Natural law was thought of by a 13th century monk called Thomas Aquinas. He was inspired by a Greek a posteriori philosopher called Aristotle who came up with the efficient cause and believed that a humans purpose is to reach eudaimonia. Aristotle's thoughts were inspired by the stoics who were a group of Greek philosophers who believed God is everywhere and in everyone there is a divine spark. Natural law is based on the belief that everyone has a natural sense that 'good is to be done and evil avoided' which somse call human nature. It is what directs our conscience and if applied with reason to a situation will lead to the right outcome.
Some people may say that natural law is the best approach to ethical desicion making, because it follows a certain set of rules, for example the primary precepts which consist of preservation of life, ordered and peaceful society, worship God, educate children and reproduce. These are seen as absolutist and this could be an advantage, because they do not change, meaning if in a situation and you're not sure what to do, people may like to have a set of rules which do not change or alter, then you can apply which ever primary precept approprite to the current situation you are in. For example you should never commit suicide, because it breaks the primary precept of preservation of life. You can then go on to link this to the secondary precepts, which consist of the rules do not murder, do not abort unborn, defend the weak and do not commit suicide. These however allow a level of flexibility and are deemed quite realistic, meaning more relativist than the primary precepets. This can also be a strength, because the secondary precepts allow more flexibility meaning you can apply it to your situation more easily. Therefore natural law would be the best approadch to ethical desicion making.
On the other hand, people may say that natural law isn't the best