Personally I believe that Natural Law has some serious weaknesses.
My first reason for this is that Natural Law depends on accepting the view that good is what is found in nature. However from here it is easy to ask whether everything found in nature is actually good. Are, for example, cancerous tumours good? They could be if their goodness is found in fulfilling hr function of said cancerous tumour however being in that situation first hand, you found find it very difficult to agree.
Aquinas assumed that all people seek to worship God, which many could perceive to be artificial instead of natural. From here he assumed that God created the world and Natural Law within it. These assumptions would not be natural ones for an Atheist to make. This is where the link between Aquinas and Aristotle come in. Aquinas introduced the five primary precepts as a way God intended for people to live whereas Aristotle said it is down to human reasoning. With this it is clear that being a Christian makes Natural Law much more accessible as there is a set guide to follow whereas with Aristotle the path can often become blurred and again, the assumption for everyone to be religious is not a natural one to make.
My third reason is with defining exactly what good is. G.E Moore argued that goodness is unanalysable and unnatural and so can not be defined by any reference to nature. So with the definition of good being unobtainable, how can we all follow a life of good following what could potentially be just an apparent good as nobody really knows exactly knows what good is, we only believe we do.
However it can be argued that even before Aquinas developed Natural Law; it was a pretty stable concept which could be followed. As previously mentioned that we don’t truly know what good is, following our reason, as Aristotle said, would become much easier. This is because what we perceive to be good is personal to the individual and