Aquinas looked particularly at Aristotle’s theory about the final cause and prime mover. The final cause being the purpose of everything and the prime mover, the thing that is pushing everything towards its final cause. From this he made 5 assumptions: Everything in nature has a purpose; God created all nature; failure to develop nature to do its purpose is an imperfection; God’s moral laws can be known through reasoning; and natural law is part of God’s divine plan. To summarise, there are certain laws of nature that we should follow in order to become perfect and reach our purpose/final cause.
The primary precepts are as follows: preservation of life, living in an ordered society, worshipping God, education and reproduction. According to Aquinas, our actions must always conform to these precepts. For example, abortion, suicide and murder are all wrong because they all go again the precept of preserving life. Whereas attending church every Sunday is something Aquinas thought we should do because it is a secondary precept of the first, worshipping God.
One more relative concept of Natural Law however, is that it can be more flexible. It recognises that all individual situations are different and we must consider this when making moral judgements. In some situations, the doctrine of double effect can be used. One example would be if a pregnant woman has cancer and the only way to save her would be for the doctor to perform a hysterectomy. This would kill the unborn child but save the mother, rather than them both dying. Natural Law says that it would be acceptable for the doctor to perform the hysterectomy because the act in itself is not wrong and the intentions of the doctor are not evil. The death of the unborn child is an unintended side effect.
To conclude, Natural Law is a deontological absolute theory that states that we should all follow the 5 primary precepts. Acts that follow the primary precepts are secondary precepts. However in some cases the doctrine of double effect can be used to justify an action which has bad consequences.