Another person to think of this Law was Aristotle. Aristotle was a Greek thinker; whose work was saved due to Islam saving all work in their scripture. Aristotle felt that what the law commanded varied from place to place, but what was "by nature" should be the same everywhere.
Aquinas Natural Law Theory came from readings from the Bible, mainly in Genesis 1 (which is about the creation of the world) and Genesis 20 (Decalogue were the precepts were generated from). ‘Natural laws are hierarchical in nature; secondary laws of nature are based on primary laws of nature...Natural laws exist because the universe has a Creator God who is logical and has imposed order on His universe’ is the extract from Genesis 1 that shows were Aquinas may have developed the idea of Natural Law.
Aquinas came up with four Laws that all interlink with each other, the first is Eternal Law- God's will and wisdom, which is revealed in, Divine Law- given in scripture and through the church, made know in, Natural law- from which, Human Law is derived. All laws echo each other, so without one the rest would not be possible; this comes to reason that everything in nature must have a purpose.
One part of Natural Law is the precepts, Primary and also Secondary which derives from the first Primary precept. There are five Primary precepts, which are written in descending order. The most important precept is self preservation and the preservation of the innocent; this could be interpenetrated by meaning that your sole needs to be pure for you/the innocent to achieve perfection. It is from this Primary precept that the Secondary precepts come from, these precepts are do not murder, do not abort, do not commit suicide and defend the defenseless. It is these that are crucial to achieving perfection, as more of these are considered evil.
The second Primary precept is the continuation of the species through reproduction. However the can be a Casuistry argument against this one. For example if there was a gay couple, every time they were intimate there is no change of contraception, which is want the second precept relies on to be completed. However Natural Law does make allowances for differences in situations.
Another argument is the double effect. This is when an action has more than one effect, which one is the real motive. For example if there was a terminal ill patient who required a strong pain-killer such as morphine, a measured does is given to end suffering, this is the real motive/first effect, however the does hastens death instead, this would be the secondary effect. This would not be classed as murder because this was not the first and real motive of the doctor, therefore the doctor would not be breaking the Secondary precept of Natural Law. So he would still be on the way to achieving perfection.