There are a variety of theories that have been developed since the time of the ancient Greeks.
At the core is the belief that God has ordered the universe in a certain way and that humans have a duty to conform to the natural law which God has made.
Therefore to keep to the natural law is morally good, to of against that law is morally bad.
It is principally a religious ethical system but not completely so. It is possible to be an atheist and still believe in natural law.
Aristotle argued that everything in nature has a function, purpose and end. Eudaimonia is the final end of all things. It is best understood as the contentment which comes when a thing achieves its purpose.
Thomas Aquinas developed the natural law theory from the writings…