach it. In the middle ages theologians took this principia ethica to argue that nature manifested the purposes of the Creator who established an order in all things. Thus they believed there were natural laws that govern human conduct and lead one to the highest Good.
The most famous advocate of the Natural Law Theory was the Christian philosopher Thomas Aquinas. Aquinas developed Aristotle's ideas and argued that the natural 'purpose' of the world is found in God. Humans are free beings and are capable of choosing to follow the 'natural law' of God which is understood through reason. He believed the 'human purpose' was, 'to reproduce, to learn, to live harmoniously in society and to worship God'.
Thus Natural law describes not only how things are but how things ought to be and this happens when things are fulfilling their natural purpose, and are unnatural when they are not. Moral laws are also grounded in Natural law. Aquinas believed human beings were created by God and that each one of us has a particular purpose that we are potentially capable of achieving. He also taught that morality is not primarily based on commands from God but on reason, which could be called philosophical investigation, which by its application can raise people to an awareness of morality.
Sin would be considered as falling short of this ideal. Although people know how they should live sometimes they become confused concerning the Ideal because they mistakenly associate apparent good with it, for example when people drink large quantities of beer which may make them feel good but at the same time it is destroying their liver which is not good, the Ideal is in fact to feel good without having to drinks lots of beer to achieve it. Aquinas believed that even though an act may be good in itself, like giving money to charity it can be done for wrong reasons.
Money may be given to charity so that the person can boast about it to others. He also taught a strong link between morality and happiness life as he did not believe the moral life, Natural Law, should be something humanity struggles to follow but something that should come naturally. The Natural Law Theory is a reasonable concept as it seems 'natural' for us to care about other people, animals and the environment and we often feel happier in ourselves for doing so, but Natural Law can also be the source of much controversy.
For instance, in pronouncing on sexual matters the Roman Catholic Church has adopted a Natural Law ethic to give guidance to its members on sex, contraception and homosexuality. Following Aquinas thus Aristotle, that because everything has a natural God-given purpose, they interpret the purpose of sex, the telos of sex, as procreation. Unhindered conception is the natural outcome of sexual activity between males and females. Anything that prohibits this natural outcome is seen as a barrier to the fulfilment of the sexual telos.
Thus contraception, oral and anal sex, masturbation and homosexuality are all understood as activities that prevent procreation from happening and are condemned as unnatural by the Catholic Church. The Catholic view of sex, using the Natural Law Theory, raises some interesting issues, one being, is the telos of sex always procreation? There seems to be confusion here between what is the case and what ought to be the case, like we could say that putting a knife into someone is a bad thing to do but ought not a surgeon do this to save someone's life?.
What about sexual intercourse as an expression of love between couples or as something enjoyable? If every sexual act is supposed to result in procreation it does not account for couples who are unable to have children. Is their relationship unnatural? Should they be having sex? What about women who get pregnant after being raped? In this context does Natural Law actually condone rape? The Natural Law ethic on sexual activity has also influenced Catholic views on Bioethics.
Artificial insemination, for example, is seen as unnatural because it does not take place within the context of a sexual act but is an 'activity' apart from the sexual act and may involve masturbation on the part of the male, but applying the Natural Law Theory one stage further, one could argue that because sexual activity is meant to produce children, within a married couple where this is not possible then the natural telos of this relationship is being hindered.
As the Catholic Church does not advocate divorce, in order that either partner may find other people with whom they can procreate, should not every possible step be taken to help this couple have children? The issue is really one of defining what the 'final' cause of something is? In this case is it that the sexual act should result in procreation or that couples should have children?
Despite looking rather absolutist in its postulates the Natural Law Theory is actually rather flexible. This can be seen where Aquinas sought to hold the notions of primary precepts in tension with secondary precepts. In this light it could be argued that Natural Law naturally leads one to Situations Ethics.